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 /*
  *  Generic process-grouping system.
  *
  *  Based originally on the cpuset system, extracted by Paul Menage
  *  Copyright (C) 2006 Google, Inc
  *
  *  Notifications support
  *  Copyright (C) 2009 Nokia Corporation
  *  Author: Kirill A. Shutemov
  *
  *  Copyright notices from the original cpuset code:
  *  --------------------------------------------------
  *  Copyright (C) 2003 BULL SA.
  *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
  *
  *  Portions derived from Patrick Mochel's sysfs code.
  *  sysfs is Copyright (c) 2001-3 Patrick Mochel
  *
  *  2003-10-10 Written by Simon Derr.
  *  2003-10-22 Updates by Stephen Hemminger.
  *  2004 May-July Rework by Paul Jackson.
  *  ---------------------------------------------------
  *
  *  This file is subject to the terms and conditions of the GNU General Public
  *  License.  See the file COPYING in the main directory of the Linux
  *  distribution for more details.
  */
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include "cgroup-internal.h"
 
 #include <linux/bpf-cgroup.h>
 #include <linux/cred.h>
 #include <linux/errno.h>
 #include <linux/init_task.h>
 #include <linux/kernel.h>
 #include <linux/magic.h>
 #include <linux/mutex.h>
 #include <linux/mount.h>
 #include <linux/pagemap.h>
 #include <linux/proc_fs.h>
 #include <linux/rcupdate.h>
 #include <linux/sched.h>
 #include <linux/sched/task.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
 #include <linux/percpu-rwsem.h>
 #include <linux/string.h>
 #include <linux/hashtable.h>
 #include <linux/idr.h>
 #include <linux/kthread.h>
 #include <linux/atomic.h>
 #include <linux/cpuset.h>
 #include <linux/proc_ns.h>
 #include <linux/nsproxy.h>
 #include <linux/file.h>
 #include <linux/fs_parser.h>
 #include <linux/sched/cputime.h>
 #include <linux/psi.h>
 #include <net/sock.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/cgroup.h>
 
 #define CGROUP_FILE_NAME_MAX        (MAX_CGROUP_TYPE_NAMELEN +  \
                      MAX_CFTYPE_NAME + 2)
 /* let's not notify more than 100 times per second */
 #define CGROUP_FILE_NOTIFY_MIN_INTV DIV_ROUND_UP(HZ, 100)
 
 /*
  * To avoid confusing the compiler (and generating warnings) with code
  * that attempts to access what would be a 0-element array (i.e. sized
  * to a potentially empty array when CGROUP_SUBSYS_COUNT == 0), this
  * constant expression can be added.
  */
 #define CGROUP_HAS_SUBSYS_CONFIG    (CGROUP_SUBSYS_COUNT > 0)
 
 /*
  * cgroup_mutex is the master lock.  Any modification to cgroup or its
  * hierarchy must be performed while holding it.
  *
  * css_set_lock protects task->cgroups pointer, the list of css_set
  * objects, and the chain of tasks off each css_set.
  *
  * These locks are exported if CONFIG_PROVE_RCU so that accessors in
  * cgroup.h can use them for lockdep annotations.
  */
 DEFINE_MUTEX(cgroup_mutex);
 DEFINE_SPINLOCK(css_set_lock);
 
 #ifdef CONFIG_PROVE_RCU
 EXPORT_SYMBOL_GPL(cgroup_mutex);
 EXPORT_SYMBOL_GPL(css_set_lock);
 #endif
 
 DEFINE_SPINLOCK(trace_cgroup_path_lock);
 char trace_cgroup_path[TRACE_CGROUP_PATH_LEN];
 static bool cgroup_debug __read_mostly;
 
 /*
  * Protects cgroup_idr and css_idr so that IDs can be released without
  * grabbing cgroup_mutex.
  */
 static DEFINE_SPINLOCK(cgroup_idr_lock);
 
 /*
  * Protects cgroup_file->kn for !self csses.  It synchronizes notifications
  * against file removal/re-creation across css hiding.
  */
 static DEFINE_SPINLOCK(cgroup_file_kn_lock);
 
 DEFINE_PERCPU_RWSEM(cgroup_threadgroup_rwsem);
 
 #define cgroup_assert_mutex_or_rcu_locked()             \
     RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&           \
                !lockdep_is_held(&cgroup_mutex),     \
                "cgroup_mutex or RCU read lock required");
 
 /*
  * cgroup destruction makes heavy use of work items and there can be a lot
  * of concurrent destructions.  Use a separate workqueue so that cgroup
  * destruction work items don't end up filling up max_active of system_wq
  * which may lead to deadlock.
  */
 static struct workqueue_struct *cgroup_destroy_wq;
 
 /* generate an array of cgroup subsystem pointers */
 #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys,
 struct cgroup_subsys *cgroup_subsys[] = {
 #include <linux/cgroup_subsys.h>
 };
 #undef SUBSYS
 
 /* array of cgroup subsystem names */
 #define SUBSYS(_x) [_x ## _cgrp_id] = #_x,
 static const char *cgroup_subsys_name[] = {
 #include <linux/cgroup_subsys.h>
 };
 #undef SUBSYS
 
 /* array of static_keys for cgroup_subsys_enabled() and cgroup_subsys_on_dfl() */
 #define SUBSYS(_x)                              \
     DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_enabled_key);         \
     DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_on_dfl_key);          \
     EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_enabled_key);          \
     EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_on_dfl_key);
 #include <linux/cgroup_subsys.h>
 #undef SUBSYS
 
 #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_enabled_key,
 static struct static_key_true *cgroup_subsys_enabled_key[] = {
 #include <linux/cgroup_subsys.h>
 };
 #undef SUBSYS
 
 #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_on_dfl_key,
 static struct static_key_true *cgroup_subsys_on_dfl_key[] = {
 #include <linux/cgroup_subsys.h>
 };
 #undef SUBSYS
 
 static DEFINE_PER_CPU(struct cgroup_rstat_cpu, cgrp_dfl_root_rstat_cpu);
 
 /* the default hierarchy */
 struct cgroup_root cgrp_dfl_root = { .cgrp.rstat_cpu = &cgrp_dfl_root_rstat_cpu };
 EXPORT_SYMBOL_GPL(cgrp_dfl_root);
 
 /*
  * The default hierarchy always exists but is hidden until mounted for the
  * first time.  This is for backward compatibility.
  */
 static bool cgrp_dfl_visible;
 
 /* some controllers are not supported in the default hierarchy */
 static u16 cgrp_dfl_inhibit_ss_mask;
 
 /* some controllers are implicitly enabled on the default hierarchy */
 static u16 cgrp_dfl_implicit_ss_mask;
 
 /* some controllers can be threaded on the default hierarchy */
 static u16 cgrp_dfl_threaded_ss_mask;
 
 /* The list of hierarchy roots */
 LIST_HEAD(cgroup_roots);
 static int cgroup_root_count;
 
 /* hierarchy ID allocation and mapping, protected by cgroup_mutex */
 static DEFINE_IDR(cgroup_hierarchy_idr);
 
 /*
  * Assign a monotonically increasing serial number to csses.  It guarantees
  * cgroups with bigger numbers are newer than those with smaller numbers.
  * Also, as csses are always appended to the parent's ->children list, it
  * guarantees that sibling csses are always sorted in the ascending serial
  * number order on the list.  Protected by cgroup_mutex.
  */
 static u64 css_serial_nr_next = 1;
 
 /*
  * These bitmasks identify subsystems with specific features to avoid
  * having to do iterative checks repeatedly.
  */
 static u16 have_fork_callback __read_mostly;
 static u16 have_exit_callback __read_mostly;
 static u16 have_release_callback __read_mostly;
 static u16 have_canfork_callback __read_mostly;
 
 /* cgroup namespace for init task */
 struct cgroup_namespace init_cgroup_ns = {
     .ns.count   = REFCOUNT_INIT(2),
     .user_ns    = &init_user_ns,
     .ns.ops     = &cgroupns_operations,
     .ns.inum    = PROC_CGROUP_INIT_INO,
     .root_cset  = &init_css_set,
 };
 
 static struct file_system_type cgroup2_fs_type;
 static struct cftype cgroup_base_files[];
 
 /* cgroup optional features */
 enum cgroup_opt_features {
 #ifdef CONFIG_PSI
     OPT_FEATURE_PRESSURE,
 #endif
     OPT_FEATURE_COUNT
 };
 
 static const char *cgroup_opt_feature_names[OPT_FEATURE_COUNT] = {
 #ifdef CONFIG_PSI
     "pressure",
 #endif
 };
 
 static u16 cgroup_feature_disable_mask __read_mostly;
 
 static int cgroup_apply_control(struct cgroup *cgrp);
 static void cgroup_finalize_control(struct cgroup *cgrp, int ret);
 static void css_task_iter_skip(struct css_task_iter *it,
                    struct task_struct *task);
 static int cgroup_destroy_locked(struct cgroup *cgrp);
 static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
                           struct cgroup_subsys *ss);
 static void css_release(struct percpu_ref *ref);
 static void kill_css(struct cgroup_subsys_state *css);
 static int cgroup_addrm_files(struct cgroup_subsys_state *css,
                   struct cgroup *cgrp, struct cftype cfts[],
                   bool is_add);
 
 /**
  * cgroup_ssid_enabled - cgroup subsys enabled test by subsys ID
  * @ssid: subsys ID of interest
  *
  * cgroup_subsys_enabled() can only be used with literal subsys names which
  * is fine for individual subsystems but unsuitable for cgroup core.  This
  * is slower static_key_enabled() based test indexed by @ssid.
  */
 bool cgroup_ssid_enabled(int ssid)
 {
     if (!CGROUP_HAS_SUBSYS_CONFIG)
         return false;
 
     return static_key_enabled(cgroup_subsys_enabled_key[ssid]);
 }
 
 /**
  * cgroup_on_dfl - test whether a cgroup is on the default hierarchy
  * @cgrp: the cgroup of interest
  *
  * The default hierarchy is the v2 interface of cgroup and this function
  * can be used to test whether a cgroup is on the default hierarchy for
  * cases where a subsystem should behave differently depending on the
  * interface version.
  *
  * List of changed behaviors:
  *
  * - Mount options "noprefix", "xattr", "clone_children", "release_agent"
  *   and "name" are disallowed.
  *
  * - When mounting an existing superblock, mount options should match.
  *
  * - rename(2) is disallowed.
  *
  * - "tasks" is removed.  Everything should be at process granularity.  Use
  *   "cgroup.procs" instead.
  *
  * - "cgroup.procs" is not sorted.  pids will be unique unless they got
  *   recycled in-between reads.
  *
  * - "release_agent" and "notify_on_release" are removed.  Replacement
  *   notification mechanism will be implemented.
  *
  * - "cgroup.clone_children" is removed.
  *
  * - "cgroup.subtree_populated" is available.  Its value is 0 if the cgroup
  *   and its descendants contain no task; otherwise, 1.  The file also
  *   generates kernfs notification which can be monitored through poll and
  *   [di]notify when the value of the file changes.
  *
  * - cpuset: tasks will be kept in empty cpusets when hotplug happens and
  *   take masks of ancestors with non-empty cpus/mems, instead of being
  *   moved to an ancestor.
  *
  * - cpuset: a task can be moved into an empty cpuset, and again it takes
  *   masks of ancestors.
  *
  * - blkcg: blk-throttle becomes properly hierarchical.
  *
  * - debug: disallowed on the default hierarchy.
  */
 bool cgroup_on_dfl(const struct cgroup *cgrp)
 {
     return cgrp->root == &cgrp_dfl_root;
 }
 
 /* IDR wrappers which synchronize using cgroup_idr_lock */
 static int cgroup_idr_alloc(struct idr *idr, void *ptr, int start, int end,
                 gfp_t gfp_mask)
 {
     int ret;
 
     idr_preload(gfp_mask);
     spin_lock_bh(&cgroup_idr_lock);
     ret = idr_alloc(idr, ptr, start, end, gfp_mask & ~__GFP_DIRECT_RECLAIM);
     spin_unlock_bh(&cgroup_idr_lock);
     idr_preload_end();
     return ret;
 }
 
 static void *cgroup_idr_replace(struct idr *idr, void *ptr, int id)
 {
     void *ret;
 
     spin_lock_bh(&cgroup_idr_lock);
     ret = idr_replace(idr, ptr, id);
     spin_unlock_bh(&cgroup_idr_lock);
     return ret;
 }
 
 static void cgroup_idr_remove(struct idr *idr, int id)
 {
     spin_lock_bh(&cgroup_idr_lock);
     idr_remove(idr, id);
     spin_unlock_bh(&cgroup_idr_lock);
 }
 
 static bool cgroup_has_tasks(struct cgroup *cgrp)
 {
     return cgrp->nr_populated_csets;
 }
 
 bool cgroup_is_threaded(struct cgroup *cgrp)
 {
     return cgrp->dom_cgrp != cgrp;
 }
 
 /* can @cgrp host both domain and threaded children? */
 static bool cgroup_is_mixable(struct cgroup *cgrp)
 {
     /*
      * Root isn't under domain level resource control exempting it from
      * the no-internal-process constraint, so it can serve as a thread
      * root and a parent of resource domains at the same time.
      */
     return !cgroup_parent(cgrp);
 }
 
 /* can @cgrp become a thread root? Should always be true for a thread root */
 static bool cgroup_can_be_thread_root(struct cgroup *cgrp)
 {
     /* mixables don't care */
     if (cgroup_is_mixable(cgrp))
         return true;
 
     /* domain roots can't be nested under threaded */
     if (cgroup_is_threaded(cgrp))
         return false;
 
     /* can only have either domain or threaded children */
     if (cgrp->nr_populated_domain_children)
         return false;
 
     /* and no domain controllers can be enabled */
     if (cgrp->subtree_control & ~cgrp_dfl_threaded_ss_mask)
         return false;
 
     return true;
 }
 
 /* is @cgrp root of a threaded subtree? */
 bool cgroup_is_thread_root(struct cgroup *cgrp)
 {
     /* thread root should be a domain */
     if (cgroup_is_threaded(cgrp))
         return false;
 
     /* a domain w/ threaded children is a thread root */
     if (cgrp->nr_threaded_children)
         return true;
 
     /*
      * A domain which has tasks and explicit threaded controllers
      * enabled is a thread root.
      */
     if (cgroup_has_tasks(cgrp) &&
         (cgrp->subtree_control & cgrp_dfl_threaded_ss_mask))
         return true;
 
     return false;
 }
 
 /* a domain which isn't connected to the root w/o brekage can't be used */
 static bool cgroup_is_valid_domain(struct cgroup *cgrp)
 {
     /* the cgroup itself can be a thread root */
     if (cgroup_is_threaded(cgrp))
         return false;
 
     /* but the ancestors can't be unless mixable */
     while ((cgrp = cgroup_parent(cgrp))) {
         if (!cgroup_is_mixable(cgrp) && cgroup_is_thread_root(cgrp))
             return false;
         if (cgroup_is_threaded(cgrp))
             return false;
     }
 
     return true;
 }
 
 /* subsystems visibly enabled on a cgroup */
 static u16 cgroup_control(struct cgroup *cgrp)
 {
     struct cgroup *parent = cgroup_parent(cgrp);
     u16 root_ss_mask = cgrp->root->subsys_mask;
 
     if (parent) {
         u16 ss_mask = parent->subtree_control;
 
         /* threaded cgroups can only have threaded controllers */
         if (cgroup_is_threaded(cgrp))
             ss_mask &= cgrp_dfl_threaded_ss_mask;
         return ss_mask;
     }
 
     if (cgroup_on_dfl(cgrp))
         root_ss_mask &= ~(cgrp_dfl_inhibit_ss_mask |
                   cgrp_dfl_implicit_ss_mask);
     return root_ss_mask;
 }
 
 /* subsystems enabled on a cgroup */
 static u16 cgroup_ss_mask(struct cgroup *cgrp)
 {
     struct cgroup *parent = cgroup_parent(cgrp);
 
     if (parent) {
         u16 ss_mask = parent->subtree_ss_mask;
 
         /* threaded cgroups can only have threaded controllers */
         if (cgroup_is_threaded(cgrp))
             ss_mask &= cgrp_dfl_threaded_ss_mask;
         return ss_mask;
     }
 
     return cgrp->root->subsys_mask;
 }
 
 /**
  * cgroup_css - obtain a cgroup's css for the specified subsystem
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest (%NULL returns @cgrp->self)
  *
  * Return @cgrp's css (cgroup_subsys_state) associated with @ss.  This
  * function must be called either under cgroup_mutex or rcu_read_lock() and
  * the caller is responsible for pinning the returned css if it wants to
  * keep accessing it outside the said locks.  This function may return
  * %NULL if @cgrp doesn't have @subsys_id enabled.
  */
 static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
                           struct cgroup_subsys *ss)
 {
     if (CGROUP_HAS_SUBSYS_CONFIG && ss)
         return rcu_dereference_check(cgrp->subsys[ss->id],
                     lockdep_is_held(&cgroup_mutex));
     else
         return &cgrp->self;
 }
 
 /**
  * cgroup_tryget_css - try to get a cgroup's css for the specified subsystem
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest
  *
  * Find and get @cgrp's css associated with @ss.  If the css doesn't exist
  * or is offline, %NULL is returned.
  */
 static struct cgroup_subsys_state *cgroup_tryget_css(struct cgroup *cgrp,
                              struct cgroup_subsys *ss)
 {
     struct cgroup_subsys_state *css;
 
     rcu_read_lock();
     css = cgroup_css(cgrp, ss);
     if (css && !css_tryget_online(css))
         css = NULL;
     rcu_read_unlock();
 
     return css;
 }
 
 /**
  * cgroup_e_css_by_mask - obtain a cgroup's effective css for the specified ss
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest (%NULL returns @cgrp->self)
  *
  * Similar to cgroup_css() but returns the effective css, which is defined
  * as the matching css of the nearest ancestor including self which has @ss
  * enabled.  If @ss is associated with the hierarchy @cgrp is on, this
  * function is guaranteed to return non-NULL css.
  */
 static struct cgroup_subsys_state *cgroup_e_css_by_mask(struct cgroup *cgrp,
                             struct cgroup_subsys *ss)
 {
     lockdep_assert_held(&cgroup_mutex);
 
     if (!ss)
         return &cgrp->self;
 
     /*
      * This function is used while updating css associations and thus
      * can't test the csses directly.  Test ss_mask.
      */
     while (!(cgroup_ss_mask(cgrp) & (1 << ss->id))) {
         cgrp = cgroup_parent(cgrp);
         if (!cgrp)
             return NULL;
     }
 
     return cgroup_css(cgrp, ss);
 }
 
 /**
  * cgroup_e_css - obtain a cgroup's effective css for the specified subsystem
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest
  *
  * Find and get the effective css of @cgrp for @ss.  The effective css is
  * defined as the matching css of the nearest ancestor including self which
  * has @ss enabled.  If @ss is not mounted on the hierarchy @cgrp is on,
  * the root css is returned, so this function always returns a valid css.
  *
  * The returned css is not guaranteed to be online, and therefore it is the
  * callers responsibility to try get a reference for it.
  */
 struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
                      struct cgroup_subsys *ss)
 {
     struct cgroup_subsys_state *css;
 
     if (!CGROUP_HAS_SUBSYS_CONFIG)
         return NULL;
 
     do {
         css = cgroup_css(cgrp, ss);
 
         if (css)
             return css;
         cgrp = cgroup_parent(cgrp);
     } while (cgrp);
 
     return init_css_set.subsys[ss->id];
 }
 
 /**
  * cgroup_get_e_css - get a cgroup's effective css for the specified subsystem
  * @cgrp: the cgroup of interest
  * @ss: the subsystem of interest
  *
  * Find and get the effective css of @cgrp for @ss.  The effective css is
  * defined as the matching css of the nearest ancestor including self which
  * has @ss enabled.  If @ss is not mounted on the hierarchy @cgrp is on,
  * the root css is returned, so this function always returns a valid css.
  * The returned css must be put using css_put().
  */
 struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgrp,
                          struct cgroup_subsys *ss)
 {
     struct cgroup_subsys_state *css;
 
     if (!CGROUP_HAS_SUBSYS_CONFIG)
         return NULL;
 
     rcu_read_lock();
 
     do {
         css = cgroup_css(cgrp, ss);
 
         if (css && css_tryget_online(css))
             goto out_unlock;
         cgrp = cgroup_parent(cgrp);
     } while (cgrp);
 
     css = init_css_set.subsys[ss->id];
     css_get(css);
 out_unlock:
     rcu_read_unlock();
     return css;
 }
 EXPORT_SYMBOL_GPL(cgroup_get_e_css);
 
 static void cgroup_get_live(struct cgroup *cgrp)
 {
     WARN_ON_ONCE(cgroup_is_dead(cgrp));
     css_get(&cgrp->self);
 }
 
 /**
  * __cgroup_task_count - count the number of tasks in a cgroup. The caller
  * is responsible for taking the css_set_lock.
  * @cgrp: the cgroup in question
  */
 int __cgroup_task_count(const struct cgroup *cgrp)
 {
     int count = 0;
     struct cgrp_cset_link *link;
 
     lockdep_assert_held(&css_set_lock);
 
     list_for_each_entry(link, &cgrp->cset_links, cset_link)
         count += link->cset->nr_tasks;
 
     return count;
 }
 
 /**
  * cgroup_task_count - count the number of tasks in a cgroup.
  * @cgrp: the cgroup in question
  */
 int cgroup_task_count(const struct cgroup *cgrp)
 {
     int count;
 
     spin_lock_irq(&css_set_lock);
     count = __cgroup_task_count(cgrp);
     spin_unlock_irq(&css_set_lock);
 
     return count;
 }
 
 struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
 {
     struct cgroup *cgrp = of->kn->parent->priv;
     struct cftype *cft = of_cft(of);
 
     /*
      * This is open and unprotected implementation of cgroup_css().
      * seq_css() is only called from a kernfs file operation which has
      * an active reference on the file.  Because all the subsystem
      * files are drained before a css is disassociated with a cgroup,
      * the matching css from the cgroup's subsys table is guaranteed to
      * be and stay valid until the enclosing operation is complete.
      */
     if (CGROUP_HAS_SUBSYS_CONFIG && cft->ss)
         return rcu_dereference_raw(cgrp->subsys[cft->ss->id]);
     else
         return &cgrp->self;
 }
 EXPORT_SYMBOL_GPL(of_css);
 
 /**
  * for_each_css - iterate all css's of a cgroup
  * @css: the iteration cursor
  * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
  * @cgrp: the target cgroup to iterate css's of
  *
  * Should be called under cgroup_[tree_]mutex.
  */
 #define for_each_css(css, ssid, cgrp)                   \
     for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)    \
         if (!((css) = rcu_dereference_check(            \
                 (cgrp)->subsys[(ssid)],         \
                 lockdep_is_held(&cgroup_mutex)))) { }   \
         else
 
 /**
  * for_each_e_css - iterate all effective css's of a cgroup
  * @css: the iteration cursor
  * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
  * @cgrp: the target cgroup to iterate css's of
  *
  * Should be called under cgroup_[tree_]mutex.
  */
 #define for_each_e_css(css, ssid, cgrp)                     \
     for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)        \
         if (!((css) = cgroup_e_css_by_mask(cgrp,            \
                            cgroup_subsys[(ssid)]))) \
             ;                           \
         else
 
 /**
  * do_each_subsys_mask - filter for_each_subsys with a bitmask
  * @ss: the iteration cursor
  * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
  * @ss_mask: the bitmask
  *
  * The block will only run for cases where the ssid-th bit (1 << ssid) of
  * @ss_mask is set.
  */
 #define do_each_subsys_mask(ss, ssid, ss_mask) do {         \
     unsigned long __ss_mask = (ss_mask);                \
     if (!CGROUP_HAS_SUBSYS_CONFIG) {                \
         (ssid) = 0;                     \
         break;                          \
     }                               \
     for_each_set_bit(ssid, &__ss_mask, CGROUP_SUBSYS_COUNT) {   \
         (ss) = cgroup_subsys[ssid];             \
         {
 
 #define while_each_subsys_mask()                    \
         }                           \
     }                               \
 } while (false)
 
 /* iterate over child cgrps, lock should be held throughout iteration */
 #define cgroup_for_each_live_child(child, cgrp)             \
     list_for_each_entry((child), &(cgrp)->self.children, self.sibling) \
         if (({ lockdep_assert_held(&cgroup_mutex);      \
                cgroup_is_dead(child); }))           \
             ;                       \
         else
 
 /* walk live descendants in pre order */
 #define cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp)      \
     css_for_each_descendant_pre((d_css), cgroup_css((cgrp), NULL))  \
         if (({ lockdep_assert_held(&cgroup_mutex);      \
                (dsct) = (d_css)->cgroup;            \
                cgroup_is_dead(dsct); }))            \
             ;                       \
         else
 
 /* walk live descendants in postorder */
 #define cgroup_for_each_live_descendant_post(dsct, d_css, cgrp)     \
     css_for_each_descendant_post((d_css), cgroup_css((cgrp), NULL)) \
         if (({ lockdep_assert_held(&cgroup_mutex);      \
                (dsct) = (d_css)->cgroup;            \
                cgroup_is_dead(dsct); }))            \
             ;                       \
         else
 
 /*
  * The default css_set - used by init and its children prior to any
  * hierarchies being mounted. It contains a pointer to the root state
  * for each subsystem. Also used to anchor the list of css_sets. Not
  * reference-counted, to improve performance when child cgroups
  * haven't been created.
  */
 struct css_set init_css_set = {
     .refcount       = REFCOUNT_INIT(1),
     .dom_cset       = &init_css_set,
     .tasks          = LIST_HEAD_INIT(init_css_set.tasks),
     .mg_tasks       = LIST_HEAD_INIT(init_css_set.mg_tasks),
     .dying_tasks        = LIST_HEAD_INIT(init_css_set.dying_tasks),
     .task_iters     = LIST_HEAD_INIT(init_css_set.task_iters),
     .threaded_csets     = LIST_HEAD_INIT(init_css_set.threaded_csets),
     .cgrp_links     = LIST_HEAD_INIT(init_css_set.cgrp_links),
     .mg_src_preload_node    = LIST_HEAD_INIT(init_css_set.mg_src_preload_node),
     .mg_dst_preload_node    = LIST_HEAD_INIT(init_css_set.mg_dst_preload_node),
     .mg_node        = LIST_HEAD_INIT(init_css_set.mg_node),
 
     /*
      * The following field is re-initialized when this cset gets linked
      * in cgroup_init().  However, let's initialize the field
      * statically too so that the default cgroup can be accessed safely
      * early during boot.
      */
     .dfl_cgrp       = &cgrp_dfl_root.cgrp,
 };
 
 static int css_set_count    = 1;    /* 1 for init_css_set */
 
 static bool css_set_threaded(struct css_set *cset)
 {
     return cset->dom_cset != cset;
 }
 
 /**
  * css_set_populated - does a css_set contain any tasks?
  * @cset: target css_set
  *
  * css_set_populated() should be the same as !!cset->nr_tasks at steady
  * state. However, css_set_populated() can be called while a task is being
  * added to or removed from the linked list before the nr_tasks is
  * properly updated. Hence, we can't just look at ->nr_tasks here.
  */
 static bool css_set_populated(struct css_set *cset)
 {
     lockdep_assert_held(&css_set_lock);
 
     return !list_empty(&cset->tasks) || !list_empty(&cset->mg_tasks);
 }
 
 /**
  * cgroup_update_populated - update the populated count of a cgroup
  * @cgrp: the target cgroup
  * @populated: inc or dec populated count
  *
  * One of the css_sets associated with @cgrp is either getting its first
  * task or losing the last.  Update @cgrp->nr_populated_* accordingly.  The
  * count is propagated towards root so that a given cgroup's
  * nr_populated_children is zero iff none of its descendants contain any
  * tasks.
  *
  * @cgrp's interface file "cgroup.populated" is zero if both
  * @cgrp->nr_populated_csets and @cgrp->nr_populated_children are zero and
  * 1 otherwise.  When the sum changes from or to zero, userland is notified
  * that the content of the interface file has changed.  This can be used to
  * detect when @cgrp and its descendants become populated or empty.
  */
 static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
 {
     struct cgroup *child = NULL;
     int adj = populated ? 1 : -1;
 
     lockdep_assert_held(&css_set_lock);
 
     do {
         bool was_populated = cgroup_is_populated(cgrp);
 
         if (!child) {
             cgrp->nr_populated_csets += adj;
         } else {
             if (cgroup_is_threaded(child))
                 cgrp->nr_populated_threaded_children += adj;
             else
                 cgrp->nr_populated_domain_children += adj;
         }
 
         if (was_populated == cgroup_is_populated(cgrp))
             break;
 
         cgroup1_check_for_release(cgrp);
         TRACE_CGROUP_PATH(notify_populated, cgrp,
                   cgroup_is_populated(cgrp));
         cgroup_file_notify(&cgrp->events_file);
 
         child = cgrp;
         cgrp = cgroup_parent(cgrp);
     } while (cgrp);
 }
 
 /**
  * css_set_update_populated - update populated state of a css_set
  * @cset: target css_set
  * @populated: whether @cset is populated or depopulated
  *
  * @cset is either getting the first task or losing the last.  Update the
  * populated counters of all associated cgroups accordingly.
  */
 static void css_set_update_populated(struct css_set *cset, bool populated)
 {
     struct cgrp_cset_link *link;
 
     lockdep_assert_held(&css_set_lock);
 
     list_for_each_entry(link, &cset->cgrp_links, cgrp_link)
         cgroup_update_populated(link->cgrp, populated);
 }
 
 /*
  * @task is leaving, advance task iterators which are pointing to it so
  * that they can resume at the next position.  Advancing an iterator might
  * remove it from the list, use safe walk.  See css_task_iter_skip() for
  * details.
  */
 static void css_set_skip_task_iters(struct css_set *cset,
                     struct task_struct *task)
 {
     struct css_task_iter *it, *pos;
 
     list_for_each_entry_safe(it, pos, &cset->task_iters, iters_node)
         css_task_iter_skip(it, task);
 }
 
 /**
  * css_set_move_task - move a task from one css_set to another
  * @task: task being moved
  * @from_cset: css_set @task currently belongs to (may be NULL)
  * @to_cset: new css_set @task is being moved to (may be NULL)
  * @use_mg_tasks: move to @to_cset->mg_tasks instead of ->tasks
  *
  * Move @task from @from_cset to @to_cset.  If @task didn't belong to any
  * css_set, @from_cset can be NULL.  If @task is being disassociated
  * instead of moved, @to_cset can be NULL.
  *
  * This function automatically handles populated counter updates and
  * css_task_iter adjustments but the caller is responsible for managing
  * @from_cset and @to_cset's reference counts.
  */
 static void css_set_move_task(struct task_struct *task,
                   struct css_set *from_cset, struct css_set *to_cset,
                   bool use_mg_tasks)
 {
     lockdep_assert_held(&css_set_lock);
 
     if (to_cset && !css_set_populated(to_cset))
         css_set_update_populated(to_cset, true);
 
     if (from_cset) {
         WARN_ON_ONCE(list_empty(&task->cg_list));
 
         css_set_skip_task_iters(from_cset, task);
         list_del_init(&task->cg_list);
         if (!css_set_populated(from_cset))
             css_set_update_populated(from_cset, false);
     } else {
         WARN_ON_ONCE(!list_empty(&task->cg_list));
     }
 
     if (to_cset) {
         /*
          * We are synchronized through cgroup_threadgroup_rwsem
          * against PF_EXITING setting such that we can't race
          * against cgroup_exit()/cgroup_free() dropping the css_set.
          */
         WARN_ON_ONCE(task->flags & PF_EXITING);
 
         cgroup_move_task(task, to_cset);
         list_add_tail(&task->cg_list, use_mg_tasks ? &to_cset->mg_tasks :
                                  &to_cset->tasks);
     }
 }
 
 /*
  * hash table for cgroup groups. This improves the performance to find
  * an existing css_set. This hash doesn't (currently) take into
  * account cgroups in empty hierarchies.
  */
 #define CSS_SET_HASH_BITS   7
 static DEFINE_HASHTABLE(css_set_table, CSS_SET_HASH_BITS);
 
 static unsigned long css_set_hash(struct cgroup_subsys_state *css[])
 {
     unsigned long key = 0UL;
     struct cgroup_subsys *ss;
     int i;
 
     for_each_subsys(ss, i)
         key += (unsigned long)css[i];
     key = (key >> 16) ^ key;
 
     return key;
 }
 
 void put_css_set_locked(struct css_set *cset)
 {
     struct cgrp_cset_link *link, *tmp_link;
     struct cgroup_subsys *ss;
     int ssid;
 
     lockdep_assert_held(&css_set_lock);
 
     if (!refcount_dec_and_test(&cset->refcount))
         return;
 
     WARN_ON_ONCE(!list_empty(&cset->threaded_csets));
 
     /* This css_set is dead. Unlink it and release cgroup and css refs */
     for_each_subsys(ss, ssid) {
         list_del(&cset->e_cset_node[ssid]);
         css_put(cset->subsys[ssid]);
     }
     hash_del(&cset->hlist);
     css_set_count--;
 
     list_for_each_entry_safe(link, tmp_link, &cset->cgrp_links, cgrp_link) {
         list_del(&link->cset_link);
         list_del(&link->cgrp_link);
         if (cgroup_parent(link->cgrp))
             cgroup_put(link->cgrp);
         kfree(link);
     }
 
     if (css_set_threaded(cset)) {
         list_del(&cset->threaded_csets_node);
         put_css_set_locked(cset->dom_cset);
     }
 
     kfree_rcu(cset, rcu_head);
 }
 
 /**
  * compare_css_sets - helper function for find_existing_css_set().
  * @cset: candidate css_set being tested
  * @old_cset: existing css_set for a task
  * @new_cgrp: cgroup that's being entered by the task
  * @template: desired set of css pointers in css_set (pre-calculated)
  *
  * Returns true if "cset" matches "old_cset" except for the hierarchy
  * which "new_cgrp" belongs to, for which it should match "new_cgrp".
  */
 static bool compare_css_sets(struct css_set *cset,
                  struct css_set *old_cset,
                  struct cgroup *new_cgrp,
                  struct cgroup_subsys_state *template[])
 {
     struct cgroup *new_dfl_cgrp;
     struct list_head *l1, *l2;
 
     /*
      * On the default hierarchy, there can be csets which are
      * associated with the same set of cgroups but different csses.
      * Let's first ensure that csses match.
      */
     if (memcmp(template, cset->subsys, sizeof(cset->subsys)))
         return false;
 
 
     /* @cset's domain should match the default cgroup's */
     if (cgroup_on_dfl(new_cgrp))
         new_dfl_cgrp = new_cgrp;
     else
         new_dfl_cgrp = old_cset->dfl_cgrp;
 
     if (new_dfl_cgrp->dom_cgrp != cset->dom_cset->dfl_cgrp)
         return false;
 
     /*
      * Compare cgroup pointers in order to distinguish between
      * different cgroups in hierarchies.  As different cgroups may
      * share the same effective css, this comparison is always
      * necessary.
      */
     l1 = &cset->cgrp_links;
     l2 = &old_cset->cgrp_links;
     while (1) {
         struct cgrp_cset_link *link1, *link2;
         struct cgroup *cgrp1, *cgrp2;
 
         l1 = l1->next;
         l2 = l2->next;
         /* See if we reached the end - both lists are equal length. */
         if (l1 == &cset->cgrp_links) {
             BUG_ON(l2 != &old_cset->cgrp_links);
             break;
         } else {
             BUG_ON(l2 == &old_cset->cgrp_links);
         }
         /* Locate the cgroups associated with these links. */
         link1 = list_entry(l1, struct cgrp_cset_link, cgrp_link);
         link2 = list_entry(l2, struct cgrp_cset_link, cgrp_link);
         cgrp1 = link1->cgrp;
         cgrp2 = link2->cgrp;
         /* Hierarchies should be linked in the same order. */
         BUG_ON(cgrp1->root != cgrp2->root);
 
         /*
          * If this hierarchy is the hierarchy of the cgroup
          * that's changing, then we need to check that this
          * css_set points to the new cgroup; if it's any other
          * hierarchy, then this css_set should point to the
          * same cgroup as the old css_set.
          */
         if (cgrp1->root == new_cgrp->root) {
             if (cgrp1 != new_cgrp)
                 return false;
         } else {
             if (cgrp1 != cgrp2)
                 return false;
         }
     }
     return true;
 }
 
 /**
  * find_existing_css_set - init css array and find the matching css_set
  * @old_cset: the css_set that we're using before the cgroup transition
  * @cgrp: the cgroup that we're moving into
  * @template: out param for the new set of csses, should be clear on entry
  */
 static struct css_set *find_existing_css_set(struct css_set *old_cset,
                     struct cgroup *cgrp,
                     struct cgroup_subsys_state *template[])
 {
     struct cgroup_root *root = cgrp->root;
     struct cgroup_subsys *ss;
     struct css_set *cset;
     unsigned long key;
     int i;
 
     /*
      * Build the set of subsystem state objects that we want to see in the
      * new css_set. While subsystems can change globally, the entries here
      * won't change, so no need for locking.
      */
     for_each_subsys(ss, i) {
         if (root->subsys_mask & (1UL << i)) {
             /*
              * @ss is in this hierarchy, so we want the
              * effective css from @cgrp.
              */
             template[i] = cgroup_e_css_by_mask(cgrp, ss);
         } else {
             /*
              * @ss is not in this hierarchy, so we don't want
              * to change the css.
              */
             template[i] = old_cset->subsys[i];
         }
     }
 
     key = css_set_hash(template);
     hash_for_each_possible(css_set_table, cset, hlist, key) {
         if (!compare_css_sets(cset, old_cset, cgrp, template))
             continue;
 
         /* This css_set matches what we need */
         return cset;
     }
 
     /* No existing cgroup group matched */
     return NULL;
 }
 
 static void free_cgrp_cset_links(struct list_head *links_to_free)
 {
     struct cgrp_cset_link *link, *tmp_link;
 
     list_for_each_entry_safe(link, tmp_link, links_to_free, cset_link) {
         list_del(&link->cset_link);
         kfree(link);
     }
 }
 
 /**
  * allocate_cgrp_cset_links - allocate cgrp_cset_links
  * @count: the number of links to allocate
  * @tmp_links: list_head the allocated links are put on
  *
  * Allocate @count cgrp_cset_link structures and chain them on @tmp_links
  * through ->cset_link.  Returns 0 on success or -errno.
  */
 static int allocate_cgrp_cset_links(int count, struct list_head *tmp_links)
 {
     struct cgrp_cset_link *link;
     int i;
 
     INIT_LIST_HEAD(tmp_links);
 
     for (i = 0; i < count; i++) {
         link = kzalloc(sizeof(*link), GFP_KERNEL);
         if (!link) {
             free_cgrp_cset_links(tmp_links);
             return -ENOMEM;
         }
         list_add(&link->cset_link, tmp_links);
     }
     return 0;
 }
 
 /**
  * link_css_set - a helper function to link a css_set to a cgroup
  * @tmp_links: cgrp_cset_link objects allocated by allocate_cgrp_cset_links()
  * @cset: the css_set to be linked
  * @cgrp: the destination cgroup
  */
 static void link_css_set(struct list_head *tmp_links, struct css_set *cset,
              struct cgroup *cgrp)
 {
     struct cgrp_cset_link *link;
 
     BUG_ON(list_empty(tmp_links));
 
     if (cgroup_on_dfl(cgrp))
         cset->dfl_cgrp = cgrp;
 
     link = list_first_entry(tmp_links, struct cgrp_cset_link, cset_link);
     link->cset = cset;
     link->cgrp = cgrp;
 
     /*
      * Always add links to the tail of the lists so that the lists are
      * in chronological order.
      */
     list_move_tail(&link->cset_link, &cgrp->cset_links);
     list_add_tail(&link->cgrp_link, &cset->cgrp_links);
 
     if (cgroup_parent(cgrp))
         cgroup_get_live(cgrp);
 }
 
 /**
  * find_css_set - return a new css_set with one cgroup updated
  * @old_cset: the baseline css_set
  * @cgrp: the cgroup to be updated
  *
  * Return a new css_set that's equivalent to @old_cset, but with @cgrp
  * substituted into the appropriate hierarchy.
  */
 static struct css_set *find_css_set(struct css_set *old_cset,
                     struct cgroup *cgrp)
 {
     struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT] = { };
     struct css_set *cset;
     struct list_head tmp_links;
     struct cgrp_cset_link *link;
     struct cgroup_subsys *ss;
     unsigned long key;
     int ssid;
 
     lockdep_assert_held(&cgroup_mutex);
 
     /* First see if we already have a cgroup group that matches
      * the desired set */
     spin_lock_irq(&css_set_lock);
     cset = find_existing_css_set(old_cset, cgrp, template);
     if (cset)
         get_css_set(cset);
     spin_unlock_irq(&css_set_lock);
 
     if (cset)
         return cset;
 
     cset = kzalloc(sizeof(*cset), GFP_KERNEL);
     if (!cset)
         return NULL;
 
     /* Allocate all the cgrp_cset_link objects that we'll need */
     if (allocate_cgrp_cset_links(cgroup_root_count, &tmp_links) < 0) {
         kfree(cset);
         return NULL;
     }
 
     refcount_set(&cset->refcount, 1);
     cset->dom_cset = cset;
     INIT_LIST_HEAD(&cset->tasks);
     INIT_LIST_HEAD(&cset->mg_tasks);
     INIT_LIST_HEAD(&cset->dying_tasks);
     INIT_LIST_HEAD(&cset->task_iters);
     INIT_LIST_HEAD(&cset->threaded_csets);
     INIT_HLIST_NODE(&cset->hlist);
     INIT_LIST_HEAD(&cset->cgrp_links);
     INIT_LIST_HEAD(&cset->mg_src_preload_node);
     INIT_LIST_HEAD(&cset->mg_dst_preload_node);
     INIT_LIST_HEAD(&cset->mg_node);
 
     /* Copy the set of subsystem state objects generated in
      * find_existing_css_set() */
     memcpy(cset->subsys, template, sizeof(cset->subsys));
 
     spin_lock_irq(&css_set_lock);
     /* Add reference counts and links from the new css_set. */
     list_for_each_entry(link, &old_cset->cgrp_links, cgrp_link) {
         struct cgroup *c = link->cgrp;
 
         if (c->root == cgrp->root)
             c = cgrp;
         link_css_set(&tmp_links, cset, c);
     }
 
     BUG_ON(!list_empty(&tmp_links));
 
     css_set_count++;
 
     /* Add @cset to the hash table */
     key = css_set_hash(cset->subsys);
     hash_add(css_set_table, &cset->hlist, key);
 
     for_each_subsys(ss, ssid) {
         struct cgroup_subsys_state *css = cset->subsys[ssid];
 
         list_add_tail(&cset->e_cset_node[ssid],
                   &css->cgroup->e_csets[ssid]);
         css_get(css);
     }
 
     spin_unlock_irq(&css_set_lock);
 
     /*
      * If @cset should be threaded, look up the matching dom_cset and
      * link them up.  We first fully initialize @cset then look for the
      * dom_cset.  It's simpler this way and safe as @cset is guaranteed
      * to stay empty until we return.
      */
     if (cgroup_is_threaded(cset->dfl_cgrp)) {
         struct css_set *dcset;
 
         dcset = find_css_set(cset, cset->dfl_cgrp->dom_cgrp);
         if (!dcset) {
             put_css_set(cset);
             return NULL;
         }
 
         spin_lock_irq(&css_set_lock);
         cset->dom_cset = dcset;
         list_add_tail(&cset->threaded_csets_node,
                   &dcset->threaded_csets);
         spin_unlock_irq(&css_set_lock);
     }
 
     return cset;
 }
 
 struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root)
 {
     struct cgroup *root_cgrp = kernfs_root_to_node(kf_root)->priv;
 
     return root_cgrp->root;
 }
 
 void cgroup_favor_dynmods(struct cgroup_root *root, bool favor)
 {
     bool favoring = root->flags & CGRP_ROOT_FAVOR_DYNMODS;
 
     /* see the comment above CGRP_ROOT_FAVOR_DYNMODS definition */
     if (favor && !favoring) {
         rcu_sync_enter(&cgroup_threadgroup_rwsem.rss);
         root->flags |= CGRP_ROOT_FAVOR_DYNMODS;
     } else if (!favor && favoring) {
         rcu_sync_exit(&cgroup_threadgroup_rwsem.rss);
         root->flags &= ~CGRP_ROOT_FAVOR_DYNMODS;
     }
 }
 
 static int cgroup_init_root_id(struct cgroup_root *root)
 {
     int id;
 
     lockdep_assert_held(&cgroup_mutex);
 
     id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, 0, 0, GFP_KERNEL);
     if (id < 0)
         return id;
 
     root->hierarchy_id = id;
     return 0;
 }
 
 static void cgroup_exit_root_id(struct cgroup_root *root)
 {
     lockdep_assert_held(&cgroup_mutex);
 
     idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
 }
 
 void cgroup_free_root(struct cgroup_root *root)
 {
     kfree(root);
 }
 
 static void cgroup_destroy_root(struct cgroup_root *root)
 {
     struct cgroup *cgrp = &root->cgrp;
     struct cgrp_cset_link *link, *tmp_link;
 
     trace_cgroup_destroy_root(root);
 
     cgroup_lock_and_drain_offline(&cgrp_dfl_root.cgrp);
 
     BUG_ON(atomic_read(&root->nr_cgrps));
     BUG_ON(!list_empty(&cgrp->self.children));
 
     /* Rebind all subsystems back to the default hierarchy */
     WARN_ON(rebind_subsystems(&cgrp_dfl_root, root->subsys_mask));
 
     /*
      * Release all the links from cset_links to this hierarchy's
      * root cgroup
      */
     spin_lock_irq(&css_set_lock);
 
     list_for_each_entry_safe(link, tmp_link, &cgrp->cset_links, cset_link) {
         list_del(&link->cset_link);
         list_del(&link->cgrp_link);
         kfree(link);
     }
 
     spin_unlock_irq(&css_set_lock);
 
     if (!list_empty(&root->root_list)) {
         list_del(&root->root_list);
         cgroup_root_count--;
     }
 
     cgroup_favor_dynmods(root, false);
     cgroup_exit_root_id(root);
 
     mutex_unlock(&cgroup_mutex);
 
     cgroup_rstat_exit(cgrp);
     kernfs_destroy_root(root->kf_root);
     cgroup_free_root(root);
 }
 
 static inline struct cgroup *__cset_cgroup_from_root(struct css_set *cset,
                         struct cgroup_root *root)
 {
     struct cgroup *res_cgroup = NULL;
 
     if (cset == &init_css_set) {
         res_cgroup = &root->cgrp;
     } else if (root == &cgrp_dfl_root) {
         res_cgroup = cset->dfl_cgrp;
     } else {
         struct cgrp_cset_link *link;
 
         list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
             struct cgroup *c = link->cgrp;
 
             if (c->root == root) {
                 res_cgroup = c;
                 break;
             }
         }
     }
 
     return res_cgroup;
 }
 
 /*
  * look up cgroup associated with current task's cgroup namespace on the
  * specified hierarchy
  */
 static struct cgroup *
 current_cgns_cgroup_from_root(struct cgroup_root *root)
 {
     struct cgroup *res = NULL;
     struct css_set *cset;
 
     lockdep_assert_held(&css_set_lock);
 
     rcu_read_lock();
 
     cset = current->nsproxy->cgroup_ns->root_cset;
     res = __cset_cgroup_from_root(cset, root);
 
     rcu_read_unlock();
 
     BUG_ON(!res);
     return res;
 }
 
 /* look up cgroup associated with given css_set on the specified hierarchy */
 static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
                         struct cgroup_root *root)
 {
     struct cgroup *res = NULL;
 
     lockdep_assert_held(&cgroup_mutex);
     lockdep_assert_held(&css_set_lock);
 
     res = __cset_cgroup_from_root(cset, root);
 
     BUG_ON(!res);
     return res;
 }
 
 /*
  * Return the cgroup for "task" from the given hierarchy. Must be
  * called with cgroup_mutex and css_set_lock held.
  */
 struct cgroup *task_cgroup_from_root(struct task_struct *task,
                      struct cgroup_root *root)
 {
     /*
      * No need to lock the task - since we hold css_set_lock the
      * task can't change groups.
      */
     return cset_cgroup_from_root(task_css_set(task), root);
 }
 
 /*
  * A task must hold cgroup_mutex to modify cgroups.
  *
  * Any task can increment and decrement the count field without lock.
  * So in general, code holding cgroup_mutex can't rely on the count
  * field not changing.  However, if the count goes to zero, then only
  * cgroup_attach_task() can increment it again.  Because a count of zero
  * means that no tasks are currently attached, therefore there is no
  * way a task attached to that cgroup can fork (the other way to
  * increment the count).  So code holding cgroup_mutex can safely
  * assume that if the count is zero, it will stay zero. Similarly, if
  * a task holds cgroup_mutex on a cgroup with zero count, it
  * knows that the cgroup won't be removed, as cgroup_rmdir()
  * needs that mutex.
  *
  * A cgroup can only be deleted if both its 'count' of using tasks
  * is zero, and its list of 'children' cgroups is empty.  Since all
  * tasks in the system use _some_ cgroup, and since there is always at
  * least one task in the system (init, pid == 1), therefore, root cgroup
  * always has either children cgroups and/or using tasks.  So we don't
  * need a special hack to ensure that root cgroup cannot be deleted.
  *
  * P.S.  One more locking exception.  RCU is used to guard the
  * update of a tasks cgroup pointer by cgroup_attach_task()
  */
 
 static struct kernfs_syscall_ops cgroup_kf_syscall_ops;
 
 static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
                   char *buf)
 {
     struct cgroup_subsys *ss = cft->ss;
 
     if (cft->ss && !(cft->flags & CFTYPE_NO_PREFIX) &&
         !(cgrp->root->flags & CGRP_ROOT_NOPREFIX)) {
         const char *dbg = (cft->flags & CFTYPE_DEBUG) ? ".__DEBUG__." : "";
 
         snprintf(buf, CGROUP_FILE_NAME_MAX, "%s%s.%s",
              dbg, cgroup_on_dfl(cgrp) ? ss->name : ss->legacy_name,
              cft->name);
     } else {
         strscpy(buf, cft->name, CGROUP_FILE_NAME_MAX);
     }
     return buf;
 }
 
 /**
  * cgroup_file_mode - deduce file mode of a control file
  * @cft: the control file in question
  *
  * S_IRUGO for read, S_IWUSR for write.
  */
 static umode_t cgroup_file_mode(const struct cftype *cft)
 {
     umode_t mode = 0;
 
     if (cft->read_u64 || cft->read_s64 || cft->seq_show)
         mode |= S_IRUGO;
 
     if (cft->write_u64 || cft->write_s64 || cft->write) {
         if (cft->flags & CFTYPE_WORLD_WRITABLE)
             mode |= S_IWUGO;
         else
             mode |= S_IWUSR;
     }
 
     return mode;
 }
 
 /**
  * cgroup_calc_subtree_ss_mask - calculate subtree_ss_mask
  * @subtree_control: the new subtree_control mask to consider
  * @this_ss_mask: available subsystems
  *
  * On the default hierarchy, a subsystem may request other subsystems to be
  * enabled together through its ->depends_on mask.  In such cases, more
  * subsystems than specified in "cgroup.subtree_control" may be enabled.
  *
  * This function calculates which subsystems need to be enabled if
  * @subtree_control is to be applied while restricted to @this_ss_mask.
  */
 static u16 cgroup_calc_subtree_ss_mask(u16 subtree_control, u16 this_ss_mask)
 {
     u16 cur_ss_mask = subtree_control;
     struct cgroup_subsys *ss;
     int ssid;
 
     lockdep_assert_held(&cgroup_mutex);
 
     cur_ss_mask |= cgrp_dfl_implicit_ss_mask;
 
     while (true) {
         u16 new_ss_mask = cur_ss_mask;
 
         do_each_subsys_mask(ss, ssid, cur_ss_mask) {
             new_ss_mask |= ss->depends_on;
         } while_each_subsys_mask();
 
         /*
          * Mask out subsystems which aren't available.  This can
          * happen only if some depended-upon subsystems were bound
          * to non-default hierarchies.
          */
         new_ss_mask &= this_ss_mask;
 
         if (new_ss_mask == cur_ss_mask)
             break;
         cur_ss_mask = new_ss_mask;
     }
 
     return cur_ss_mask;
 }
 
 /**
  * cgroup_kn_unlock - unlocking helper for cgroup kernfs methods
  * @kn: the kernfs_node being serviced
  *
  * This helper undoes cgroup_kn_lock_live() and should be invoked before
  * the method finishes if locking succeeded.  Note that once this function
  * returns the cgroup returned by cgroup_kn_lock_live() may become
  * inaccessible any time.  If the caller intends to continue to access the
  * cgroup, it should pin it before invoking this function.
  */
 void cgroup_kn_unlock(struct kernfs_node *kn)
 {
     struct cgroup *cgrp;
 
     if (kernfs_type(kn) == KERNFS_DIR)
         cgrp = kn->priv;
     else
         cgrp = kn->parent->priv;
 
     mutex_unlock(&cgroup_mutex);
 
     kernfs_unbreak_active_protection(kn);
     cgroup_put(cgrp);
 }
 
 /**
  * cgroup_kn_lock_live - locking helper for cgroup kernfs methods
  * @kn: the kernfs_node being serviced
  * @drain_offline: perform offline draining on the cgroup
  *
  * This helper is to be used by a cgroup kernfs method currently servicing
  * @kn.  It breaks the active protection, performs cgroup locking and
  * verifies that the associated cgroup is alive.  Returns the cgroup if
  * alive; otherwise, %NULL.  A successful return should be undone by a
  * matching cgroup_kn_unlock() invocation.  If @drain_offline is %true, the
  * cgroup is drained of offlining csses before return.
  *
  * Any cgroup kernfs method implementation which requires locking the
  * associated cgroup should use this helper.  It avoids nesting cgroup
  * locking under kernfs active protection and allows all kernfs operations
  * including self-removal.
  */
 struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn, bool drain_offline)
 {
     struct cgroup *cgrp;
 
     if (kernfs_type(kn) == KERNFS_DIR)
         cgrp = kn->priv;
     else
         cgrp = kn->parent->priv;
 
     /*
      * We're gonna grab cgroup_mutex which nests outside kernfs
      * active_ref.  cgroup liveliness check alone provides enough
      * protection against removal.  Ensure @cgrp stays accessible and
      * break the active_ref protection.
      */
     if (!cgroup_tryget(cgrp))
         return NULL;
     kernfs_break_active_protection(kn);
 
     if (drain_offline)
         cgroup_lock_and_drain_offline(cgrp);
     else
         mutex_lock(&cgroup_mutex);
 
     if (!cgroup_is_dead(cgrp))
         return cgrp;
 
     cgroup_kn_unlock(kn);
     return NULL;
 }
 
 static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
 {
     char name[CGROUP_FILE_NAME_MAX];
 
     lockdep_assert_held(&cgroup_mutex);
 
     if (cft->file_offset) {
         struct cgroup_subsys_state *css = cgroup_css(cgrp, cft->ss);
         struct cgroup_file *cfile = (void *)css + cft->file_offset;
 
         spin_lock_irq(&cgroup_file_kn_lock);
         cfile->kn = NULL;
         spin_unlock_irq(&cgroup_file_kn_lock);
 
         del_timer_sync(&cfile->notify_timer);
     }
 
     kernfs_remove_by_name(cgrp->kn, cgroup_file_name(cgrp, cft, name));
 }
 
 /**
  * css_clear_dir - remove subsys files in a cgroup directory
  * @css: target css
  */
 static void css_clear_dir(struct cgroup_subsys_state *css)
 {
     struct cgroup *cgrp = css->cgroup;
     struct cftype *cfts;
 
     if (!(css->flags & CSS_VISIBLE))
         return;
 
     css->flags &= ~CSS_VISIBLE;
 
     if (!css->ss) {
         if (cgroup_on_dfl(cgrp))
             cfts = cgroup_base_files;
         else
             cfts = cgroup1_base_files;
 
         cgroup_addrm_files(css, cgrp, cfts, false);
     } else {
         list_for_each_entry(cfts, &css->ss->cfts, node)
             cgroup_addrm_files(css, cgrp, cfts, false);
     }
 }
 
 /**
  * css_populate_dir - create subsys files in a cgroup directory
  * @css: target css
  *
  * On failure, no file is added.
  */
 static int css_populate_dir(struct cgroup_subsys_state *css)
 {
     struct cgroup *cgrp = css->cgroup;
     struct cftype *cfts, *failed_cfts;
     int ret;
 
     if ((css->flags & CSS_VISIBLE) || !cgrp->kn)
         return 0;
 
     if (!css->ss) {
         if (cgroup_on_dfl(cgrp))
             cfts = cgroup_base_files;
         else
             cfts = cgroup1_base_files;
 
         ret = cgroup_addrm_files(&cgrp->self, cgrp, cfts, true);
         if (ret < 0)
             return ret;
     } else {
         list_for_each_entry(cfts, &css->ss->cfts, node) {
             ret = cgroup_addrm_files(css, cgrp, cfts, true);
             if (ret < 0) {
                 failed_cfts = cfts;
                 goto err;
             }
         }
     }
 
     css->flags |= CSS_VISIBLE;
 
     return 0;
 err:
     list_for_each_entry(cfts, &css->ss->cfts, node) {
         if (cfts == failed_cfts)
             break;
         cgroup_addrm_files(css, cgrp, cfts, false);
     }
     return ret;
 }
 
 int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
 {
     struct cgroup *dcgrp = &dst_root->cgrp;
     struct cgroup_subsys *ss;
     int ssid, i, ret;
     u16 dfl_disable_ss_mask = 0;
 
     lockdep_assert_held(&cgroup_mutex);
 
     do_each_subsys_mask(ss, ssid, ss_mask) {
         /*
          * If @ss has non-root csses attached to it, can't move.
          * If @ss is an implicit controller, it is exempt from this
          * rule and can be stolen.
          */
         if (css_next_child(NULL, cgroup_css(&ss->root->cgrp, ss)) &&
             !ss->implicit_on_dfl)
             return -EBUSY;
 
         /* can't move between two non-dummy roots either */
         if (ss->root != &cgrp_dfl_root && dst_root != &cgrp_dfl_root)
             return -EBUSY;
 
         /*
          * Collect ssid's that need to be disabled from default
          * hierarchy.
          */
         if (ss->root == &cgrp_dfl_root)
             dfl_disable_ss_mask |= 1 << ssid;
 
     } while_each_subsys_mask();
 
     if (dfl_disable_ss_mask) {
         struct cgroup *scgrp = &cgrp_dfl_root.cgrp;
 
         /*
          * Controllers from default hierarchy that need to be rebound
          * are all disabled together in one go.
          */
         cgrp_dfl_root.subsys_mask &= ~dfl_disable_ss_mask;
         WARN_ON(cgroup_apply_control(scgrp));
         cgroup_finalize_control(scgrp, 0);
     }
 
     do_each_subsys_mask(ss, ssid, ss_mask) {
         struct cgroup_root *src_root = ss->root;
         struct cgroup *scgrp = &src_root->cgrp;
         struct cgroup_subsys_state *css = cgroup_css(scgrp, ss);
         struct css_set *cset;
 
         WARN_ON(!css || cgroup_css(dcgrp, ss));
 
         if (src_root != &cgrp_dfl_root) {
             /* disable from the source */
             src_root->subsys_mask &= ~(1 << ssid);
             WARN_ON(cgroup_apply_control(scgrp));
             cgroup_finalize_control(scgrp, 0);
         }
 
         /* rebind */
         RCU_INIT_POINTER(scgrp->subsys[ssid], NULL);
         rcu_assign_pointer(dcgrp->subsys[ssid], css);
         ss->root = dst_root;
         css->cgroup = dcgrp;
 
         spin_lock_irq(&css_set_lock);
         hash_for_each(css_set_table, i, cset, hlist)
             list_move_tail(&cset->e_cset_node[ss->id],
                        &dcgrp->e_csets[ss->id]);
         spin_unlock_irq(&css_set_lock);
 
         if (ss->css_rstat_flush) {
             list_del_rcu(&css->rstat_css_node);
             synchronize_rcu();
             list_add_rcu(&css->rstat_css_node,
                      &dcgrp->rstat_css_list);
         }
 
         /* default hierarchy doesn't enable controllers by default */
         dst_root->subsys_mask |= 1 << ssid;
         if (dst_root == &cgrp_dfl_root) {
             static_branch_enable(cgroup_subsys_on_dfl_key[ssid]);
         } else {
             dcgrp->subtree_control |= 1 << ssid;
             static_branch_disable(cgroup_subsys_on_dfl_key[ssid]);
         }
 
         ret = cgroup_apply_control(dcgrp);
         if (ret)
             pr_warn("partial failure to rebind %s controller (err=%d)\n",
                 ss->name, ret);
 
         if (ss->bind)
             ss->bind(css);
     } while_each_subsys_mask();
 
     kernfs_activate(dcgrp->kn);
     return 0;
 }
 
 int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
              struct kernfs_root *kf_root)
 {
     int len = 0;
     char *buf = NULL;
     struct cgroup_root *kf_cgroot = cgroup_root_from_kf(kf_root);
     struct cgroup *ns_cgroup;
 
     buf = kmalloc(PATH_MAX, GFP_KERNEL);
     if (!buf)
         return -ENOMEM;
 
     spin_lock_irq(&css_set_lock);
     ns_cgroup = current_cgns_cgroup_from_root(kf_cgroot);
     len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX);
     spin_unlock_irq(&css_set_lock);
 
     if (len >= PATH_MAX)
         len = -ERANGE;
     else if (len > 0) {
         seq_escape(sf, buf, " \t\n\\");
         len = 0;
     }
     kfree(buf);
     return len;
 }
 
 enum cgroup2_param {
     Opt_nsdelegate,
     Opt_favordynmods,
     Opt_memory_localevents,
     Opt_memory_recursiveprot,
     nr__cgroup2_params
 };
 
 static const struct fs_parameter_spec cgroup2_fs_parameters[] = {
     fsparam_flag("nsdelegate",      Opt_nsdelegate),
     fsparam_flag("favordynmods",        Opt_favordynmods),
     fsparam_flag("memory_localevents",  Opt_memory_localevents),
     fsparam_flag("memory_recursiveprot",    Opt_memory_recursiveprot),
     {}
 };
 
 static int cgroup2_parse_param(struct fs_context *fc, struct fs_parameter *param)
 {
     struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
     struct fs_parse_result result;
     int opt;
 
     opt = fs_parse(fc, cgroup2_fs_parameters, param, &result);
     if (opt < 0)
         return opt;
 
     switch (opt) {
     case Opt_nsdelegate:
         ctx->flags |= CGRP_ROOT_NS_DELEGATE;
         return 0;
     case Opt_favordynmods:
         ctx->flags |= CGRP_ROOT_FAVOR_DYNMODS;
         return 0;
     case Opt_memory_localevents:
         ctx->flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
         return 0;
     case Opt_memory_recursiveprot:
         ctx->flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
         return 0;
     }
     return -EINVAL;
 }
 
 static void apply_cgroup_root_flags(unsigned int root_flags)
 {
     if (current->nsproxy->cgroup_ns == &init_cgroup_ns) {
         if (root_flags & CGRP_ROOT_NS_DELEGATE)
             cgrp_dfl_root.flags |= CGRP_ROOT_NS_DELEGATE;
         else
             cgrp_dfl_root.flags &= ~CGRP_ROOT_NS_DELEGATE;
 
         cgroup_favor_dynmods(&cgrp_dfl_root,
                      root_flags & CGRP_ROOT_FAVOR_DYNMODS);
 
         if (root_flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
             cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
         else
             cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_LOCAL_EVENTS;
 
         if (root_flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)
             cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
         else
             cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_RECURSIVE_PROT;
     }
 }
 
 static int cgroup_show_options(struct seq_file *seq, struct kernfs_root *kf_root)
 {
     if (cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE)
         seq_puts(seq, ",nsdelegate");
     if (cgrp_dfl_root.flags & CGRP_ROOT_FAVOR_DYNMODS)
         seq_puts(seq, ",favordynmods");
     if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
         seq_puts(seq, ",memory_localevents");
     if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)
         seq_puts(seq, ",memory_recursiveprot");
     return 0;
 }
 
 static int cgroup_reconfigure(struct fs_context *fc)
 {
     struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
 
     apply_cgroup_root_flags(ctx->flags);
     return 0;
 }
 
 static void init_cgroup_housekeeping(struct cgroup *cgrp)
 {
     struct cgroup_subsys *ss;
     int ssid;
 
     INIT_LIST_HEAD(&cgrp->self.sibling);
     INIT_LIST_HEAD(&cgrp->self.children);
     INIT_LIST_HEAD(&cgrp->cset_links);
     INIT_LIST_HEAD(&cgrp->pidlists);
     mutex_init(&cgrp->pidlist_mutex);
     cgrp->self.cgroup = cgrp;
     cgrp->self.flags |= CSS_ONLINE;
     cgrp->dom_cgrp = cgrp;
     cgrp->max_descendants = INT_MAX;
     cgrp->max_depth = INT_MAX;
     INIT_LIST_HEAD(&cgrp->rstat_css_list);
     prev_cputime_init(&cgrp->prev_cputime);
 
     for_each_subsys(ss, ssid)
         INIT_LIST_HEAD(&cgrp->e_csets[ssid]);
 
     init_waitqueue_head(&cgrp->offline_waitq);
     INIT_WORK(&cgrp->release_agent_work, cgroup1_release_agent);
 }
 
 void init_cgroup_root(struct cgroup_fs_context *ctx)
 {
     struct cgroup_root *root = ctx->root;
     struct cgroup *cgrp = &root->cgrp;
 
     INIT_LIST_HEAD(&root->root_list);
     atomic_set(&root->nr_cgrps, 1);
     cgrp->root = root;
     init_cgroup_housekeeping(cgrp);
 
     /* DYNMODS must be modified through cgroup_favor_dynmods() */
     root->flags = ctx->flags & ~CGRP_ROOT_FAVOR_DYNMODS;
     if (ctx->release_agent)
         strscpy(root->release_agent_path, ctx->release_agent, PATH_MAX);
     if (ctx->name)
         strscpy(root->name, ctx->name, MAX_CGROUP_ROOT_NAMELEN);
     if (ctx->cpuset_clone_children)
         set_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->cgrp.flags);
 }
 
 int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
 {
     LIST_HEAD(tmp_links);
     struct cgroup *root_cgrp = &root->cgrp;
     struct kernfs_syscall_ops *kf_sops;
     struct css_set *cset;
     int i, ret;
 
     lockdep_assert_held(&cgroup_mutex);
 
     ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release,
                   0, GFP_KERNEL);
     if (ret)
         goto out;
 
     /*
      * We're accessing css_set_count without locking css_set_lock here,
      * but that's OK - it can only be increased by someone holding
      * cgroup_lock, and that's us.  Later rebinding may disable
      * controllers on the default hierarchy and thus create new csets,
      * which can't be more than the existing ones.  Allocate 2x.
      */
     ret = allocate_cgrp_cset_links(2 * css_set_count, &tmp_links);
     if (ret)
         goto cancel_ref;
 
     ret = cgroup_init_root_id(root);
     if (ret)
         goto cancel_ref;
 
     kf_sops = root == &cgrp_dfl_root ?
         &cgroup_kf_syscall_ops : &cgroup1_kf_syscall_ops;
 
     root->kf_root = kernfs_create_root(kf_sops,
                        KERNFS_ROOT_CREATE_DEACTIVATED |
                        KERNFS_ROOT_SUPPORT_EXPORTOP |
                        KERNFS_ROOT_SUPPORT_USER_XATTR,
                        root_cgrp);
     if (IS_ERR(root->kf_root)) {
         ret = PTR_ERR(root->kf_root);
         goto exit_root_id;
     }
     root_cgrp->kn = kernfs_root_to_node(root->kf_root);
     WARN_ON_ONCE(cgroup_ino(root_cgrp) != 1);
     root_cgrp->ancestor_ids[0] = cgroup_id(root_cgrp);
 
     ret = css_populate_dir(&root_cgrp->self);
     if (ret)
         goto destroy_root;
 
     ret = cgroup_rstat_init(root_cgrp);
     if (ret)
         goto destroy_root;
 
     ret = rebind_subsystems(root, ss_mask);
     if (ret)
         goto exit_stats;
 
     ret = cgroup_bpf_inherit(root_cgrp);
     WARN_ON_ONCE(ret);
 
     trace_cgroup_setup_root(root);
 
     /*
      * There must be no failure case after here, since rebinding takes
      * care of subsystems' refcounts, which are explicitly dropped in
      * the failure exit path.
      */
     list_add(&root->root_list, &cgroup_roots);
     cgroup_root_count++;
 
     /*
      * Link the root cgroup in this hierarchy into all the css_set
      * objects.
      */
     spin_lock_irq(&css_set_lock);
     hash_for_each(css_set_table, i, cset, hlist) {
         link_css_set(&tmp_links, cset, root_cgrp);
         if (css_set_populated(cset))
             cgroup_update_populated(root_cgrp, true);
     }
     spin_unlock_irq(&css_set_lock);
 
     BUG_ON(!list_empty(&root_cgrp->self.children));
     BUG_ON(atomic_read(&root->nr_cgrps) != 1);
 
     ret = 0;
     goto out;
 
 exit_stats:
     cgroup_rstat_exit(root_cgrp);
 destroy_root:
     kernfs_destroy_root(root->kf_root);
     root->kf_root = NULL;
 exit_root_id:
     cgroup_exit_root_id(root);
 cancel_ref:
     percpu_ref_exit(&root_cgrp->self.refcnt);
 out:
     free_cgrp_cset_links(&tmp_links);
     return ret;
 }
 
 int cgroup_do_get_tree(struct fs_context *fc)
 {
     struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
     int ret;
 
     ctx->kfc.root = ctx->root->kf_root;
     if (fc->fs_type == &cgroup2_fs_type)
         ctx->kfc.magic = CGROUP2_SUPER_MAGIC;
     else
         ctx->kfc.magic = CGROUP_SUPER_MAGIC;
     ret = kernfs_get_tree(fc);
 
     /*
      * In non-init cgroup namespace, instead of root cgroup's dentry,
      * we return the dentry corresponding to the cgroupns->root_cgrp.
      */
     if (!ret && ctx->ns != &init_cgroup_ns) {
         struct dentry *nsdentry;
         struct super_block *sb = fc->root->d_sb;
         struct cgroup *cgrp;
 
         mutex_lock(&cgroup_mutex);
         spin_lock_irq(&css_set_lock);
 
         cgrp = cset_cgroup_from_root(ctx->ns->root_cset, ctx->root);
 
         spin_unlock_irq(&css_set_lock);
         mutex_unlock(&cgroup_mutex);
 
         nsdentry = kernfs_node_dentry(cgrp->kn, sb);
         dput(fc->root);
         if (IS_ERR(nsdentry)) {
             deactivate_locked_super(sb);
             ret = PTR_ERR(nsdentry);
             nsdentry = NULL;
         }
         fc->root = nsdentry;
     }
 
     if (!ctx->kfc.new_sb_created)
         cgroup_put(&ctx->root->cgrp);
 
     return ret;
 }
 
 /*
  * Destroy a cgroup filesystem context.
  */
 static void cgroup_fs_context_free(struct fs_context *fc)
 {
     struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
 
     kfree(ctx->name);
     kfree(ctx->release_agent);
     put_cgroup_ns(ctx->ns);
     kernfs_free_fs_context(fc);
     kfree(ctx);
 }
 
 static int cgroup_get_tree(struct fs_context *fc)
 {
     struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
     int ret;
 
     cgrp_dfl_visible = true;
     cgroup_get_live(&cgrp_dfl_root.cgrp);
     ctx->root = &cgrp_dfl_root;
 
     ret = cgroup_do_get_tree(fc);
     if (!ret)
         apply_cgroup_root_flags(ctx->flags);
     return ret;
 }
 
 static const struct fs_context_operations cgroup_fs_context_ops = {
     .free       = cgroup_fs_context_free,
     .parse_param    = cgroup2_parse_param,
     .get_tree   = cgroup_get_tree,
     .reconfigure    = cgroup_reconfigure,
 };
 
 static const struct fs_context_operations cgroup1_fs_context_ops = {
     .free       = cgroup_fs_context_free,
     .parse_param    = cgroup1_parse_param,
     .get_tree   = cgroup1_get_tree,
     .reconfigure    = cgroup1_reconfigure,
 };
 
 /*
  * Initialise the cgroup filesystem creation/reconfiguration context.  Notably,
  * we select the namespace we're going to use.
  */
 static int cgroup_init_fs_context(struct fs_context *fc)
 {
     struct cgroup_fs_context *ctx;
 
     ctx = kzalloc(sizeof(struct cgroup_fs_context), GFP_KERNEL);
     if (!ctx)
         return -ENOMEM;
 
     ctx->ns = current->nsproxy->cgroup_ns;
     get_cgroup_ns(ctx->ns);
     fc->fs_private = &ctx->kfc;
     if (fc->fs_type == &cgroup2_fs_type)
         fc->ops = &cgroup_fs_context_ops;
     else
         fc->ops = &cgroup1_fs_context_ops;
     put_user_ns(fc->user_ns);
     fc->user_ns = get_user_ns(ctx->ns->user_ns);
     fc->global = true;
 
 #ifdef CONFIG_CGROUP_FAVOR_DYNMODS
     ctx->flags |= CGRP_ROOT_FAVOR_DYNMODS;
 #endif
     return 0;
 }
 
 static void cgroup_kill_sb(struct super_block *sb)
 {
     struct kernfs_root *kf_root = kernfs_root_from_sb(sb);
     struct cgroup_root *root = cgroup_root_from_kf(kf_root);
 
     /*
      * If @root doesn't have any children, start killing it.
      * This prevents new mounts by disabling percpu_ref_tryget_live().
      *
      * And don't kill the default root.
      */
     if (list_empty(&root->cgrp.self.children) && root != &cgrp_dfl_root &&
         !percpu_ref_is_dying(&root->cgrp.self.refcnt)) {
         cgroup_bpf_offline(&root->cgrp);
         percpu_ref_kill(&root->cgrp.self.refcnt);
     }
     cgroup_put(&root->cgrp);
     kernfs_kill_sb(sb);
 }
 
 struct file_system_type cgroup_fs_type = {
     .name           = "cgroup",
     .init_fs_context    = cgroup_init_fs_context,
     .parameters     = cgroup1_fs_parameters,
     .kill_sb        = cgroup_kill_sb,
     .fs_flags       = FS_USERNS_MOUNT,
 };
 
 static struct file_system_type cgroup2_fs_type = {
     .name           = "cgroup2",
     .init_fs_context    = cgroup_init_fs_context,
     .parameters     = cgroup2_fs_parameters,
     .kill_sb        = cgroup_kill_sb,
     .fs_flags       = FS_USERNS_MOUNT,
 };
 
 #ifdef CONFIG_CPUSETS
 static const struct fs_context_operations cpuset_fs_context_ops = {
     .get_tree   = cgroup1_get_tree,
     .free       = cgroup_fs_context_free,
 };
 
 /*
  * This is ugly, but preserves the userspace API for existing cpuset
  * users. If someone tries to mount the "cpuset" filesystem, we
  * silently switch it to mount "cgroup" instead
  */
 static int cpuset_init_fs_context(struct fs_context *fc)
 {
     char *agent = kstrdup("/sbin/cpuset_release_agent", GFP_USER);
     struct cgroup_fs_context *ctx;
     int err;
 
     err = cgroup_init_fs_context(fc);
     if (err) {
         kfree(agent);
         return err;
     }
 
     fc->ops = &cpuset_fs_context_ops;
 
     ctx = cgroup_fc2context(fc);
     ctx->subsys_mask = 1 << cpuset_cgrp_id;
     ctx->flags |= CGRP_ROOT_NOPREFIX;
     ctx->release_agent = agent;
 
     get_filesystem(&cgroup_fs_type);
     put_filesystem(fc->fs_type);
     fc->fs_type = &cgroup_fs_type;
 
     return 0;
 }
 
 static struct file_system_type cpuset_fs_type = {
     .name           = "cpuset",
     .init_fs_context    = cpuset_init_fs_context,
     .fs_flags       = FS_USERNS_MOUNT,
 };
 #endif
 
 int cgroup_path_ns_locked(struct cgroup *cgrp, char *buf, size_t buflen,
               struct cgroup_namespace *ns)
 {
     struct cgroup *root = cset_cgroup_from_root(ns->root_cset, cgrp->root);
 
     return kernfs_path_from_node(cgrp->kn, root->kn, buf, buflen);
 }
 
 int cgroup_path_ns(struct cgroup *cgrp, char *buf, size_t buflen,
            struct cgroup_namespace *ns)
 {
     int ret;
 
     mutex_lock(&cgroup_mutex);
     spin_lock_irq(&css_set_lock);
 
     ret = cgroup_path_ns_locked(cgrp, buf, buflen, ns);
 
     spin_unlock_irq(&css_set_lock);
     mutex_unlock(&cgroup_mutex);
 
     return ret;
 }
 EXPORT_SYMBOL_GPL(cgroup_path_ns);
 
 /**
  * task_cgroup_path - cgroup path of a task in the first cgroup hierarchy
  * @task: target task
  * @buf: the buffer to write the path into
  * @buflen: the length of the buffer
  *
  * Determine @task's cgroup on the first (the one with the lowest non-zero
  * hierarchy_id) cgroup hierarchy and copy its path into @buf.  This
  * function grabs cgroup_mutex and shouldn't be used inside locks used by
  * cgroup controller callbacks.
  *
  * Return value is the same as kernfs_path().
  */
 int task_cgroup_path(struct task_struct *task, char *buf, size_t buflen)
 {
     struct cgroup_root *root;
     struct cgroup *cgrp;
     int hierarchy_id = 1;
     int ret;
 
     mutex_lock(&cgroup_mutex);
     spin_lock_irq(&css_set_lock);
 
     root = idr_get_next(&cgroup_hierarchy_idr, &hierarchy_id);
 
     if (root) {
         cgrp = task_cgroup_from_root(task, root);
         ret = cgroup_path_ns_locked(cgrp, buf, buflen, &init_cgroup_ns);
     } else {
         /* if no hierarchy exists, everyone is in "/" */
         ret = strlcpy(buf, "/", buflen);
     }
 
     spin_unlock_irq(&css_set_lock);
     mutex_unlock(&cgroup_mutex);
     return ret;
 }
 EXPORT_SYMBOL_GPL(task_cgroup_path);
 
 /**
  * cgroup_attach_lock - Lock for ->attach()
  * @lock_threadgroup: whether to down_write cgroup_threadgroup_rwsem
  *
  * cgroup migration sometimes needs to stabilize threadgroups against forks and
  * exits by write-locking cgroup_threadgroup_rwsem. However, some ->attach()
  * implementations (e.g. cpuset), also need to disable CPU hotplug.
  * Unfortunately, letting ->attach() operations acquire cpus_read_lock() can
  * lead to deadlocks.
  *
  * Bringing up a CPU may involve creating and destroying tasks which requires
  * read-locking threadgroup_rwsem, so threadgroup_rwsem nests inside
  * cpus_read_lock(). If we call an ->attach() which acquires the cpus lock while
  * write-locking threadgroup_rwsem, the locking order is reversed and we end up
  * waiting for an on-going CPU hotplug operation which in turn is waiting for
  * the threadgroup_rwsem to be released to create new tasks. For more details:
  *
  *   http://lkml.kernel.org/r/20220711174629.uehfmqegcwn2lqzu@wubuntu
  *
  * Resolve the situation by always acquiring cpus_read_lock() before optionally
  * write-locking cgroup_threadgroup_rwsem. This allows ->attach() to assume that
  * CPU hotplug is disabled on entry.
  */
 static void cgroup_attach_lock(bool lock_threadgroup)
 {
     cpus_read_lock();
     if (lock_threadgroup)
         percpu_down_write(&cgroup_threadgroup_rwsem);
 }
 
 /**
  * cgroup_attach_unlock - Undo cgroup_attach_lock()
  * @lock_threadgroup: whether to up_write cgroup_threadgroup_rwsem
  */
 static void cgroup_attach_unlock(bool lock_threadgroup)
 {
     if (lock_threadgroup)
         percpu_up_write(&cgroup_threadgroup_rwsem);
     cpus_read_unlock();
 }
 
 /**
  * cgroup_migrate_add_task - add a migration target task to a migration context
  * @task: target task
  * @mgctx: target migration context
  *
  * Add @task, which is a migration target, to @mgctx->tset.  This function
  * becomes noop if @task doesn't need to be migrated.  @task's css_set
  * should have been added as a migration source and @task->cg_list will be
  * moved from the css_set's tasks list to mg_tasks one.
  */
 static void cgroup_migrate_add_task(struct task_struct *task,
                     struct cgroup_mgctx *mgctx)
 {
     struct css_set *cset;
 
     lockdep_assert_held(&css_set_lock);
 
     /* @task either already exited or can't exit until the end */
     if (task->flags & PF_EXITING)
         return;
 
     /* cgroup_threadgroup_rwsem protects racing against forks */
     WARN_ON_ONCE(list_empty(&task->cg_list));
 
     cset = task_css_set(task);
     if (!cset->mg_src_cgrp)
         return;
 
     mgctx->tset.nr_tasks++;
 
     list_move_tail(&task->cg_list, &cset->mg_tasks);
     if (list_empty(&cset->mg_node))
         list_add_tail(&cset->mg_node,
                   &mgctx->tset.src_csets);
     if (list_empty(&cset->mg_dst_cset->mg_node))
         list_add_tail(&cset->mg_dst_cset->mg_node,
                   &mgctx->tset.dst_csets);
 }
 
 /**
  * cgroup_taskset_first - reset taskset and return the first task
  * @tset: taskset of interest
  * @dst_cssp: output variable for the destination css
  *
  * @tset iteration is initialized and the first task is returned.
  */
 struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset,
                      struct cgroup_subsys_state **dst_cssp)
 {
     tset->cur_cset = list_first_entry(tset->csets, struct css_set, mg_node);
     tset->cur_task = NULL;
 
     return cgroup_taskset_next(tset, dst_cssp);
 }
 
 /**
  * cgroup_taskset_next - iterate to the next task in taskset
  * @tset: taskset of interest
  * @dst_cssp: output variable for the destination css
  *
  * Return the next task in @tset.  Iteration must have been initialized
  * with cgroup_taskset_first().
  */
 struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset,
                     struct cgroup_subsys_state **dst_cssp)
 {
     struct css_set *cset = tset->cur_cset;
     struct task_struct *task = tset->cur_task;
 
     while (CGROUP_HAS_SUBSYS_CONFIG && &cset->mg_node != tset->csets) {
         if (!task)
             task = list_first_entry(&cset->mg_tasks,
                         struct task_struct, cg_list);
         else
             task = list_next_entry(task, cg_list);
 
         if (&task->cg_list != &cset->mg_tasks) {
             tset->cur_cset = cset;
             tset->cur_task = task;
 
             /*
              * This function may be called both before and
              * after cgroup_taskset_migrate().  The two cases
              * can be distinguished by looking at whether @cset
              * has its ->mg_dst_cset set.
              */
             if (cset->mg_dst_cset)
                 *dst_cssp = cset->mg_dst_cset->subsys[tset->ssid];
             else
                 *dst_cssp = cset->subsys[tset->ssid];
 
             return task;
         }
 
         cset = list_next_entry(cset, mg_node);
         task = NULL;
     }
 
     return NULL;
 }
 
 /**
  * cgroup_migrate_execute - migrate a taskset
  * @mgctx: migration context
  *
  * Migrate tasks in @mgctx as setup by migration preparation functions.
  * This function fails iff one of the ->can_attach callbacks fails and
  * guarantees that either all or none of the tasks in @mgctx are migrated.
  * @mgctx is consumed regardless of success.
  */
 static int cgroup_migrate_execute(struct cgroup_mgctx *mgctx)
 {
     struct cgroup_taskset *tset = &mgctx->tset;
     struct cgroup_subsys *ss;
     struct task_struct *task, *tmp_task;
     struct css_set *cset, *tmp_cset;
     int ssid, failed_ssid, ret;
 
     /* check that we can legitimately attach to the cgroup */
     if (tset->nr_tasks) {
         do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
             if (ss->can_attach) {
                 tset->ssid = ssid;
                 ret = ss->can_attach(tset);
                 if (ret) {
                     failed_ssid = ssid;
                     goto out_cancel_attach;
                 }
             }
         } while_each_subsys_mask();
     }
 
     /*
      * Now that we're guaranteed success, proceed to move all tasks to
      * the new cgroup.  There are no failure cases after here, so this
      * is the commit point.
      */
     spin_lock_irq(&css_set_lock);
     list_for_each_entry(cset, &tset->src_csets, mg_node) {
         list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list) {
             struct css_set *from_cset = task_css_set(task);
             struct css_set *to_cset = cset->mg_dst_cset;
 
             get_css_set(to_cset);
             to_cset->nr_tasks++;
             css_set_move_task(task, from_cset, to_cset, true);
             from_cset->nr_tasks--;
             /*
              * If the source or destination cgroup is frozen,
              * the task might require to change its state.
              */
             cgroup_freezer_migrate_task(task, from_cset->dfl_cgrp,
                             to_cset->dfl_cgrp);
             put_css_set_locked(from_cset);
 
         }
     }
     spin_unlock_irq(&css_set_lock);
 
     /*
      * Migration is committed, all target tasks are now on dst_csets.
      * Nothing is sensitive to fork() after this point.  Notify
      * controllers that migration is complete.
      */
     tset->csets = &tset->dst_csets;
 
     if (tset->nr_tasks) {
         do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
             if (ss->attach) {
                 tset->ssid = ssid;
                 ss->attach(tset);
             }
         } while_each_subsys_mask();
     }
 
     ret = 0;
     goto out_release_tset;
 
 out_cancel_attach:
     if (tset->nr_tasks) {
         do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
             if (ssid == failed_ssid)
                 break;
             if (ss->cancel_attach) {
                 tset->ssid = ssid;
                 ss->cancel_attach(tset);
             }
         } while_each_subsys_mask();
     }
 out_release_tset:
     spin_lock_irq(&css_set_lock);
     list_splice_init(&tset->dst_csets, &tset->src_csets);
     list_for_each_entry_safe(cset, tmp_cset, &tset->src_csets, mg_node) {
         list_splice_tail_init(&cset->mg_tasks, &cset->tasks);
         list_del_init(&cset->mg_node);
     }
     spin_unlock_irq(&css_set_lock);
 
     /*
      * Re-initialize the cgroup_taskset structure in case it is reused
      * again in another cgroup_migrate_add_task()/cgroup_migrate_execute()
      * iteration.
      */
     tset->nr_tasks = 0;
     tset->csets    = &tset->src_csets;
     return ret;
 }
 
 /**
  * cgroup_migrate_vet_dst - verify whether a cgroup can be migration destination
  * @dst_cgrp: destination cgroup to test
  *
  * On the default hierarchy, except for the mixable, (possible) thread root
  * and threaded cgroups, subtree_control must be zero for migration
  * destination cgroups with tasks so that child cgroups don't compete
  * against tasks.
  */
 int cgroup_migrate_vet_dst(struct cgroup *dst_cgrp)
 {
     /* v1 doesn't have any restriction */
     if (!cgroup_on_dfl(dst_cgrp))
         return 0;
 
     /* verify @dst_cgrp can host resources */
     if (!cgroup_is_valid_domain(dst_cgrp->dom_cgrp))
         return -EOPNOTSUPP;
 
     /*
      * If @dst_cgrp is already or can become a thread root or is
      * threaded, it doesn't matter.
      */
     if (cgroup_can_be_thread_root(dst_cgrp) || cgroup_is_threaded(dst_cgrp))
         return 0;
 
     /* apply no-internal-process constraint */
     if (dst_cgrp->subtree_control)
         return -EBUSY;
 
     return 0;
 }
 
 /**
  * cgroup_migrate_finish - cleanup after attach
  * @mgctx: migration context
  *
  * Undo cgroup_migrate_add_src() and cgroup_migrate_prepare_dst().  See
  * those functions for details.
  */
 void cgroup_migrate_finish(struct cgroup_mgctx *mgctx)
 {
     struct css_set *cset, *tmp_cset;
 
     lockdep_assert_held(&cgroup_mutex);
 
     spin_lock_irq(&css_set_lock);
 
     list_for_each_entry_safe(cset, tmp_cset, &mgctx->preloaded_src_csets,
                  mg_src_preload_node) {
         cset->mg_src_cgrp = NULL;
         cset->mg_dst_cgrp = NULL;
         cset->mg_dst_cset = NULL;
         list_del_init(&cset->mg_src_preload_node);
         put_css_set_locked(cset);
     }
 
     list_for_each_entry_safe(cset, tmp_cset, &mgctx->preloaded_dst_csets,
                  mg_dst_preload_node) {
         cset->mg_src_cgrp = NULL;
         cset->mg_dst_cgrp = NULL;
         cset->mg_dst_cset = NULL;
         list_del_init(&cset->mg_dst_preload_node);
         put_css_set_locked(cset);
     }
 
     spin_unlock_irq(&css_set_lock);
 }
 
 /**
  * cgroup_migrate_add_src - add a migration source css_set
  * @src_cset: the source css_set to add
  * @dst_cgrp: the destination cgroup
  * @mgctx: migration context
  *
  * Tasks belonging to @src_cset are about to be migrated to @dst_cgrp.  Pin
  * @src_cset and add it to @mgctx->src_csets, which should later be cleaned
  * up by cgroup_migrate_finish().
  *
  * This function may be called without holding cgroup_threadgroup_rwsem
  * even if the target is a process.  Threads may be created and destroyed
  * but as long as cgroup_mutex is not dropped, no new css_set can be put
  * into play and the preloaded css_sets are guaranteed to cover all
  * migrations.
  */
 void cgroup_migrate_add_src(struct css_set *src_cset,
                 struct cgroup *dst_cgrp,
                 struct cgroup_mgctx *mgctx)
 {
     struct cgroup *src_cgrp;
 
     lockdep_assert_held(&cgroup_mutex);
     lockdep_assert_held(&css_set_lock);
 
     /*
      * If ->dead, @src_set is associated with one or more dead cgroups
      * and doesn't contain any migratable tasks.  Ignore it early so
      * that the rest of migration path doesn't get confused by it.
      */
     if (src_cset->dead)
         return;
 
     if (!list_empty(&src_cset->mg_src_preload_node))
         return;
 
     src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);
 
     WARN_ON(src_cset->mg_src_cgrp);
     WARN_ON(src_cset->mg_dst_cgrp);
     WARN_ON(!list_empty(&src_cset->mg_tasks));
     WARN_ON(!list_empty(&src_cset->mg_node));
 
     src_cset->mg_src_cgrp = src_cgrp;
     src_cset->mg_dst_cgrp = dst_cgrp;
     get_css_set(src_cset);
     list_add_tail(&src_cset->mg_src_preload_node, &mgctx->preloaded_src_csets);
 }
 
 /**
  * cgroup_migrate_prepare_dst - prepare destination css_sets for migration
  * @mgctx: migration context
  *
  * Tasks are about to be moved and all the source css_sets have been
  * preloaded to @mgctx->preloaded_src_csets.  This function looks up and
  * pins all destination css_sets, links each to its source, and append them
  * to @mgctx->preloaded_dst_csets.
  *
  * This function must be called after cgroup_migrate_add_src() has been
  * called on each migration source css_set.  After migration is performed
  * using cgroup_migrate(), cgroup_migrate_finish() must be called on
  * @mgctx.
  */
 int cgroup_migrate_prepare_dst(struct cgroup_mgctx *mgctx)
 {
     struct css_set *src_cset, *tmp_cset;
 
     lockdep_assert_held(&cgroup_mutex);
 
     /* look up the dst cset for each src cset and link it to src */
     list_for_each_entry_safe(src_cset, tmp_cset, &mgctx->preloaded_src_csets,
                  mg_src_preload_node) {
         struct css_set *dst_cset;
         struct cgroup_subsys *ss;
         int ssid;
 
         dst_cset = find_css_set(src_cset, src_cset->mg_dst_cgrp);
         if (!dst_cset)
             return -ENOMEM;
 
         WARN_ON_ONCE(src_cset->mg_dst_cset || dst_cset->mg_dst_cset);
 
         /*
          * If src cset equals dst, it's noop.  Drop the src.
          * cgroup_migrate() will skip the cset too.  Note that we
          * can't handle src == dst as some nodes are used by both.
          */
         if (src_cset == dst_cset) {
             src_cset->mg_src_cgrp = NULL;
             src_cset->mg_dst_cgrp = NULL;
             list_del_init(&src_cset->mg_src_preload_node);
             put_css_set(src_cset);
             put_css_set(dst_cset);
             continue;
         }
 
         src_cset->mg_dst_cset = dst_cset;
 
         if (list_empty(&dst_cset->mg_dst_preload_node))
             list_add_tail(&dst_cset->mg_dst_preload_node,
                       &mgctx->preloaded_dst_csets);
         else
             put_css_set(dst_cset);
 
         for_each_subsys(ss, ssid)
             if (src_cset->subsys[ssid] != dst_cset->subsys[ssid])
                 mgctx->ss_mask |= 1 << ssid;
     }
 
     return 0;
 }
 
 /**
  * cgroup_migrate - migrate a process or task to a cgroup
  * @leader: the leader of the process or the task to migrate
  * @threadgroup: whether @leader points to the whole process or a single task
  * @mgctx: migration context
  *
  * Migrate a process or task denoted by @leader.  If migrating a process,
  * the caller must be holding cgroup_threadgroup_rwsem.  The caller is also
  * responsible for invoking cgroup_migrate_add_src() and
  * cgroup_migrate_prepare_dst() on the targets before invoking this
  * function and following up with cgroup_migrate_finish().
  *
  * As long as a controller's ->can_attach() doesn't fail, this function is
  * guaranteed to succeed.  This means that, excluding ->can_attach()
  * failure, when migrating multiple targets, the success or failure can be
  * decided for all targets by invoking group_migrate_prepare_dst() before
  * actually starting migrating.
  */
 int cgroup_migrate(struct task_struct *leader, bool threadgroup,
            struct cgroup_mgctx *mgctx)
 {
     struct task_struct *task;
 
     /*
      * Prevent freeing of tasks while we take a snapshot. Tasks that are
      * already PF_EXITING could be freed from underneath us unless we
      * take an rcu_read_lock.
      */
     spin_lock_irq(&css_set_lock);
     rcu_read_lock();
     task = leader;
     do {
         cgroup_migrate_add_task(task, mgctx);
         if (!threadgroup)
             break;
     } while_each_thread(leader, task);
     rcu_read_unlock();
     spin_unlock_irq(&css_set_lock);
 
     return cgroup_migrate_execute(mgctx);
 }
 
 /**
  * cgroup_attach_task - attach a task or a whole threadgroup to a cgroup
  * @dst_cgrp: the cgroup to attach to
  * @leader: the task or the leader of the threadgroup to be attached
  * @threadgroup: attach the whole threadgroup?
  *
  * Call holding cgroup_mutex and cgroup_threadgroup_rwsem.
  */
 int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
                bool threadgroup)
 {
     DEFINE_CGROUP_MGCTX(mgctx);
     struct task_struct *task;
     int ret = 0;
 
     /* look up all src csets */
     spin_lock_irq(&css_set_lock);
     rcu_read_lock();
     task = leader;
     do {
         cgroup_migrate_add_src(task_css_set(task), dst_cgrp, &mgctx);
         if (!threadgroup)
             break;
     } while_each_thread(leader, task);
     rcu_read_unlock();
     spin_unlock_irq(&css_set_lock);
 
     /* prepare dst csets and commit */
     ret = cgroup_migrate_prepare_dst(&mgctx);
     if (!ret)
         ret = cgroup_migrate(leader, threadgroup, &mgctx);
 
     cgroup_migrate_finish(&mgctx);
 
     if (!ret)
         TRACE_CGROUP_PATH(attach_task, dst_cgrp, leader, threadgroup);
 
     return ret;
 }
 
 struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
                          bool *threadgroup_locked)
 {
     struct task_struct *tsk;
     pid_t pid;
 
     if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
         return ERR_PTR(-EINVAL);
 
     /*
      * If we migrate a single thread, we don't care about threadgroup
      * stability. If the thread is `current`, it won't exit(2) under our
      * hands or change PID through exec(2). We exclude
      * cgroup_update_dfl_csses and other cgroup_{proc,thread}s_write
      * callers by cgroup_mutex.
      * Therefore, we can skip the global lock.
      */
     lockdep_assert_held(&cgroup_mutex);
     *threadgroup_locked = pid || threadgroup;
     cgroup_attach_lock(*threadgroup_locked);
 
     rcu_read_lock();
     if (pid) {
         tsk = find_task_by_vpid(pid);
         if (!tsk) {
             tsk = ERR_PTR(-ESRCH);
             goto out_unlock_threadgroup;
         }
     } else {
         tsk = current;
     }
 
     if (threadgroup)
         tsk = tsk->group_leader;
 
     /*
      * kthreads may acquire PF_NO_SETAFFINITY during initialization.
      * If userland migrates such a kthread to a non-root cgroup, it can
      * become trapped in a cpuset, or RT kthread may be born in a
      * cgroup with no rt_runtime allocated.  Just say no.
      */
     if (tsk->no_cgroup_migration || (tsk->flags & PF_NO_SETAFFINITY)) {
         tsk = ERR_PTR(-EINVAL);
         goto out_unlock_threadgroup;
     }
 
     get_task_struct(tsk);
     goto out_unlock_rcu;
 
 out_unlock_threadgroup:
     cgroup_attach_unlock(*threadgroup_locked);
     *threadgroup_locked = false;
 out_unlock_rcu:
     rcu_read_unlock();
     return tsk;
 }
 
 void cgroup_procs_write_finish(struct task_struct *task, bool threadgroup_locked)
 {
     struct cgroup_subsys *ss;
     int ssid;
 
     /* release reference from cgroup_procs_write_start() */
     put_task_struct(task);
 
     cgroup_attach_unlock(threadgroup_locked);
 
     for_each_subsys(ss, ssid)
         if (ss->post_attach)
             ss->post_attach();
 }
 
 static void cgroup_print_ss_mask(struct seq_file *seq, u16 ss_mask)
 {
     struct cgroup_subsys *ss;
     bool printed = false;
     int ssid;
 
     do_each_subsys_mask(ss, ssid, ss_mask) {
         if (printed)
             seq_putc(seq, ' ');
         seq_puts(seq, ss->name);
         printed = true;
     } while_each_subsys_mask();
     if (printed)
         seq_putc(seq, '\n');
 }
 
 /* show controllers which are enabled from the parent */
 static int cgroup_controllers_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
 
     cgroup_print_ss_mask(seq, cgroup_control(cgrp));
     return 0;
 }
 
 /* show controllers which are enabled for a given cgroup's children */
 static int cgroup_subtree_control_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
 
     cgroup_print_ss_mask(seq, cgrp->subtree_control);
     return 0;
 }
 
 /**
  * cgroup_update_dfl_csses - update css assoc of a subtree in default hierarchy
  * @cgrp: root of the subtree to update csses for
  *
  * @cgrp's control masks have changed and its subtree's css associations
  * need to be updated accordingly.  This function looks up all css_sets
  * which are attached to the subtree, creates the matching updated css_sets
  * and migrates the tasks to the new ones.
  */
 static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 {
     DEFINE_CGROUP_MGCTX(mgctx);
     struct cgroup_subsys_state *d_css;
     struct cgroup *dsct;
     struct css_set *src_cset;
     bool has_tasks;
     int ret;
 
     lockdep_assert_held(&cgroup_mutex);
 
     /* look up all csses currently attached to @cgrp's subtree */
     spin_lock_irq(&css_set_lock);
     cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
         struct cgrp_cset_link *link;
 
         /*
          * As cgroup_update_dfl_csses() is only called by
          * cgroup_apply_control(). The csses associated with the
          * given cgrp will not be affected by changes made to
          * its subtree_control file. We can skip them.
          */
         if (dsct == cgrp)
             continue;
 
         list_for_each_entry(link, &dsct->cset_links, cset_link)
             cgroup_migrate_add_src(link->cset, dsct, &mgctx);
     }
     spin_unlock_irq(&css_set_lock);
 
     /*
      * We need to write-lock threadgroup_rwsem while migrating tasks.
      * However, if there are no source csets for @cgrp, changing its
      * controllers isn't gonna produce any task migrations and the
      * write-locking can be skipped safely.
      */
     has_tasks = !list_empty(&mgctx.preloaded_src_csets);
     cgroup_attach_lock(has_tasks);
 
     /* NULL dst indicates self on default hierarchy */
     ret = cgroup_migrate_prepare_dst(&mgctx);
     if (ret)
         goto out_finish;
 
     spin_lock_irq(&css_set_lock);
     list_for_each_entry(src_cset, &mgctx.preloaded_src_csets,
                 mg_src_preload_node) {
         struct task_struct *task, *ntask;
 
         /* all tasks in src_csets need to be migrated */
         list_for_each_entry_safe(task, ntask, &src_cset->tasks, cg_list)
             cgroup_migrate_add_task(task, &mgctx);
     }
     spin_unlock_irq(&css_set_lock);
 
     ret = cgroup_migrate_execute(&mgctx);
 out_finish:
     cgroup_migrate_finish(&mgctx);
     cgroup_attach_unlock(has_tasks);
     return ret;
 }
 
 /**
  * cgroup_lock_and_drain_offline - lock cgroup_mutex and drain offlined csses
  * @cgrp: root of the target subtree
  *
  * Because css offlining is asynchronous, userland may try to re-enable a
  * controller while the previous css is still around.  This function grabs
  * cgroup_mutex and drains the previous css instances of @cgrp's subtree.
  */
 void cgroup_lock_and_drain_offline(struct cgroup *cgrp)
     __acquires(&cgroup_mutex)
 {
     struct cgroup *dsct;
     struct cgroup_subsys_state *d_css;
     struct cgroup_subsys *ss;
     int ssid;
 
 restart:
     mutex_lock(&cgroup_mutex);
 
     cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
         for_each_subsys(ss, ssid) {
             struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
             DEFINE_WAIT(wait);
 
             if (!css || !percpu_ref_is_dying(&css->refcnt))
                 continue;
 
             cgroup_get_live(dsct);
             prepare_to_wait(&dsct->offline_waitq, &wait,
                     TASK_UNINTERRUPTIBLE);
 
             mutex_unlock(&cgroup_mutex);
             schedule();
             finish_wait(&dsct->offline_waitq, &wait);
 
             cgroup_put(dsct);
             goto restart;
         }
     }
 }
 
 /**
  * cgroup_save_control - save control masks and dom_cgrp of a subtree
  * @cgrp: root of the target subtree
  *
  * Save ->subtree_control, ->subtree_ss_mask and ->dom_cgrp to the
  * respective old_ prefixed fields for @cgrp's subtree including @cgrp
  * itself.
  */
 static void cgroup_save_control(struct cgroup *cgrp)
 {
     struct cgroup *dsct;
     struct cgroup_subsys_state *d_css;
 
     cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
         dsct->old_subtree_control = dsct->subtree_control;
         dsct->old_subtree_ss_mask = dsct->subtree_ss_mask;
         dsct->old_dom_cgrp = dsct->dom_cgrp;
     }
 }
 
 /**
  * cgroup_propagate_control - refresh control masks of a subtree
  * @cgrp: root of the target subtree
  *
  * For @cgrp and its subtree, ensure ->subtree_ss_mask matches
  * ->subtree_control and propagate controller availability through the
  * subtree so that descendants don't have unavailable controllers enabled.
  */
 static void cgroup_propagate_control(struct cgroup *cgrp)
 {
     struct cgroup *dsct;
     struct cgroup_subsys_state *d_css;
 
     cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
         dsct->subtree_control &= cgroup_control(dsct);
         dsct->subtree_ss_mask =
             cgroup_calc_subtree_ss_mask(dsct->subtree_control,
                             cgroup_ss_mask(dsct));
     }
 }
 
 /**
  * cgroup_restore_control - restore control masks and dom_cgrp of a subtree
  * @cgrp: root of the target subtree
  *
  * Restore ->subtree_control, ->subtree_ss_mask and ->dom_cgrp from the
  * respective old_ prefixed fields for @cgrp's subtree including @cgrp
  * itself.
  */
 static void cgroup_restore_control(struct cgroup *cgrp)
 {
     struct cgroup *dsct;
     struct cgroup_subsys_state *d_css;
 
     cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
         dsct->subtree_control = dsct->old_subtree_control;
         dsct->subtree_ss_mask = dsct->old_subtree_ss_mask;
         dsct->dom_cgrp = dsct->old_dom_cgrp;
     }
 }
 
 static bool css_visible(struct cgroup_subsys_state *css)
 {
     struct cgroup_subsys *ss = css->ss;
     struct cgroup *cgrp = css->cgroup;
 
     if (cgroup_control(cgrp) & (1 << ss->id))
         return true;
     if (!(cgroup_ss_mask(cgrp) & (1 << ss->id)))
         return false;
     return cgroup_on_dfl(cgrp) && ss->implicit_on_dfl;
 }
 
 /**
  * cgroup_apply_control_enable - enable or show csses according to control
  * @cgrp: root of the target subtree
  *
  * Walk @cgrp's subtree and create new csses or make the existing ones
  * visible.  A css is created invisible if it's being implicitly enabled
  * through dependency.  An invisible css is made visible when the userland
  * explicitly enables it.
  *
  * Returns 0 on success, -errno on failure.  On failure, csses which have
  * been processed already aren't cleaned up.  The caller is responsible for
  * cleaning up with cgroup_apply_control_disable().
  */
 static int cgroup_apply_control_enable(struct cgroup *cgrp)
 {
     struct cgroup *dsct;
     struct cgroup_subsys_state *d_css;
     struct cgroup_subsys *ss;
     int ssid, ret;
 
     cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
         for_each_subsys(ss, ssid) {
             struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
 
             if (!(cgroup_ss_mask(dsct) & (1 << ss->id)))
                 continue;
 
             if (!css) {
                 css = css_create(dsct, ss);
                 if (IS_ERR(css))
                     return PTR_ERR(css);
             }
 
             WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));
 
             if (css_visible(css)) {
                 ret = css_populate_dir(css);
                 if (ret)
                     return ret;
             }
         }
     }
 
     return 0;
 }
 
 /**
  * cgroup_apply_control_disable - kill or hide csses according to control
  * @cgrp: root of the target subtree
  *
  * Walk @cgrp's subtree and kill and hide csses so that they match
  * cgroup_ss_mask() and cgroup_visible_mask().
  *
  * A css is hidden when the userland requests it to be disabled while other
  * subsystems are still depending on it.  The css must not actively control
  * resources and be in the vanilla state if it's made visible again later.
  * Controllers which may be depended upon should provide ->css_reset() for
  * this purpose.
  */
 static void cgroup_apply_control_disable(struct cgroup *cgrp)
 {
     struct cgroup *dsct;
     struct cgroup_subsys_state *d_css;
     struct cgroup_subsys *ss;
     int ssid;
 
     cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
         for_each_subsys(ss, ssid) {
             struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
 
             if (!css)
                 continue;
 
             WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));
 
             if (css->parent &&
                 !(cgroup_ss_mask(dsct) & (1 << ss->id))) {
                 kill_css(css);
             } else if (!css_visible(css)) {
                 css_clear_dir(css);
                 if (ss->css_reset)
                     ss->css_reset(css);
             }
         }
     }
 }
 
 /**
  * cgroup_apply_control - apply control mask updates to the subtree
  * @cgrp: root of the target subtree
  *
  * subsystems can be enabled and disabled in a subtree using the following
  * steps.
  *
  * 1. Call cgroup_save_control() to stash the current state.
  * 2. Update ->subtree_control masks in the subtree as desired.
  * 3. Call cgroup_apply_control() to apply the changes.
  * 4. Optionally perform other related operations.
  * 5. Call cgroup_finalize_control() to finish up.
  *
  * This function implements step 3 and propagates the mask changes
  * throughout @cgrp's subtree, updates csses accordingly and perform
  * process migrations.
  */
 static int cgroup_apply_control(struct cgroup *cgrp)
 {
     int ret;
 
     cgroup_propagate_control(cgrp);
 
     ret = cgroup_apply_control_enable(cgrp);
     if (ret)
         return ret;
 
     /*
      * At this point, cgroup_e_css_by_mask() results reflect the new csses
      * making the following cgroup_update_dfl_csses() properly update
      * css associations of all tasks in the subtree.
      */
     ret = cgroup_update_dfl_csses(cgrp);
     if (ret)
         return ret;
 
     return 0;
 }
 
 /**
  * cgroup_finalize_control - finalize control mask update
  * @cgrp: root of the target subtree
  * @ret: the result of the update
  *
  * Finalize control mask update.  See cgroup_apply_control() for more info.
  */
 static void cgroup_finalize_control(struct cgroup *cgrp, int ret)
 {
     if (ret) {
         cgroup_restore_control(cgrp);
         cgroup_propagate_control(cgrp);
     }
 
     cgroup_apply_control_disable(cgrp);
 }
 
 static int cgroup_vet_subtree_control_enable(struct cgroup *cgrp, u16 enable)
 {
     u16 domain_enable = enable & ~cgrp_dfl_threaded_ss_mask;
 
     /* if nothing is getting enabled, nothing to worry about */
     if (!enable)
         return 0;
 
     /* can @cgrp host any resources? */
     if (!cgroup_is_valid_domain(cgrp->dom_cgrp))
         return -EOPNOTSUPP;
 
     /* mixables don't care */
     if (cgroup_is_mixable(cgrp))
         return 0;
 
     if (domain_enable) {
         /* can't enable domain controllers inside a thread subtree */
         if (cgroup_is_thread_root(cgrp) || cgroup_is_threaded(cgrp))
             return -EOPNOTSUPP;
     } else {
         /*
          * Threaded controllers can handle internal competitions
          * and are always allowed inside a (prospective) thread
          * subtree.
          */
         if (cgroup_can_be_thread_root(cgrp) || cgroup_is_threaded(cgrp))
             return 0;
     }
 
     /*
      * Controllers can't be enabled for a cgroup with tasks to avoid
      * child cgroups competing against tasks.
      */
     if (cgroup_has_tasks(cgrp))
         return -EBUSY;
 
     return 0;
 }
 
 /* change the enabled child controllers for a cgroup in the default hierarchy */
 static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
                         char *buf, size_t nbytes,
                         loff_t off)
 {
     u16 enable = 0, disable = 0;
     struct cgroup *cgrp, *child;
     struct cgroup_subsys *ss;
     char *tok;
     int ssid, ret;
 
     /*
      * Parse input - space separated list of subsystem names prefixed
      * with either + or -.
      */
     buf = strstrip(buf);
     while ((tok = strsep(&buf, " "))) {
         if (tok[0] == '\0')
             continue;
         do_each_subsys_mask(ss, ssid, ~cgrp_dfl_inhibit_ss_mask) {
             if (!cgroup_ssid_enabled(ssid) ||
                 strcmp(tok + 1, ss->name))
                 continue;
 
             if (*tok == '+') {
                 enable |= 1 << ssid;
                 disable &= ~(1 << ssid);
             } else if (*tok == '-') {
                 disable |= 1 << ssid;
                 enable &= ~(1 << ssid);
             } else {
                 return -EINVAL;
             }
             break;
         } while_each_subsys_mask();
         if (ssid == CGROUP_SUBSYS_COUNT)
             return -EINVAL;
     }
 
     cgrp = cgroup_kn_lock_live(of->kn, true);
     if (!cgrp)
         return -ENODEV;
 
     for_each_subsys(ss, ssid) {
         if (enable & (1 << ssid)) {
             if (cgrp->subtree_control & (1 << ssid)) {
                 enable &= ~(1 << ssid);
                 continue;
             }
 
             if (!(cgroup_control(cgrp) & (1 << ssid))) {
                 ret = -ENOENT;
                 goto out_unlock;
             }
         } else if (disable & (1 << ssid)) {
             if (!(cgrp->subtree_control & (1 << ssid))) {
                 disable &= ~(1 << ssid);
                 continue;
             }
 
             /* a child has it enabled? */
             cgroup_for_each_live_child(child, cgrp) {
                 if (child->subtree_control & (1 << ssid)) {
                     ret = -EBUSY;
                     goto out_unlock;
                 }
             }
         }
     }
 
     if (!enable && !disable) {
         ret = 0;
         goto out_unlock;
     }
 
     ret = cgroup_vet_subtree_control_enable(cgrp, enable);
     if (ret)
         goto out_unlock;
 
     /* save and update control masks and prepare csses */
     cgroup_save_control(cgrp);
 
     cgrp->subtree_control |= enable;
     cgrp->subtree_control &= ~disable;
 
     ret = cgroup_apply_control(cgrp);
     cgroup_finalize_control(cgrp, ret);
     if (ret)
         goto out_unlock;
 
     kernfs_activate(cgrp->kn);
 out_unlock:
     cgroup_kn_unlock(of->kn);
     return ret ?: nbytes;
 }
 
 /**
  * cgroup_enable_threaded - make @cgrp threaded
  * @cgrp: the target cgroup
  *
  * Called when "threaded" is written to the cgroup.type interface file and
  * tries to make @cgrp threaded and join the parent's resource domain.
  * This function is never called on the root cgroup as cgroup.type doesn't
  * exist on it.
  */
 static int cgroup_enable_threaded(struct cgroup *cgrp)
 {
     struct cgroup *parent = cgroup_parent(cgrp);
     struct cgroup *dom_cgrp = parent->dom_cgrp;
     struct cgroup *dsct;
     struct cgroup_subsys_state *d_css;
     int ret;
 
     lockdep_assert_held(&cgroup_mutex);
 
     /* noop if already threaded */
     if (cgroup_is_threaded(cgrp))
         return 0;
 
     /*
      * If @cgroup is populated or has domain controllers enabled, it
      * can't be switched.  While the below cgroup_can_be_thread_root()
      * test can catch the same conditions, that's only when @parent is
      * not mixable, so let's check it explicitly.
      */
     if (cgroup_is_populated(cgrp) ||
         cgrp->subtree_control & ~cgrp_dfl_threaded_ss_mask)
         return -EOPNOTSUPP;
 
     /* we're joining the parent's domain, ensure its validity */
     if (!cgroup_is_valid_domain(dom_cgrp) ||
         !cgroup_can_be_thread_root(dom_cgrp))
         return -EOPNOTSUPP;
 
     /*
      * The following shouldn't cause actual migrations and should
      * always succeed.
      */
     cgroup_save_control(cgrp);
 
     cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp)
         if (dsct == cgrp || cgroup_is_threaded(dsct))
             dsct->dom_cgrp = dom_cgrp;
 
     ret = cgroup_apply_control(cgrp);
     if (!ret)
         parent->nr_threaded_children++;
 
     cgroup_finalize_control(cgrp, ret);
     return ret;
 }
 
 static int cgroup_type_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
 
     if (cgroup_is_threaded(cgrp))
         seq_puts(seq, "threaded\n");
     else if (!cgroup_is_valid_domain(cgrp))
         seq_puts(seq, "domain invalid\n");
     else if (cgroup_is_thread_root(cgrp))
         seq_puts(seq, "domain threaded\n");
     else
         seq_puts(seq, "domain\n");
 
     return 0;
 }
 
 static ssize_t cgroup_type_write(struct kernfs_open_file *of, char *buf,
                  size_t nbytes, loff_t off)
 {
     struct cgroup *cgrp;
     int ret;
 
     /* only switching to threaded mode is supported */
     if (strcmp(strstrip(buf), "threaded"))
         return -EINVAL;
 
     /* drain dying csses before we re-apply (threaded) subtree control */
     cgrp = cgroup_kn_lock_live(of->kn, true);
     if (!cgrp)
         return -ENOENT;
 
     /* threaded can only be enabled */
     ret = cgroup_enable_threaded(cgrp);
 
     cgroup_kn_unlock(of->kn);
     return ret ?: nbytes;
 }
 
 static int cgroup_max_descendants_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
     int descendants = READ_ONCE(cgrp->max_descendants);
 
     if (descendants == INT_MAX)
         seq_puts(seq, "max\n");
     else
         seq_printf(seq, "%d\n", descendants);
 
     return 0;
 }
 
 static ssize_t cgroup_max_descendants_write(struct kernfs_open_file *of,
                        char *buf, size_t nbytes, loff_t off)
 {
     struct cgroup *cgrp;
     int descendants;
     ssize_t ret;
 
     buf = strstrip(buf);
     if (!strcmp(buf, "max")) {
         descendants = INT_MAX;
     } else {
         ret = kstrtoint(buf, 0, &descendants);
         if (ret)
             return ret;
     }
 
     if (descendants < 0)
         return -ERANGE;
 
     cgrp = cgroup_kn_lock_live(of->kn, false);
     if (!cgrp)
         return -ENOENT;
 
     cgrp->max_descendants = descendants;
 
     cgroup_kn_unlock(of->kn);
 
     return nbytes;
 }
 
 static int cgroup_max_depth_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
     int depth = READ_ONCE(cgrp->max_depth);
 
     if (depth == INT_MAX)
         seq_puts(seq, "max\n");
     else
         seq_printf(seq, "%d\n", depth);
 
     return 0;
 }
 
 static ssize_t cgroup_max_depth_write(struct kernfs_open_file *of,
                       char *buf, size_t nbytes, loff_t off)
 {
     struct cgroup *cgrp;
     ssize_t ret;
     int depth;
 
     buf = strstrip(buf);
     if (!strcmp(buf, "max")) {
         depth = INT_MAX;
     } else {
         ret = kstrtoint(buf, 0, &depth);
         if (ret)
             return ret;
     }
 
     if (depth < 0)
         return -ERANGE;
 
     cgrp = cgroup_kn_lock_live(of->kn, false);
     if (!cgrp)
         return -ENOENT;
 
     cgrp->max_depth = depth;
 
     cgroup_kn_unlock(of->kn);
 
     return nbytes;
 }
 
 static int cgroup_events_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
 
     seq_printf(seq, "populated %d\n", cgroup_is_populated(cgrp));
     seq_printf(seq, "frozen %d\n", test_bit(CGRP_FROZEN, &cgrp->flags));
 
     return 0;
 }
 
 static int cgroup_stat_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgroup = seq_css(seq)->cgroup;
 
     seq_printf(seq, "nr_descendants %d\n",
            cgroup->nr_descendants);
     seq_printf(seq, "nr_dying_descendants %d\n",
            cgroup->nr_dying_descendants);
 
     return 0;
 }
 
 static int __maybe_unused cgroup_extra_stat_show(struct seq_file *seq,
                          struct cgroup *cgrp, int ssid)
 {
     struct cgroup_subsys *ss = cgroup_subsys[ssid];
     struct cgroup_subsys_state *css;
     int ret;
 
     if (!ss->css_extra_stat_show)
         return 0;
 
     css = cgroup_tryget_css(cgrp, ss);
     if (!css)
         return 0;
 
     ret = ss->css_extra_stat_show(seq, css);
     css_put(css);
     return ret;
 }
 
 static int cpu_stat_show(struct seq_file *seq, void *v)
 {
     struct cgroup __maybe_unused *cgrp = seq_css(seq)->cgroup;
     int ret = 0;
 
     cgroup_base_stat_cputime_show(seq);
 #ifdef CONFIG_CGROUP_SCHED
     ret = cgroup_extra_stat_show(seq, cgrp, cpu_cgrp_id);
 #endif
     return ret;
 }
 
 #ifdef CONFIG_PSI
 static int cgroup_io_pressure_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
     struct psi_group *psi = cgroup_ino(cgrp) == 1 ? &psi_system : cgrp->psi;
 
     return psi_show(seq, psi, PSI_IO);
 }
 static int cgroup_memory_pressure_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
     struct psi_group *psi = cgroup_ino(cgrp) == 1 ? &psi_system : cgrp->psi;
 
     return psi_show(seq, psi, PSI_MEM);
 }
 static int cgroup_cpu_pressure_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
     struct psi_group *psi = cgroup_ino(cgrp) == 1 ? &psi_system : cgrp->psi;
 
     return psi_show(seq, psi, PSI_CPU);
 }
 
 static ssize_t cgroup_pressure_write(struct kernfs_open_file *of, char *buf,
                       size_t nbytes, enum psi_res res)
 {
     struct cgroup_file_ctx *ctx = of->priv;
     struct psi_trigger *new;
     struct cgroup *cgrp;
     struct psi_group *psi;
 
     cgrp = cgroup_kn_lock_live(of->kn, false);
     if (!cgrp)
         return -ENODEV;
 
     cgroup_get(cgrp);
     cgroup_kn_unlock(of->kn);
 
     /* Allow only one trigger per file descriptor */
     if (ctx->psi.trigger) {
         cgroup_put(cgrp);
         return -EBUSY;
     }
 
     psi = cgroup_ino(cgrp) == 1 ? &psi_system : cgrp->psi;
     new = psi_trigger_create(psi, buf, res);
     if (IS_ERR(new)) {
         cgroup_put(cgrp);
         return PTR_ERR(new);
     }
 
     smp_store_release(&ctx->psi.trigger, new);
     cgroup_put(cgrp);
 
     return nbytes;
 }
 
 static ssize_t cgroup_io_pressure_write(struct kernfs_open_file *of,
                       char *buf, size_t nbytes,
                       loff_t off)
 {
     return cgroup_pressure_write(of, buf, nbytes, PSI_IO);
 }
 
 static ssize_t cgroup_memory_pressure_write(struct kernfs_open_file *of,
                       char *buf, size_t nbytes,
                       loff_t off)
 {
     return cgroup_pressure_write(of, buf, nbytes, PSI_MEM);
 }
 
 static ssize_t cgroup_cpu_pressure_write(struct kernfs_open_file *of,
                       char *buf, size_t nbytes,
                       loff_t off)
 {
     return cgroup_pressure_write(of, buf, nbytes, PSI_CPU);
 }
 
 static __poll_t cgroup_pressure_poll(struct kernfs_open_file *of,
                       poll_table *pt)
 {
     struct cgroup_file_ctx *ctx = of->priv;
 
     return psi_trigger_poll(&ctx->psi.trigger, of->file, pt);
 }
 
 static void cgroup_pressure_release(struct kernfs_open_file *of)
 {
     struct cgroup_file_ctx *ctx = of->priv;
 
     psi_trigger_destroy(ctx->psi.trigger);
 }
 
 bool cgroup_psi_enabled(void)
 {
     return (cgroup_feature_disable_mask & (1 << OPT_FEATURE_PRESSURE)) == 0;
 }
 
 #else /* CONFIG_PSI */
 bool cgroup_psi_enabled(void)
 {
     return false;
 }
 
 #endif /* CONFIG_PSI */
 
 static int cgroup_freeze_show(struct seq_file *seq, void *v)
 {
     struct cgroup *cgrp = seq_css(seq)->cgroup;
 
     seq_printf(seq, "%d\n", cgrp->freezer.freeze);
 
     return 0;
 }
 
 static ssize_t cgroup_freeze_write(struct kernfs_open_file *of,
                    char *buf, size_t nbytes, loff_t off)
 {
     struct cgroup *cgrp;
     ssize_t ret;
     int freeze;
 
     ret = kstrtoint(strstrip(buf), 0, &freeze);
     if (ret)
         return ret;
 
     if (freeze < 0 || freeze > 1)
         return -ERANGE;
 
     cgrp = cgroup_kn_lock_live(of->kn, false);
     if (!cgrp)
         return -ENOENT;
 
     cgroup_freeze(cgrp, freeze);
 
     cgroup_kn_unlock(of->kn);
 
     return nbytes;
 }
 
 static void __cgroup_kill(struct cgroup *cgrp)
 {
     struct css_task_iter it;
     struct task_struct *task;
 
     lockdep_assert_held(&cgroup_mutex);
 
     spin_lock_irq(&css_set_lock);
     set_bit(CGRP_KILL, &cgrp->flags);
     spin_unlock_irq(&css_set_lock);
 
     css_task_iter_start(&cgrp->self, CSS_TASK_ITER_PROCS | CSS_TASK_ITER_THREADED, &it);
     while ((task = css_task_iter_next(&it))) {
         /* Ignore kernel threads here. */
         if (task->flags & PF_KTHREAD)
             continue;
 
         /* Skip tasks that are already dying. */
         if (__fatal_signal_pending(task))
             continue;
 
         send_sig(SIGKILL, task, 0);
     }
     css_task_iter_end(&it);
 
     spin_lock_irq(&css_set_lock);
     clear_bit(CGRP_KILL, &cgrp->flags);
     spin_unlock_irq(&css_set_lock);
 }
 
 static void cgroup_kill(struct cgroup *cgrp)
 {
     struct cgroup_subsys_state *css;
     struct cgroup *dsct;
 
     lockdep_assert_held(&cgroup_mutex);
 
     cgroup_for_each_live_descendant_pre(dsct, css, cgrp)
         __cgroup_kill(dsct);
 }
 
 static ssize_t cgroup_kill_write(struct kernfs_open_file *of, char *buf,
                  size_t nbytes, loff_t off)
 {
     ssize_t ret = 0;
     int kill;
     struct cgroup *cgrp;
 
     ret = kstrtoint(strstrip(buf), 0, &kill);
     if (ret)
         return ret;
 
     if (kill != 1)
         return -ERANGE;
 
     cgrp = cgroup_kn_lock_live(of->kn, false);
     if (!cgrp)
         return -ENOENT;
 
     /*
      * Killing is a process directed operation, i.e. the whole thread-group
      * is taken down so act like we do for cgroup.procs and only make this
      * writable in non-threaded cgroups.
      */
     if (cgroup_is_threaded(cgrp))
         ret = -EOPNOTSUPP;
     else
         cgroup_kill(cgrp);
 
     cgroup_kn_unlock(of->kn);
 
     return ret ?: nbytes;
 }
 
 static int cgroup_file_open(struct kernfs_open_file *of)
 {
     struct cftype *cft = of_cft(of);
     struct cgroup_file_ctx *ctx;
     int ret;
 
     ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
     if (!ctx)
         return -ENOMEM;
 
     ctx->ns = current->nsproxy->cgroup_ns;
     get_cgroup_ns(ctx->ns);
     of->priv = ctx;
 
     if (!cft->open)
         return 0;
 
     ret = cft->open(of);
     if (ret) {
         put_cgroup_ns(ctx->ns);
         kfree(ctx);
     }
     return ret;
 }
 
 static void cgroup_file_release(struct kernfs_open_file *of)
 {
     struct cftype *cft = of_cft(of);
     struct cgroup_file_ctx *ctx = of->priv;
 
     if (cft->release)
         cft->release(of);
     put_cgroup_ns(ctx->ns);
     kfree(ctx);
 }
 
 static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
                  size_t nbytes, loff_t off)
 {
     struct cgroup_file_ctx *ctx = of->priv;
     struct cgroup *cgrp = of->kn->parent->priv;
     struct cftype *cft = of_cft(of);
     struct cgroup_subsys_state *css;
     int ret;
 
     if (!nbytes)
         return 0;
 
     /*
      * If namespaces are delegation boundaries, disallow writes to
      * files in an non-init namespace root from inside the namespace
      * except for the files explicitly marked delegatable -
      * cgroup.procs and cgroup.subtree_control.
      */
     if ((cgrp->root->flags & CGRP_ROOT_NS_DELEGATE) &&
         !(cft->flags & CFTYPE_NS_DELEGATABLE) &&
         ctx->ns != &init_cgroup_ns && ctx->ns->root_cset->dfl_cgrp == cgrp)
         return -EPERM;
 
     if (cft->write)
         return cft->write(of, buf, nbytes, off);
 
     /*
      * kernfs guarantees that a file isn't deleted with operations in
      * flight, which means that the matching css is and stays alive and
      * doesn't need to be pinned.  The RCU locking is not necessary
      * either.  It's just for the convenience of using cgroup_css().
      */
     rcu_read_lock();
     css = cgroup_css(cgrp, cft->ss);
     rcu_read_unlock();
 
     if (cft->write_u64) {
         unsigned long long v;
         ret = kstrtoull(buf, 0, &v);
         if (!ret)
             ret = cft->write_u64(css, cft, v);
     } else if (cft->write_s64) {
         long long v;
         ret = kstrtoll(buf, 0, &v);
         if (!ret)
             ret = cft->write_s64(css, cft, v);
     } else {
         ret = -EINVAL;
     }
 
     return ret ?: nbytes;
 }
 
 static __poll_t cgroup_file_poll(struct kernfs_open_file *of, poll_table *pt)
 {
     struct cftype *cft = of_cft(of);
 
     if (cft->poll)
         return cft->poll(of, pt);
 
     return kernfs_generic_poll(of, pt);
 }
 
 static void *cgroup_seqfile_start(struct seq_file *seq, loff_t *ppos)
 {
     return seq_cft(seq)->seq_start(seq, ppos);
 }
 
 static void *cgroup_seqfile_next(struct seq_file *seq, void *v, loff_t *ppos)
 {
     return seq_cft(seq)->seq_next(seq, v, ppos);
 }
 
 static void cgroup_seqfile_stop(struct seq_file *seq, void *v)
 {
     if (seq_cft(seq)->seq_stop)
         seq_cft(seq)->seq_stop(seq, v);
 }
 
 static int cgroup_seqfile_show(struct seq_file *m, void *arg)
 {
     struct cftype *cft = seq_cft(m);
     struct cgroup_subsys_state *css = seq_css(m);
 
     if (cft->seq_show)
         return cft->seq_show(m, arg);
 
     if (cft->read_u64)
         seq_printf(m, "%llu\n", cft->read_u64(css, cft));
     else if (cft->read_s64)
         seq_printf(m, "%lld\n", cft->read_s64(css, cft));
     else
         return -EINVAL;
     return 0;
 }
 
 static struct kernfs_ops cgroup_kf_single_ops = {
     .atomic_write_len   = PAGE_SIZE,
     .open           = cgroup_file_open,
     .release        = cgroup_file_release,
     .write          = cgroup_file_write,
     .poll           = cgroup_file_poll,
     .seq_show       = cgroup_seqfile_show,
 };
 
 static struct kernfs_ops cgroup_kf_ops = {
     .atomic_write_len   = PAGE_SIZE,
     .open           = cgroup_file_open,
     .release        = cgroup_file_release,
     .write          = cgroup_file_write,
     .poll           = cgroup_file_poll,
     .seq_start      = cgroup_seqfile_start,
     .seq_next       = cgroup_seqfile_next,
     .seq_stop       = cgroup_seqfile_stop,
     .seq_show       = cgroup_seqfile_show,
 };
 
 /* set uid and gid of cgroup dirs and files to that of the creator */
 static int cgroup_kn_set_ugid(struct kernfs_node *kn)
 {
     struct iattr iattr = { .ia_valid = ATTR_UID | ATTR_GID,
                    .ia_uid = current_fsuid(),
                    .ia_gid = current_fsgid(), };
 
     if (uid_eq(iattr.ia_uid, GLOBAL_ROOT_UID) &&
         gid_eq(iattr.ia_gid, GLOBAL_ROOT_GID))
         return 0;
 
     return kernfs_setattr(kn, &iattr);
 }
 
 static void cgroup_file_notify_timer(struct timer_list *timer)
 {
     cgroup_file_notify(container_of(timer, struct cgroup_file,
                     notify_timer));
 }
 
 static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
                struct cftype *cft)
 {
     char name[CGROUP_FILE_NAME_MAX];
     struct kernfs_node *kn;
     struct lock_class_key *key = NULL;
     int ret;
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
     key = &cft->lockdep_key;
 #endif
     kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
                   cgroup_file_mode(cft),
                   GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
                   0, cft->kf_ops, cft,
                   NULL, key);
     if (IS_ERR(kn))
         return PTR_ERR(kn);
 
     ret = cgroup_kn_set_ugid(kn);
     if (ret) {
         kernfs_remove(kn);
         return ret;
     }
 
     if (cft->file_offset) {
         struct cgroup_file *cfile = (void *)css + cft->file_offset;
 
         timer_setup(&cfile->notify_timer, cgroup_file_notify_timer, 0);
 
         spin_lock_irq(&cgroup_file_kn_lock);
         cfile->kn = kn;
         spin_unlock_irq(&cgroup_file_kn_lock);
     }
 
     return 0;
 }
 
 /**
  * cgroup_addrm_files - add or remove files to a cgroup directory
  * @css: the target css
  * @cgrp: the target cgroup (usually css->cgroup)
  * @cfts: array of cftypes to be added
  * @is_add: whether to add or remove
  *
  * Depending on @is_add, add or remove files defined by @cfts on @cgrp.
  * For removals, this function never fails.
  */
 static int cgroup_addrm_files(struct cgroup_subsys_state *css,
                   struct cgroup *cgrp, struct cftype cfts[],
                   bool is_add)
 {
     struct cftype *cft, *cft_end = NULL;
     int ret = 0;
 
     lockdep_assert_held(&cgroup_mutex);
 
 restart:
     for (cft = cfts; cft != cft_end && cft->name[0] != '\0'; cft++) {
         /* does cft->flags tell us to skip this file on @cgrp? */
         if ((cft->flags & CFTYPE_PRESSURE) && !cgroup_psi_enabled())
             continue;
         if ((cft->flags & __CFTYPE_ONLY_ON_DFL) && !cgroup_on_dfl(cgrp))
             continue;
         if ((cft->flags & __CFTYPE_NOT_ON_DFL) && cgroup_on_dfl(cgrp))
             continue;
         if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgroup_parent(cgrp))
             continue;
         if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgroup_parent(cgrp))
             continue;
         if ((cft->flags & CFTYPE_DEBUG) && !cgroup_debug)
             continue;
         if (is_add) {
             ret = cgroup_add_file(css, cgrp, cft);
             if (ret) {
                 pr_warn("%s: failed to add %s, err=%d\n",
                     __func__, cft->name, ret);
                 cft_end = cft;
                 is_add = false;
                 goto restart;
             }
         } else {
             cgroup_rm_file(cgrp, cft);
         }
     }
     return ret;
 }
 
 static int cgroup_apply_cftypes(struct cftype *cfts, bool is_add)
 {
     struct cgroup_subsys *ss = cfts[0].ss;
     struct cgroup *root = &ss->root->cgrp;
     struct cgroup_subsys_state *css;
     int ret = 0;
 
     lockdep_assert_held(&cgroup_mutex);
 
     /* add/rm files for all cgroups created before */
     css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
         struct cgroup *cgrp = css->cgroup;
 
         if (!(css->flags & CSS_VISIBLE))
             continue;
 
         ret = cgroup_addrm_files(css, cgrp, cfts, is_add);
         if (ret)
             break;
     }
 
     if (is_add && !ret)
         kernfs_activate(root->kn);
     return ret;
 }
 
 static void cgroup_exit_cftypes(struct cftype *cfts)
 {
     struct cftype *cft;
 
     for (cft = cfts; cft->name[0] != '\0'; cft++) {
         /* free copy for custom atomic_write_len, see init_cftypes() */
         if (cft->max_write_len && cft->max_write_len != PAGE_SIZE)
             kfree(cft->kf_ops);
         cft->kf_ops = NULL;
         cft->ss = NULL;
 
         /* revert flags set by cgroup core while adding @cfts */
         cft->flags &= ~(__CFTYPE_ONLY_ON_DFL | __CFTYPE_NOT_ON_DFL);
     }
 }
 
 static int cgroup_init_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 {
     struct cftype *cft;
 
     for (cft = cfts; cft->name[0] != '\0'; cft++) {
         struct kernfs_ops *kf_ops;
 
         WARN_ON(cft->ss || cft->kf_ops);
 
         if ((cft->flags & CFTYPE_PRESSURE) && !cgroup_psi_enabled())
             continue;
 
         if (cft->seq_start)
             kf_ops = &cgroup_kf_ops;
         else
             kf_ops = &cgroup_kf_single_ops;
 
         /*
          * Ugh... if @cft wants a custom max_write_len, we need to
          * make a copy of kf_ops to set its atomic_write_len.
          */
         if (cft->max_write_len && cft->max_write_len != PAGE_SIZE) {
             kf_ops = kmemdup(kf_ops, sizeof(*kf_ops), GFP_KERNEL);
             if (!kf_ops) {
                 cgroup_exit_cftypes(cfts);
                 return -ENOMEM;
             }
             kf_ops->atomic_write_len = cft->max_write_len;
         }
 
         cft->kf_ops = kf_ops;
         cft->ss = ss;
     }
 
     return 0;
 }
 
 static int cgroup_rm_cftypes_locked(struct cftype *cfts)
 {
     lockdep_assert_held(&cgroup_mutex);
 
     if (!cfts || !cfts[0].ss)
         return -ENOENT;
 
     list_del(&cfts->node);
     cgroup_apply_cftypes(cfts, false);
     cgroup_exit_cftypes(cfts);
     return 0;
 }
 
 /**
  * cgroup_rm_cftypes - remove an array of cftypes from a subsystem
  * @cfts: zero-length name terminated array of cftypes
  *
  * Unregister @cfts.  Files described by @cfts are removed from all
  * existing cgroups and all future cgroups won't have them either.  This
  * function can be called anytime whether @cfts' subsys is attached or not.
  *
  * Returns 0 on successful unregistration, -ENOENT if @cfts is not
  * registered.
  */
 int cgroup_rm_cftypes(struct cftype *cfts)
 {
     int ret;
 
     mutex_lock(&cgroup_mutex);
     ret = cgroup_rm_cftypes_locked(cfts);
     mutex_unlock(&cgroup_mutex);
     return ret;
 }
 
 /**
  * cgroup_add_cftypes - add an array of cftypes to a subsystem
  * @ss: target cgroup subsystem
  * @cfts: zero-length name terminated array of cftypes
  *
  * Register @cfts to @ss.  Files described by @cfts are created for all
  * existing cgroups to which @ss is attached and all future cgroups will
  * have them too.  This function can be called anytime whether @ss is
  * attached or not.
  *
  * Returns 0 on successful registration, -errno on failure.  Note that this
  * function currently returns 0 as long as @cfts registration is successful
  * even if some file creation attempts on existing cgroups fail.
  */
 static int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 {
     int ret;
 
     if (!cgroup_ssid_enabled(ss->id))
         return 0;
 
     if (!cfts || cfts[0].name[0] == '\0')
         return 0;
 
     ret = cgroup_init_cftypes(ss, cfts);
     if (ret)
         return ret;
 
     mutex_lock(&cgroup_mutex);
 
     list_add_tail(&cfts->node, &ss->cfts);
     ret = cgroup_apply_cftypes(cfts, true);
     if (ret)
         cgroup_rm_cftypes_locked(cfts);
 
     mutex_unlock(&cgroup_mutex);
     return ret;
 }
 
 /**
  * cgroup_add_dfl_cftypes - add an array of cftypes for default hierarchy
  * @ss: target cgroup subsystem
  * @cfts: zero-length name terminated array of cftypes
  *
  * Similar to cgroup_add_cftypes() but the added files are only used for
  * the default hierarchy.
  */
 int cgroup_add_dfl_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 {
     struct cftype *cft;
 
     for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
         cft->flags |= __CFTYPE_ONLY_ON_DFL;
     return cgroup_add_cftypes(ss, cfts);
 }
 
 /**
  * cgroup_add_legacy_cftypes - add an array of cftypes for legacy hierarchies
  * @ss: target cgroup subsystem
  * @cfts: zero-length name terminated array of cftypes
  *
  * Similar to cgroup_add_cftypes() but the added files are only used for
  * the legacy hierarchies.
  */
 int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 {
     struct cftype *cft;
 
     for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
         cft->flags |= __CFTYPE_NOT_ON_DFL;
     return cgroup_add_cftypes(ss, cfts);
 }
 
 /**
  * cgroup_file_notify - generate a file modified event for a cgroup_file
  * @cfile: target cgroup_file
  *
  * @cfile must have been obtained by setting cftype->file_offset.
  */
 void cgroup_file_notify(struct cgroup_file *cfile)
 {
     unsigned long flags;
 
     spin_lock_irqsave(&cgroup_file_kn_lock, flags);
     if (cfile->kn) {
         unsigned long last = cfile->notified_at;
         unsigned long next = last + CGROUP_FILE_NOTIFY_MIN_INTV;
 
         if (time_in_range(jiffies, last, next)) {
             timer_reduce(&cfile->notify_timer, next);
         } else {
             kernfs_notify(cfile->kn);
             cfile->notified_at = jiffies;
         }
     }
     spin_unlock_irqrestore(&cgroup_file_kn_lock, flags);
 }
 
 /**
  * css_next_child - find the next child of a given css
  * @pos: the current position (%NULL to initiate traversal)
  * @parent: css whose children to walk
  *
  * This function returns the next child of @parent and should be called
  * under either cgroup_mutex or RCU read lock.  The only requirement is
  * that @parent and @pos are accessible.  The next sibling is guaranteed to
  * be returned regardless of their states.
  *
  * If a subsystem synchronizes ->css_online() and the start of iteration, a
  * css which finished ->css_online() is guaranteed to be visible in the
  * future iterations and will stay visible until the last reference is put.
  * A css which hasn't finished ->css_online() or already finished
  * ->css_offline() may show up during traversal.  It's each subsystem's
  * responsibility to synchronize against on/offlining.
  */
 struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos,
                        struct cgroup_subsys_state *parent)
 {
     struct cgroup_subsys_state *next;
 
     cgroup_assert_mutex_or_rcu_locked();
 
     /*
      * @pos could already have been unlinked from the sibling list.
      * Once a cgroup is removed, its ->sibling.next is no longer
      * updated when its next sibling changes.  CSS_RELEASED is set when
      * @pos is taken off list, at which time its next pointer is valid,
      * and, as releases are serialized, the one pointed to by the next
      * pointer is guaranteed to not have started release yet.  This
      * implies that if we observe !CSS_RELEASED on @pos in this RCU
      * critical section, the one pointed to by its next pointer is
      * guaranteed to not have finished its RCU grace period even if we
      * have dropped rcu_read_lock() in-between iterations.
      *
      * If @pos has CSS_RELEASED set, its next pointer can't be
      * dereferenced; however, as each css is given a monotonically
      * increasing unique serial number and always appended to the
      * sibling list, the next one can be found by walking the parent's
      * children until the first css with higher serial number than
      * @pos's.  While this path can be slower, it happens iff iteration
      * races against release and the race window is very small.
      */
     if (!pos) {
         next = list_entry_rcu(parent->children.next, struct cgroup_subsys_state, sibling);
     } else if (likely(!(pos->flags & CSS_RELEASED))) {
         next = list_entry_rcu(pos->sibling.next, struct cgroup_subsys_state, sibling);
     } else {
         list_for_each_entry_rcu(next, &parent->children, sibling,
                     lockdep_is_held(&cgroup_mutex))
             if (next->serial_nr > pos->serial_nr)
                 break;
     }
 
     /*
      * @next, if not pointing to the head, can be dereferenced and is
      * the next sibling.
      */
     if (&next->sibling != &parent->children)
         return next;
     return NULL;
 }
 
 /**
  * css_next_descendant_pre - find the next descendant for pre-order walk
  * @pos: the current position (%NULL to initiate traversal)
  * @root: css whose descendants to walk
  *
  * To be used by css_for_each_descendant_pre().  Find the next descendant
  * to visit for pre-order traversal of @root's descendants.  @root is
  * included in the iteration and the first node to be visited.
  *
  * While this function requires cgroup_mutex or RCU read locking, it
  * doesn't require the whole traversal to be contained in a single critical
  * section.  This function will return the correct next descendant as long
  * as both @pos and @root are accessible and @pos is a descendant of @root.
  *
  * If a subsystem synchronizes ->css_online() and the start of iteration, a
  * css which finished ->css_online() is guaranteed to be visible in the
  * future iterations and will stay visible until the last reference is put.
  * A css which hasn't finished ->css_online() or already finished
  * ->css_offline() may show up during traversal.  It's each subsystem's
  * responsibility to synchronize against on/offlining.
  */
 struct cgroup_subsys_state *
 css_next_descendant_pre(struct cgroup_subsys_state *pos,
             struct cgroup_subsys_state *root)
 {
     struct cgroup_subsys_state *next;
 
     cgroup_assert_mutex_or_rcu_locked();
 
     /* if first iteration, visit @root */
     if (!pos)
         return root;
 
     /* visit the first child if exists */
     next = css_next_child(NULL, pos);
     if (next)
         return next;
 
     /* no child, visit my or the closest ancestor's next sibling */
     while (pos != root) {
         next = css_next_child(pos, pos->parent);
         if (next)
             return next;
         pos = pos->parent;
     }
 
     return NULL;
 }
 EXPORT_SYMBOL_GPL(css_next_descendant_pre);
 
 /**
  * css_rightmost_descendant - return the rightmost descendant of a css
  * @pos: css of interest
  *
  * Return the rightmost descendant of @pos.  If there's no descendant, @pos
  * is returned.  This can be used during pre-order traversal to skip
  * subtree of @pos.
  *
  * While this function requires cgroup_mutex or RCU read locking, it
  * doesn't require the whole traversal to be contained in a single critical
  * section.  This function will return the correct rightmost descendant as
  * long as @pos is accessible.
  */
 struct cgroup_subsys_state *
 css_rightmost_descendant(struct cgroup_subsys_state *pos)
 {
     struct cgroup_subsys_state *last, *tmp;
 
     cgroup_assert_mutex_or_rcu_locked();
 
     do {
         last = pos;
         /* ->prev isn't RCU safe, walk ->next till the end */
         pos = NULL;
         css_for_each_child(tmp, last)
             pos = tmp;
     } while (pos);
 
     return last;
 }
 
 static struct cgroup_subsys_state *
 css_leftmost_descendant(struct cgroup_subsys_state *pos)
 {
     struct cgroup_subsys_state *last;
 
     do {
         last = pos;
         pos = css_next_child(NULL, pos);
     } while (pos);
 
     return last;
 }
 
 /**
  * css_next_descendant_post - find the next descendant for post-order walk
  * @pos: the current position (%NULL to initiate traversal)
  * @root: css whose descendants to walk
  *
  * To be used by css_for_each_descendant_post().  Find the next descendant
  * to visit for post-order traversal of @root's descendants.  @root is
  * included in the iteration and the last node to be visited.
  *
  * While this function requires cgroup_mutex or RCU read locking, it
  * doesn't require the whole traversal to be contained in a single critical
  * section.  This function will return the correct next descendant as long
  * as both @pos and @cgroup are accessible and @pos is a descendant of
  * @cgroup.
  *
  * If a subsystem synchronizes ->css_online() and the start of iteration, a
  * css which finished ->css_online() is guaranteed to be visible in the
  * future iterations and will stay visible until the last reference is put.
  * A css which hasn't finished ->css_online() or already finished
  * ->css_offline() may show up during traversal.  It's each subsystem's
  * responsibility to synchronize against on/offlining.
  */
 struct cgroup_subsys_state *
 css_next_descendant_post(struct cgroup_subsys_state *pos,
              struct cgroup_subsys_state *root)
 {
     struct cgroup_subsys_state *next;
 
     cgroup_assert_mutex_or_rcu_locked();
 
     /* if first iteration, visit leftmost descendant which may be @root */
     if (!pos)
         return css_leftmost_descendant(root);
 
     /* if we visited @root, we're done */
     if (pos == root)
         return NULL;
 
     /* if there's an unvisited sibling, visit its leftmost descendant */
     next = css_next_child(pos, pos->parent);
     if (next)
         return css_leftmost_descendant(next);
 
     /* no sibling left, visit parent */
     return pos->parent;
 }
 
 /**
  * css_has_online_children - does a css have online children
  * @css: the target css
  *
  * Returns %true if @css has any online children; otherwise, %false.  This
  * function can be called from any context but the caller is responsible
  * for synchronizing against on/offlining as necessary.
  */
 bool css_has_online_children(struct cgroup_subsys_state *css)
 {
     struct cgroup_subsys_state *child;
     bool ret = false;
 
     rcu_read_lock();
     css_for_each_child(child, css) {
         if (child->flags & CSS_ONLINE) {
             ret = true;
             break;
         }
     }
     rcu_read_unlock();
     return ret;
 }
 
 static struct css_set *css_task_iter_next_css_set(struct css_task_iter *it)
 {
     struct list_head *l;
     struct cgrp_cset_link *link;
     struct css_set *cset;
 
     lockdep_assert_held(&css_set_lock);
 
     /* find the next threaded cset */
     if (it->tcset_pos) {
         l = it->tcset_pos->next;
 
         if (l != it->tcset_head) {
             it->tcset_pos = l;
             return container_of(l, struct css_set,
                         threaded_csets_node);
         }
 
         it->tcset_pos = NULL;
     }
 
     /* find the next cset */
     l = it->cset_pos;
     l = l->next;
     if (l == it->cset_head) {
         it->cset_pos = NULL;
         return NULL;
     }
 
     if (it->ss) {
         cset = container_of(l, struct css_set, e_cset_node[it->ss->id]);
     } else {
         link = list_entry(l, struct cgrp_cset_link, cset_link);
         cset = link->cset;
     }
 
     it->cset_pos = l;
 
     /* initialize threaded css_set walking */
     if (it->flags & CSS_TASK_ITER_THREADED) {
         if (it->cur_dcset)
             put_css_set_locked(it->cur_dcset);
         it->cur_dcset = cset;
         get_css_set(cset);
 
         it->tcset_head = &cset->threaded_csets;
         it->tcset_pos = &cset->threaded_csets;
     }
 
     return cset;
 }
 
 /**
  * css_task_iter_advance_css_set - advance a task iterator to the next css_set
  * @it: the iterator to advance
  *
  * Advance @it to the next css_set to walk.
  */
 static void css_task_iter_advance_css_set(struct css_task_iter *it)
 {
     struct css_set *cset;
 
     lockdep_assert_held(&css_set_lock);
 
     /* Advance to the next non-empty css_set and find first non-empty tasks list*/
     while ((cset = css_task_iter_next_css_set(it))) {
         if (!list_empty(&cset->tasks)) {
             it->cur_tasks_head = &cset->tasks;
             break;
         } else if (!list_empty(&cset->mg_tasks)) {
             it->cur_tasks_head = &cset->mg_tasks;
             break;
         } else if (!list_empty(&cset->dying_tasks)) {
             it->cur_tasks_head = &cset->dying_tasks;
             break;
         }
     }
     if (!cset) {
         it->task_pos = NULL;
         return;
     }
     it->task_pos = it->cur_tasks_head->next;
 
     /*
      * We don't keep css_sets locked across iteration steps and thus
      * need to take steps to ensure that iteration can be resumed after
      * the lock is re-acquired.  Iteration is performed at two levels -
      * css_sets and tasks in them.
      *
      * Once created, a css_set never leaves its cgroup lists, so a
      * pinned css_set is guaranteed to stay put and we can resume
      * iteration afterwards.
      *
      * Tasks may leave @cset across iteration steps.  This is resolved
      * by registering each iterator with the css_set currently being
      * walked and making css_set_move_task() advance iterators whose
      * next task is leaving.
      */
     if (it->cur_cset) {
         list_del(&it->iters_node);
         put_css_set_locked(it->cur_cset);
     }
     get_css_set(cset);
     it->cur_cset = cset;
     list_add(&it->iters_node, &cset->task_iters);
 }
 
 static void css_task_iter_skip(struct css_task_iter *it,
                    struct task_struct *task)
 {
     lockdep_assert_held(&css_set_lock);
 
     if (it->task_pos == &task->cg_list) {
         it->task_pos = it->task_pos->next;
         it->flags |= CSS_TASK_ITER_SKIPPED;
     }
 }
 
 static void css_task_iter_advance(struct css_task_iter *it)
 {
     struct task_struct *task;
 
     lockdep_assert_held(&css_set_lock);
 repeat:
     if (it->task_pos) {
         /*
          * Advance iterator to find next entry. We go through cset
          * tasks, mg_tasks and dying_tasks, when consumed we move onto
          * the next cset.
          */
         if (it->flags & CSS_TASK_ITER_SKIPPED)
             it->flags &= ~CSS_TASK_ITER_SKIPPED;
         else
             it->task_pos = it->task_pos->next;
 
         if (it->task_pos == &it->cur_cset->tasks) {
             it->cur_tasks_head = &it->cur_cset->mg_tasks;
             it->task_pos = it->cur_tasks_head->next;
         }
         if (it->task_pos == &it->cur_cset->mg_tasks) {
             it->cur_tasks_head = &it->cur_cset->dying_tasks;
             it->task_pos = it->cur_tasks_head->next;
         }
         if (it->task_pos == &it->cur_cset->dying_tasks)
             css_task_iter_advance_css_set(it);
     } else {
         /* called from start, proceed to the first cset */
         css_task_iter_advance_css_set(it);
     }
 
     if (!it->task_pos)
         return;
 
     task = list_entry(it->task_pos, struct task_struct, cg_list);
 
     if (it->flags & CSS_TASK_ITER_PROCS) {
         /* if PROCS, skip over tasks which aren't group leaders */
         if (!thread_group_leader(task))
             goto repeat;
 
         /* and dying leaders w/o live member threads */
         if (it->cur_tasks_head == &it->cur_cset->dying_tasks &&
             !atomic_read(&task->signal->live))
             goto repeat;
     } else {
         /* skip all dying ones */
         if (it->cur_tasks_head == &it->cur_cset->dying_tasks)
             goto repeat;
     }
 }
 
 /**
  * css_task_iter_start - initiate task iteration
  * @css: the css to walk tasks of
  * @flags: CSS_TASK_ITER_* flags
  * @it: the task iterator to use
  *
  * Initiate iteration through the tasks of @css.  The caller can call
  * css_task_iter_next() to walk through the tasks until the function
  * returns NULL.  On completion of iteration, css_task_iter_end() must be
  * called.
  */
 void css_task_iter_start(struct cgroup_subsys_state *css, unsigned int flags,
              struct css_task_iter *it)
 {
     memset(it, 0, sizeof(*it));
 
     spin_lock_irq(&css_set_lock);
 
     it->ss = css->ss;
     it->flags = flags;
 
     if (CGROUP_HAS_SUBSYS_CONFIG && it->ss)
         it->cset_pos = &css->cgroup->e_csets[css->ss->id];
     else
         it->cset_pos = &css->cgroup->cset_links;
 
     it->cset_head = it->cset_pos;
 
     css_task_iter_advance(it);
 
     spin_unlock_irq(&css_set_lock);
 }
 
 /**
  * css_task_iter_next - return the next task for the iterator
  * @it: the task iterator being iterated
  *
  * The "next" function for task iteration.  @it should have been
  * initialized via css_task_iter_start().  Returns NULL when the iteration
  * reaches the end.
  */
 struct task_struct *css_task_iter_next(struct css_task_iter *it)
 {
     if (it->cur_task) {
         put_task_struct(it->cur_task);
         it->cur_task = NULL;
     }
 
     spin_lock_irq(&css_set_lock);
 
     /* @it may be half-advanced by skips, finish advancing */
     if (it->flags & CSS_TASK_ITER_SKIPPED)
         css_task_iter_advance(it);
 
     if (it->task_pos) {
         it->cur_task = list_entry(it->task_pos, struct task_struct,
                       cg_list);
         get_task_struct(it->cur_task);
         css_task_iter_advance(it);
     }
 
     spin_unlock_irq(&css_set_lock);
 
     return it->cur_task;
 }
 
 /**
  * css_task_iter_end - finish task iteration
  * @it: the task iterator to finish
  *
  * Finish task iteration started by css_task_iter_start().
  */
 void css_task_iter_end(struct css_task_iter *it)
 {
     if (it->cur_cset) {
         spin_lock_irq(&css_set_lock);
         list_del(&it->iters_node);
         put_css_set_locked(it->cur_cset);
         spin_unlock_irq(&css_set_lock);
     }
 
     if (it->cur_dcset)
         put_css_set(it->cur_dcset);
 
     if (it->cur_task)
         put_task_struct(it->cur_task);
 }
 
 static void cgroup_procs_release(struct kernfs_open_file *of)
 {
     struct cgroup_file_ctx *ctx = of->priv;
 
     if (ctx->procs.started)
         css_task_iter_end(&ctx->procs.iter);
 }
 
 static void *cgroup_procs_next(struct seq_file *s, void *v, loff_t *pos)
 {
     struct kernfs_open_file *of = s->private;
     struct cgroup_file_ctx *ctx = of->priv;
 
     if (pos)
         (*pos)++;
 
     return css_task_iter_next(&ctx->procs.iter);
 }
 
 static void *__cgroup_procs_start(struct seq_file *s, loff_t *pos,
                   unsigned int iter_flags)
 {
     struct kernfs_open_file *of = s->private;
     struct cgroup *cgrp = seq_css(s)->cgroup;
     struct cgroup_file_ctx *ctx = of->priv;
     struct css_task_iter *it = &ctx->procs.iter;
 
     /*
      * When a seq_file is seeked, it's always traversed sequentially
      * from position 0, so we can simply keep iterating on !0 *pos.
      */
     if (!ctx->procs.started) {
         if (WARN_ON_ONCE((*pos)))
             return ERR_PTR(-EINVAL);
         css_task_iter_start(&cgrp->self, iter_flags, it);
         ctx->procs.started = true;
     } else if (!(*pos)) {
         css_task_iter_end(it);
         css_task_iter_start(&cgrp->self, iter_flags, it);
     } else
         return it->cur_task;
 
     return cgroup_procs_next(s, NULL, NULL);
 }
 
 static void *cgroup_procs_start(struct seq_file *s, loff_t *pos)
 {
     struct cgroup *cgrp = seq_css(s)->cgroup;
 
     /*
      * All processes of a threaded subtree belong to the domain cgroup
      * of the subtree.  Only threads can be distributed across the
      * subtree.  Reject reads on cgroup.procs in the subtree proper.
      * They're always empty anyway.
      */
     if (cgroup_is_threaded(cgrp))
         return ERR_PTR(-EOPNOTSUPP);
 
     return __cgroup_procs_start(s, pos, CSS_TASK_ITER_PROCS |
                         CSS_TASK_ITER_THREADED);
 }
 
 static int cgroup_procs_show(struct seq_file *s, void *v)
 {
     seq_printf(s, "%d\n", task_pid_vnr(v));
     return 0;
 }
 
 static int cgroup_may_write(const struct cgroup *cgrp, struct super_block *sb)
 {
     int ret;
     struct inode *inode;
 
     lockdep_assert_held(&cgroup_mutex);
 
     inode = kernfs_get_inode(sb, cgrp->procs_file.kn);
     if (!inode)
         return -ENOMEM;
 
     ret = inode_permission(&init_user_ns, inode, MAY_WRITE);
     iput(inode);
     return ret;
 }
 
 static int cgroup_procs_write_permission(struct cgroup *src_cgrp,
                      struct cgroup *dst_cgrp,
                      struct super_block *sb,
                      struct cgroup_namespace *ns)
 {
     struct cgroup *com_cgrp = src_cgrp;
     int ret;
 
     lockdep_assert_held(&cgroup_mutex);
 
     /* find the common ancestor */
     while (!cgroup_is_descendant(dst_cgrp, com_cgrp))
         com_cgrp = cgroup_parent(com_cgrp);
 
     /* %current should be authorized to migrate to the common ancestor */
     ret = cgroup_may_write(com_cgrp, sb);
     if (ret)
         return ret;
 
     /*
      * If namespaces are delegation boundaries, %current must be able
      * to see both source and destination cgroups from its namespace.
      */
     if ((cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE) &&
         (!cgroup_is_descendant(src_cgrp, ns->root_cset->dfl_cgrp) ||
          !cgroup_is_descendant(dst_cgrp, ns->root_cset->dfl_cgrp)))
         return -ENOENT;
 
     return 0;
 }
 
 static int cgroup_attach_permissions(struct cgroup *src_cgrp,
                      struct cgroup *dst_cgrp,
                      struct super_block *sb, bool threadgroup,
                      struct cgroup_namespace *ns)
 {
     int ret = 0;
 
     ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb, ns);
     if (ret)
         return ret;
 
     ret = cgroup_migrate_vet_dst(dst_cgrp);
     if (ret)
         return ret;
 
     if (!threadgroup && (src_cgrp->dom_cgrp != dst_cgrp->dom_cgrp))
         ret = -EOPNOTSUPP;
 
     return ret;
 }
 
 static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
                     bool threadgroup)
 {
     struct cgroup_file_ctx *ctx = of->priv;
     struct cgroup *src_cgrp, *dst_cgrp;
     struct task_struct *task;
     const struct cred *saved_cred;
     ssize_t ret;
     bool threadgroup_locked;
 
     dst_cgrp = cgroup_kn_lock_live(of->kn, false);
     if (!dst_cgrp)
         return -ENODEV;
 
     task = cgroup_procs_write_start(buf, threadgroup, &threadgroup_locked);
     ret = PTR_ERR_OR_ZERO(task);
     if (ret)
         goto out_unlock;
 
     /* find the source cgroup */
     spin_lock_irq(&css_set_lock);
     src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
     spin_unlock_irq(&css_set_lock);
 
     /*
      * Process and thread migrations follow same delegation rule. Check
      * permissions using the credentials from file open to protect against
      * inherited fd attacks.
      */
     saved_cred = override_creds(of->file->f_cred);
     ret = cgroup_attach_permissions(src_cgrp, dst_cgrp,
                     of->file->f_path.dentry->d_sb,
                     threadgroup, ctx->ns);
     revert_creds(saved_cred);
     if (ret)
         goto out_finish;
 
     ret = cgroup_attach_task(dst_cgrp, task, threadgroup);
 
 out_finish:
     cgroup_procs_write_finish(task, threadgroup_locked);
 out_unlock:
     cgroup_kn_unlock(of->kn);
 
     return ret;
 }
 
 static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
                   char *buf, size_t nbytes, loff_t off)
 {
     return __cgroup_procs_write(of, buf, true) ?: nbytes;
 }
 
 static void *cgroup_threads_start(struct seq_file *s, loff_t *pos)
 {
     return __cgroup_procs_start(s, pos, 0);
 }
 
 static ssize_t cgroup_threads_write(struct kernfs_open_file *of,
                     char *buf, size_t nbytes, loff_t off)
 {
     return __cgroup_procs_write(of, buf, false) ?: nbytes;
 }
 
 /* cgroup core interface files for the default hierarchy */
 static struct cftype cgroup_base_files[] = {
     {
         .name = "cgroup.type",
         .flags = CFTYPE_NOT_ON_ROOT,
         .seq_show = cgroup_type_show,
         .write = cgroup_type_write,
     },
     {
         .name = "cgroup.procs",
         .flags = CFTYPE_NS_DELEGATABLE,
         .file_offset = offsetof(struct cgroup, procs_file),
         .release = cgroup_procs_release,
         .seq_start = cgroup_procs_start,
         .seq_next = cgroup_procs_next,
         .seq_show = cgroup_procs_show,
         .write = cgroup_procs_write,
     },
     {
         .name = "cgroup.threads",
         .flags = CFTYPE_NS_DELEGATABLE,
         .release = cgroup_procs_release,
         .seq_start = cgroup_threads_start,
         .seq_next = cgroup_procs_next,
         .seq_show = cgroup_procs_show,
         .write = cgroup_threads_write,
     },
     {
         .name = "cgroup.controllers",
         .seq_show = cgroup_controllers_show,
     },
     {
         .name = "cgroup.subtree_control",
         .flags = CFTYPE_NS_DELEGATABLE,
         .seq_show = cgroup_subtree_control_show,
         .write = cgroup_subtree_control_write,
     },
     {
         .name = "cgroup.events",
         .flags = CFTYPE_NOT_ON_ROOT,
         .file_offset = offsetof(struct cgroup, events_file),
         .seq_show = cgroup_events_show,
     },
     {
         .name = "cgroup.max.descendants",
         .seq_show = cgroup_max_descendants_show,
         .write = cgroup_max_descendants_write,
     },
     {
         .name = "cgroup.max.depth",
         .seq_show = cgroup_max_depth_show,
         .write = cgroup_max_depth_write,
     },
     {
         .name = "cgroup.stat",
         .seq_show = cgroup_stat_show,
     },
     {
         .name = "cgroup.freeze",
         .flags = CFTYPE_NOT_ON_ROOT,
         .seq_show = cgroup_freeze_show,
         .write = cgroup_freeze_write,
     },
     {
         .name = "cgroup.kill",
         .flags = CFTYPE_NOT_ON_ROOT,
         .write = cgroup_kill_write,
     },
     {
         .name = "cpu.stat",
         .seq_show = cpu_stat_show,
     },
 #ifdef CONFIG_PSI
     {
         .name = "io.pressure",
         .flags = CFTYPE_PRESSURE,
         .seq_show = cgroup_io_pressure_show,
         .write = cgroup_io_pressure_write,
         .poll = cgroup_pressure_poll,
         .release = cgroup_pressure_release,
     },
     {
         .name = "memory.pressure",
         .flags = CFTYPE_PRESSURE,
         .seq_show = cgroup_memory_pressure_show,
         .write = cgroup_memory_pressure_write,
         .poll = cgroup_pressure_poll,
         .release = cgroup_pressure_release,
     },
     {
         .name = "cpu.pressure",
         .flags = CFTYPE_PRESSURE,
         .seq_show = cgroup_cpu_pressure_show,
         .write = cgroup_cpu_pressure_write,
         .poll = cgroup_pressure_poll,
         .release = cgroup_pressure_release,
     },
 #endif /* CONFIG_PSI */
     { } /* terminate */
 };
 
 /*
  * css destruction is four-stage process.
  *
  * 1. Destruction starts.  Killing of the percpu_ref is initiated.
  *    Implemented in kill_css().
  *
  * 2. When the percpu_ref is confirmed to be visible as killed on all CPUs
  *    and thus css_tryget_online() is guaranteed to fail, the css can be
  *    offlined by invoking offline_css().  After offlining, the base ref is
  *    put.  Implemented in css_killed_work_fn().
  *
  * 3. When the percpu_ref reaches zero, the only possible remaining
  *    accessors are inside RCU read sections.  css_release() schedules the
  *    RCU callback.
  *
  * 4. After the grace period, the css can be freed.  Implemented in
  *    css_free_work_fn().
  *
  * It is actually hairier because both step 2 and 4 require process context
  * and thus involve punting to css->destroy_work adding two additional
  * steps to the already complex sequence.
  */
 static void css_free_rwork_fn(struct work_struct *work)
 {
     struct cgroup_subsys_state *css = container_of(to_rcu_work(work),
                 struct cgroup_subsys_state, destroy_rwork);
     struct cgroup_subsys *ss = css->ss;
     struct cgroup *cgrp = css->cgroup;
 
     percpu_ref_exit(&css->refcnt);
 
     if (ss) {
         /* css free path */
         struct cgroup_subsys_state *parent = css->parent;
         int id = css->id;
 
         ss->css_free(css);
         cgroup_idr_remove(&ss->css_idr, id);
         cgroup_put(cgrp);
 
         if (parent)
             css_put(parent);
     } else {
         /* cgroup free path */
         atomic_dec(&cgrp->root->nr_cgrps);
         cgroup1_pidlist_destroy_all(cgrp);
         cancel_work_sync(&cgrp->release_agent_work);
 
         if (cgroup_parent(cgrp)) {
             /*
              * We get a ref to the parent, and put the ref when
              * this cgroup is being freed, so it's guaranteed
              * that the parent won't be destroyed before its
              * children.
              */
             cgroup_put(cgroup_parent(cgrp));
             kernfs_put(cgrp->kn);
             psi_cgroup_free(cgrp);
             cgroup_rstat_exit(cgrp);
             kfree(cgrp);
         } else {
             /*
              * This is root cgroup's refcnt reaching zero,
              * which indicates that the root should be
              * released.
              */
             cgroup_destroy_root(cgrp->root);
         }
     }
 }
 
 static void css_release_work_fn(struct work_struct *work)
 {
     struct cgroup_subsys_state *css =
         container_of(work, struct cgroup_subsys_state, destroy_work);
     struct cgroup_subsys *ss = css->ss;
     struct cgroup *cgrp = css->cgroup;
 
     mutex_lock(&cgroup_mutex);
 
     css->flags |= CSS_RELEASED;
     list_del_rcu(&css->sibling);
 
     if (ss) {
         /* css release path */
         if (!list_empty(&css->rstat_css_node)) {
             cgroup_rstat_flush(cgrp);
             list_del_rcu(&css->rstat_css_node);
         }
 
         cgroup_idr_replace(&ss->css_idr, NULL, css->id);
         if (ss->css_released)
             ss->css_released(css);
     } else {
         struct cgroup *tcgrp;
 
         /* cgroup release path */
         TRACE_CGROUP_PATH(release, cgrp);
 
         cgroup_rstat_flush(cgrp);
 
         spin_lock_irq(&css_set_lock);
         for (tcgrp = cgroup_parent(cgrp); tcgrp;
              tcgrp = cgroup_parent(tcgrp))
             tcgrp->nr_dying_descendants--;
         spin_unlock_irq(&css_set_lock);
 
         /*
          * There are two control paths which try to determine
          * cgroup from dentry without going through kernfs -
          * cgroupstats_build() and css_tryget_online_from_dir().
          * Those are supported by RCU protecting clearing of
          * cgrp->kn->priv backpointer.
          */
         if (cgrp->kn)
             RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv,
                      NULL);
     }
 
     mutex_unlock(&cgroup_mutex);
 
     INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
     queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
 }
 
 static void css_release(struct percpu_ref *ref)
 {
     struct cgroup_subsys_state *css =
         container_of(ref, struct cgroup_subsys_state, refcnt);
 
     INIT_WORK(&css->destroy_work, css_release_work_fn);
     queue_work(cgroup_destroy_wq, &css->destroy_work);
 }
 
 static void init_and_link_css(struct cgroup_subsys_state *css,
                   struct cgroup_subsys *ss, struct cgroup *cgrp)
 {
     lockdep_assert_held(&cgroup_mutex);
 
     cgroup_get_live(cgrp);
 
     memset(css, 0, sizeof(*css));
     css->cgroup = cgrp;
     css->ss = ss;
     css->id = -1;
     INIT_LIST_HEAD(&css->sibling);
     INIT_LIST_HEAD(&css->children);
     INIT_LIST_HEAD(&css->rstat_css_node);
     css->serial_nr = css_serial_nr_next++;
     atomic_set(&css->online_cnt, 0);
 
     if (cgroup_parent(cgrp)) {
         css->parent = cgroup_css(cgroup_parent(cgrp), ss);
         css_get(css->parent);
     }
 
     if (ss->css_rstat_flush)
         list_add_rcu(&css->rstat_css_node, &cgrp->rstat_css_list);
 
     BUG_ON(cgroup_css(cgrp, ss));
 }
 
 /* invoke ->css_online() on a new CSS and mark it online if successful */
 static int online_css(struct cgroup_subsys_state *css)
 {
     struct cgroup_subsys *ss = css->ss;
     int ret = 0;
 
     lockdep_assert_held(&cgroup_mutex);
 
     if (ss->css_online)
         ret = ss->css_online(css);
     if (!ret) {
         css->flags |= CSS_ONLINE;
         rcu_assign_pointer(css->cgroup->subsys[ss->id], css);
 
         atomic_inc(&css->online_cnt);
         if (css->parent)
             atomic_inc(&css->parent->online_cnt);
     }
     return ret;
 }
 
 /* if the CSS is online, invoke ->css_offline() on it and mark it offline */
 static void offline_css(struct cgroup_subsys_state *css)
 {
     struct cgroup_subsys *ss = css->ss;
 
     lockdep_assert_held(&cgroup_mutex);
 
     if (!(css->flags & CSS_ONLINE))
         return;
 
     if (ss->css_offline)
         ss->css_offline(css);
 
     css->flags &= ~CSS_ONLINE;
     RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL);
 
     wake_up_all(&css->cgroup->offline_waitq);
 }
 
 /**
  * css_create - create a cgroup_subsys_state
  * @cgrp: the cgroup new css will be associated with
  * @ss: the subsys of new css
  *
  * Create a new css associated with @cgrp - @ss pair.  On success, the new
  * css is online and installed in @cgrp.  This function doesn't create the
  * interface files.  Returns 0 on success, -errno on failure.
  */
 static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
                           struct cgroup_subsys *ss)
 {
     struct cgroup *parent = cgroup_parent(cgrp);
     struct cgroup_subsys_state *parent_css = cgroup_css(parent, ss);
     struct cgroup_subsys_state *css;
     int err;
 
     lockdep_assert_held(&cgroup_mutex);
 
     css = ss->css_alloc(parent_css);
     if (!css)
         css = ERR_PTR(-ENOMEM);
     if (IS_ERR(css))
         return css;
 
     init_and_link_css(css, ss, cgrp);
 
     err = percpu_ref_init(&css->refcnt, css_release, 0, GFP_KERNEL);
     if (err)
         goto err_free_css;
 
     err = cgroup_idr_alloc(&ss->css_idr, NULL, 2, 0, GFP_KERNEL);
     if (err < 0)
         goto err_free_css;
     css->id = err;
 
     /* @css is ready to be brought online now, make it visible */
     list_add_tail_rcu(&css->sibling, &parent_css->children);
     cgroup_idr_replace(&ss->css_idr, css, css->id);
 
     err = online_css(css);
     if (err)
         goto err_list_del;
 
     return css;
 
 err_list_del:
     list_del_rcu(&css->sibling);
 err_free_css:
     list_del_rcu(&css->rstat_css_node);
     INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
     queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
     return ERR_PTR(err);
 }
 
 /*
  * The returned cgroup is fully initialized including its control mask, but
  * it isn't associated with its kernfs_node and doesn't have the control
  * mask applied.
  */
 static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
                     umode_t mode)
 {
     struct cgroup_root *root = parent->root;
     struct cgroup *cgrp, *tcgrp;
     struct kernfs_node *kn;
     int level = parent->level + 1;
     int ret;
 
     /* allocate the cgroup and its ID, 0 is reserved for the root */
     cgrp = kzalloc(struct_size(cgrp, ancestor_ids, (level + 1)),
                GFP_KERNEL);
     if (!cgrp)
         return ERR_PTR(-ENOMEM);
 
     ret = percpu_ref_init(&cgrp->self.refcnt, css_release, 0, GFP_KERNEL);
     if (ret)
         goto out_free_cgrp;
 
     ret = cgroup_rstat_init(cgrp);
     if (ret)
         goto out_cancel_ref;
 
     /* create the directory */
     kn = kernfs_create_dir(parent->kn, name, mode, cgrp);
     if (IS_ERR(kn)) {
         ret = PTR_ERR(kn);
         goto out_stat_exit;
     }
     cgrp->kn = kn;
 
     init_cgroup_housekeeping(cgrp);
 
     cgrp->self.parent = &parent->self;
     cgrp->root = root;
     cgrp->level = level;
 
     ret = psi_cgroup_alloc(cgrp);
     if (ret)
         goto out_kernfs_remove;
 
     ret = cgroup_bpf_inherit(cgrp);
     if (ret)
         goto out_psi_free;
 
     /*
      * New cgroup inherits effective freeze counter, and
      * if the parent has to be frozen, the child has too.
      */
     cgrp->freezer.e_freeze = parent->freezer.e_freeze;
     if (cgrp->freezer.e_freeze) {
         /*
          * Set the CGRP_FREEZE flag, so when a process will be
          * attached to the child cgroup, it will become frozen.
          * At this point the new cgroup is unpopulated, so we can
          * consider it frozen immediately.
          */
         set_bit(CGRP_FREEZE, &cgrp->flags);
         set_bit(CGRP_FROZEN, &cgrp->flags);
     }
 
     spin_lock_irq(&css_set_lock);
     for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) {
         cgrp->ancestor_ids[tcgrp->level] = cgroup_id(tcgrp);
 
         if (tcgrp != cgrp) {
             tcgrp->nr_descendants++;
 
             /*
              * If the new cgroup is frozen, all ancestor cgroups
              * get a new frozen descendant, but their state can't
              * change because of this.
              */
             if (cgrp->freezer.e_freeze)
                 tcgrp->freezer.nr_frozen_descendants++;
         }
     }
     spin_unlock_irq(&css_set_lock);
 
     if (notify_on_release(parent))
         set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
 
     if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &parent->flags))
         set_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);
 
     cgrp->self.serial_nr = css_serial_nr_next++;
 
     /* allocation complete, commit to creation */
     list_add_tail_rcu(&cgrp->self.sibling, &cgroup_parent(cgrp)->self.children);
     atomic_inc(&root->nr_cgrps);
     cgroup_get_live(parent);
 
     /*
      * On the default hierarchy, a child doesn't automatically inherit
      * subtree_control from the parent.  Each is configured manually.
      */
     if (!cgroup_on_dfl(cgrp))
         cgrp->subtree_control = cgroup_control(cgrp);
 
     cgroup_propagate_control(cgrp);
 
     return cgrp;
 
 out_psi_free:
     psi_cgroup_free(cgrp);
 out_kernfs_remove:
     kernfs_remove(cgrp->kn);
 out_stat_exit:
     cgroup_rstat_exit(cgrp);
 out_cancel_ref:
     percpu_ref_exit(&cgrp->self.refcnt);
 out_free_cgrp:
     kfree(cgrp);
     return ERR_PTR(ret);
 }
 
 static bool cgroup_check_hierarchy_limits(struct cgroup *parent)
 {
     struct cgroup *cgroup;
     int ret = false;
     int level = 1;
 
     lockdep_assert_held(&cgroup_mutex);
 
     for (cgroup = parent; cgroup; cgroup = cgroup_parent(cgroup)) {
         if (cgroup->nr_descendants >= cgroup->max_descendants)
             goto fail;
 
         if (level > cgroup->max_depth)
             goto fail;
 
         level++;
     }
 
     ret = true;
 fail:
     return ret;
 }
 
 int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
 {
     struct cgroup *parent, *cgrp;
     int ret;
 
     /* do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable */
     if (strchr(name, '\n'))
         return -EINVAL;
 
     parent = cgroup_kn_lock_live(parent_kn, false);
     if (!parent)
         return -ENODEV;
 
     if (!cgroup_check_hierarchy_limits(parent)) {
         ret = -EAGAIN;
         goto out_unlock;
     }
 
     cgrp = cgroup_create(parent, name, mode);
     if (IS_ERR(cgrp)) {
         ret = PTR_ERR(cgrp);
         goto out_unlock;
     }
 
     /*
      * This extra ref will be put in cgroup_free_fn() and guarantees
      * that @cgrp->kn is always accessible.
      */
     kernfs_get(cgrp->kn);
 
     ret = cgroup_kn_set_ugid(cgrp->kn);
     if (ret)
         goto out_destroy;
 
     ret = css_populate_dir(&cgrp->self);
     if (ret)
         goto out_destroy;
 
     ret = cgroup_apply_control_enable(cgrp);
     if (ret)
         goto out_destroy;
 
     TRACE_CGROUP_PATH(mkdir, cgrp);
 
     /* let's create and online css's */
     kernfs_activate(cgrp->kn);
 
     ret = 0;
     goto out_unlock;
 
 out_destroy:
     cgroup_destroy_locked(cgrp);
 out_unlock:
     cgroup_kn_unlock(parent_kn);
     return ret;
 }
 
 /*
  * This is called when the refcnt of a css is confirmed to be killed.
  * css_tryget_online() is now guaranteed to fail.  Tell the subsystem to
  * initiate destruction and put the css ref from kill_css().
  */
 static void css_killed_work_fn(struct work_struct *work)
 {
     struct cgroup_subsys_state *css =
         container_of(work, struct cgroup_subsys_state, destroy_work);
 
     mutex_lock(&cgroup_mutex);
 
     do {
         offline_css(css);
         css_put(css);
         /* @css can't go away while we're holding cgroup_mutex */
         css = css->parent;
     } while (css && atomic_dec_and_test(&css->online_cnt));
 
     mutex_unlock(&cgroup_mutex);
 }
 
 /* css kill confirmation processing requires process context, bounce */
 static void css_killed_ref_fn(struct percpu_ref *ref)
 {
     struct cgroup_subsys_state *css =
         container_of(ref, struct cgroup_subsys_state, refcnt);
 
     if (atomic_dec_and_test(&css->online_cnt)) {
         INIT_WORK(&css->destroy_work, css_killed_work_fn);
         queue_work(cgroup_destroy_wq, &css->destroy_work);
     }
 }
 
 /**
  * kill_css - destroy a css
  * @css: css to destroy
  *
  * This function initiates destruction of @css by removing cgroup interface
  * files and putting its base reference.  ->css_offline() will be invoked
  * asynchronously once css_tryget_online() is guaranteed to fail and when
  * the reference count reaches zero, @css will be released.
  */
 static void kill_css(struct cgroup_subsys_state *css)
 {
     lockdep_assert_held(&cgroup_mutex);
 
     if (css->flags & CSS_DYING)
         return;
 
     css->flags |= CSS_DYING;
 
     /*
      * This must happen before css is disassociated with its cgroup.
      * See seq_css() for details.
      */
     css_clear_dir(css);
 
     /*
      * Killing would put the base ref, but we need to keep it alive
      * until after ->css_offline().
      */
     css_get(css);
 
     /*
      * cgroup core guarantees that, by the time ->css_offline() is
      * invoked, no new css reference will be given out via
      * css_tryget_online().  We can't simply call percpu_ref_kill() and
      * proceed to offlining css's because percpu_ref_kill() doesn't
      * guarantee that the ref is seen as killed on all CPUs on return.
      *
      * Use percpu_ref_kill_and_confirm() to get notifications as each
      * css is confirmed to be seen as killed on all CPUs.
      */
     percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
 }
 
 /**
  * cgroup_destroy_locked - the first stage of cgroup destruction
  * @cgrp: cgroup to be destroyed
  *
  * css's make use of percpu refcnts whose killing latency shouldn't be
  * exposed to userland and are RCU protected.  Also, cgroup core needs to
  * guarantee that css_tryget_online() won't succeed by the time
  * ->css_offline() is invoked.  To satisfy all the requirements,
  * destruction is implemented in the following two steps.
  *
  * s1. Verify @cgrp can be destroyed and mark it dying.  Remove all
  *     userland visible parts and start killing the percpu refcnts of
  *     css's.  Set up so that the next stage will be kicked off once all
  *     the percpu refcnts are confirmed to be killed.
  *
  * s2. Invoke ->css_offline(), mark the cgroup dead and proceed with the
  *     rest of destruction.  Once all cgroup references are gone, the
  *     cgroup is RCU-freed.
  *
  * This function implements s1.  After this step, @cgrp is gone as far as
  * the userland is concerned and a new cgroup with the same name may be
  * created.  As cgroup doesn't care about the names internally, this
  * doesn't cause any problem.
  */
 static int cgroup_destroy_locked(struct cgroup *cgrp)
     __releases(&cgroup_mutex) __acquires(&cgroup_mutex)
 {
     struct cgroup *tcgrp, *parent = cgroup_parent(cgrp);
     struct cgroup_subsys_state *css;
     struct cgrp_cset_link *link;
     int ssid;
 
     lockdep_assert_held(&cgroup_mutex);
 
     /*
      * Only migration can raise populated from zero and we're already
      * holding cgroup_mutex.
      */
     if (cgroup_is_populated(cgrp))
         return -EBUSY;
 
     /*
      * Make sure there's no live children.  We can't test emptiness of
      * ->self.children as dead children linger on it while being
      * drained; otherwise, "rmdir parent/child parent" may fail.
      */
     if (css_has_online_children(&cgrp->self))
         return -EBUSY;
 
     /*
      * Mark @cgrp and the associated csets dead.  The former prevents
      * further task migration and child creation by disabling
      * cgroup_lock_live_group().  The latter makes the csets ignored by
      * the migration path.
      */
     cgrp->self.flags &= ~CSS_ONLINE;
 
     spin_lock_irq(&css_set_lock);
     list_for_each_entry(link, &cgrp->cset_links, cset_link)
         link->cset->dead = true;
     spin_unlock_irq(&css_set_lock);
 
     /* initiate massacre of all css's */
     for_each_css(css, ssid, cgrp)
         kill_css(css);
 
     /* clear and remove @cgrp dir, @cgrp has an extra ref on its kn */
     css_clear_dir(&cgrp->self);
     kernfs_remove(cgrp->kn);
 
     if (cgroup_is_threaded(cgrp))
         parent->nr_threaded_children--;
 
     spin_lock_irq(&css_set_lock);
     for (tcgrp = cgroup_parent(cgrp); tcgrp; tcgrp = cgroup_parent(tcgrp)) {
         tcgrp->nr_descendants--;
         tcgrp->nr_dying_descendants++;
         /*
          * If the dying cgroup is frozen, decrease frozen descendants
          * counters of ancestor cgroups.
          */
         if (test_bit(CGRP_FROZEN, &cgrp->flags))
             tcgrp->freezer.nr_frozen_descendants--;
     }
     spin_unlock_irq(&css_set_lock);
 
     cgroup1_check_for_release(parent);
 
     cgroup_bpf_offline(cgrp);
 
     /* put the base reference */
     percpu_ref_kill(&cgrp->self.refcnt);
 
     return 0;
 };
 
 int cgroup_rmdir(struct kernfs_node *kn)
 {
     struct cgroup *cgrp;
     int ret = 0;
 
     cgrp = cgroup_kn_lock_live(kn, false);
     if (!cgrp)
         return 0;
 
     ret = cgroup_destroy_locked(cgrp);
     if (!ret)
         TRACE_CGROUP_PATH(rmdir, cgrp);
 
     cgroup_kn_unlock(kn);
     return ret;
 }
 
 static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
     .show_options       = cgroup_show_options,
     .mkdir          = cgroup_mkdir,
     .rmdir          = cgroup_rmdir,
     .show_path      = cgroup_show_path,
 };
 
 static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
 {
     struct cgroup_subsys_state *css;
 
     pr_debug("Initializing cgroup subsys %s\n", ss->name);
 
     mutex_lock(&cgroup_mutex);
 
     idr_init(&ss->css_idr);
     INIT_LIST_HEAD(&ss->cfts);
 
     /* Create the root cgroup state for this subsystem */
     ss->root = &cgrp_dfl_root;
     css = ss->css_alloc(NULL);
     /* We don't handle early failures gracefully */
     BUG_ON(IS_ERR(css));
     init_and_link_css(css, ss, &cgrp_dfl_root.cgrp);
 
     /*
      * Root csses are never destroyed and we can't initialize
      * percpu_ref during early init.  Disable refcnting.
      */
     css->flags |= CSS_NO_REF;
 
     if (early) {
         /* allocation can't be done safely during early init */
         css->id = 1;
     } else {
         css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2, GFP_KERNEL);
         BUG_ON(css->id < 0);
     }
 
     /* Update the init_css_set to contain a subsys
      * pointer to this state - since the subsystem is
      * newly registered, all tasks and hence the
      * init_css_set is in the subsystem's root cgroup. */
     init_css_set.subsys[ss->id] = css;
 
     have_fork_callback |= (bool)ss->fork << ss->id;
     have_exit_callback |= (bool)ss->exit << ss->id;
     have_release_callback |= (bool)ss->release << ss->id;
     have_canfork_callback |= (bool)ss->can_fork << ss->id;
 
     /* At system boot, before all subsystems have been
      * registered, no tasks have been forked, so we don't
      * need to invoke fork callbacks here. */
     BUG_ON(!list_empty(&init_task.tasks));
 
     BUG_ON(online_css(css));
 
     mutex_unlock(&cgroup_mutex);
 }
 
 /**
  * cgroup_init_early - cgroup initialization at system boot
  *
  * Initialize cgroups at system boot, and initialize any
  * subsystems that request early init.
  */
 int __init cgroup_init_early(void)
 {
     static struct cgroup_fs_context __initdata ctx;
     struct cgroup_subsys *ss;
     int i;
 
     ctx.root = &cgrp_dfl_root;
     init_cgroup_root(&ctx);
     cgrp_dfl_root.cgrp.self.flags |= CSS_NO_REF;
 
     RCU_INIT_POINTER(init_task.cgroups, &init_css_set);
 
     for_each_subsys(ss, i) {
         WARN(!ss->css_alloc || !ss->css_free || ss->name || ss->id,
              "invalid cgroup_subsys %d:%s css_alloc=%p css_free=%p id:name=%d:%s\n",
              i, cgroup_subsys_name[i], ss->css_alloc, ss->css_free,
              ss->id, ss->name);
         WARN(strlen(cgroup_subsys_name[i]) > MAX_CGROUP_TYPE_NAMELEN,
              "cgroup_subsys_name %s too long\n", cgroup_subsys_name[i]);
 
         ss->id = i;
         ss->name = cgroup_subsys_name[i];
         if (!ss->legacy_name)
             ss->legacy_name = cgroup_subsys_name[i];
 
         if (ss->early_init)
             cgroup_init_subsys(ss, true);
     }
     return 0;
 }
 
 /**
  * cgroup_init - cgroup initialization
  *
  * Register cgroup filesystem and /proc file, and initialize
  * any subsystems that didn't request early init.
  */
 int __init cgroup_init(void)
 {
     struct cgroup_subsys *ss;
     int ssid;
 
     BUILD_BUG_ON(CGROUP_SUBSYS_COUNT > 16);
     BUG_ON(cgroup_init_cftypes(NULL, cgroup_base_files));
     BUG_ON(cgroup_init_cftypes(NULL, cgroup1_base_files));
 
     cgroup_rstat_boot();
 
     get_user_ns(init_cgroup_ns.user_ns);
 
     mutex_lock(&cgroup_mutex);
 
     /*
      * Add init_css_set to the hash table so that dfl_root can link to
      * it during init.
      */
     hash_add(css_set_table, &init_css_set.hlist,
          css_set_hash(init_css_set.subsys));
 
     BUG_ON(cgroup_setup_root(&cgrp_dfl_root, 0));
 
     mutex_unlock(&cgroup_mutex);
 
     for_each_subsys(ss, ssid) {
         if (ss->early_init) {
             struct cgroup_subsys_state *css =
                 init_css_set.subsys[ss->id];
 
             css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2,
                            GFP_KERNEL);
             BUG_ON(css->id < 0);
         } else {
             cgroup_init_subsys(ss, false);
         }
 
         list_add_tail(&init_css_set.e_cset_node[ssid],
                   &cgrp_dfl_root.cgrp.e_csets[ssid]);
 
         /*
          * Setting dfl_root subsys_mask needs to consider the
          * disabled flag and cftype registration needs kmalloc,
          * both of which aren't available during early_init.
          */
         if (!cgroup_ssid_enabled(ssid))
             continue;
 
         if (cgroup1_ssid_disabled(ssid))
             printk(KERN_INFO "Disabling %s control group subsystem in v1 mounts\n",
                    ss->name);
 
         cgrp_dfl_root.subsys_mask |= 1 << ss->id;
 
         /* implicit controllers must be threaded too */
         WARN_ON(ss->implicit_on_dfl && !ss->threaded);
 
         if (ss->implicit_on_dfl)
             cgrp_dfl_implicit_ss_mask |= 1 << ss->id;
         else if (!ss->dfl_cftypes)
             cgrp_dfl_inhibit_ss_mask |= 1 << ss->id;
 
         if (ss->threaded)
             cgrp_dfl_threaded_ss_mask |= 1 << ss->id;
 
         if (ss->dfl_cftypes == ss->legacy_cftypes) {
             WARN_ON(cgroup_add_cftypes(ss, ss->dfl_cftypes));
         } else {
             WARN_ON(cgroup_add_dfl_cftypes(ss, ss->dfl_cftypes));
             WARN_ON(cgroup_add_legacy_cftypes(ss, ss->legacy_cftypes));
         }
 
         if (ss->bind)
             ss->bind(init_css_set.subsys[ssid]);
 
         mutex_lock(&cgroup_mutex);
         css_populate_dir(init_css_set.subsys[ssid]);
         mutex_unlock(&cgroup_mutex);
     }
 
     /* init_css_set.subsys[] has been updated, re-hash */
     hash_del(&init_css_set.hlist);
     hash_add(css_set_table, &init_css_set.hlist,
          css_set_hash(init_css_set.subsys));
 
     WARN_ON(sysfs_create_mount_point(fs_kobj, "cgroup"));
     WARN_ON(register_filesystem(&cgroup_fs_type));
     WARN_ON(register_filesystem(&cgroup2_fs_type));
     WARN_ON(!proc_create_single("cgroups", 0, NULL, proc_cgroupstats_show));
 #ifdef CONFIG_CPUSETS
     WARN_ON(register_filesystem(&cpuset_fs_type));
 #endif
 
     return 0;
 }
 
 static int __init cgroup_wq_init(void)
 {
     /*
      * There isn't much point in executing destruction path in
      * parallel.  Good chunk is serialized with cgroup_mutex anyway.
      * Use 1 for @max_active.
      *
      * We would prefer to do this in cgroup_init() above, but that
      * is called before init_workqueues(): so leave this until after.
      */
     cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
     BUG_ON(!cgroup_destroy_wq);
     return 0;
 }
 core_initcall(cgroup_wq_init);
 
 void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen)
 {
     struct kernfs_node *kn;
 
     kn = kernfs_find_and_get_node_by_id(cgrp_dfl_root.kf_root, id);
     if (!kn)
         return;
     kernfs_path(kn, buf, buflen);
     kernfs_put(kn);
 }
 
 /*
  * cgroup_get_from_id : get the cgroup associated with cgroup id
  * @id: cgroup id
  * On success return the cgrp, on failure return NULL
  */
 struct cgroup *cgroup_get_from_id(u64 id)
 {
     struct kernfs_node *kn;
     struct cgroup *cgrp = NULL;
 
     kn = kernfs_find_and_get_node_by_id(cgrp_dfl_root.kf_root, id);
     if (!kn)
         goto out;
 
     if (kernfs_type(kn) != KERNFS_DIR)
         goto put;
 
     rcu_read_lock();
 
     cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
     if (cgrp && !cgroup_tryget(cgrp))
         cgrp = NULL;
 
     rcu_read_unlock();
 put:
     kernfs_put(kn);
 out:
     return cgrp;
 }
 EXPORT_SYMBOL_GPL(cgroup_get_from_id);
 
 /*
  * proc_cgroup_show()
  *  - Print task's cgroup paths into seq_file, one line for each hierarchy
  *  - Used for /proc/<pid>/cgroup.
  */
 int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
              struct pid *pid, struct task_struct *tsk)
 {
     char *buf;
     int retval;
     struct cgroup_root *root;
 
     retval = -ENOMEM;
     buf = kmalloc(PATH_MAX, GFP_KERNEL);
     if (!buf)
         goto out;
 
     mutex_lock(&cgroup_mutex);
     spin_lock_irq(&css_set_lock);
 
     for_each_root(root) {
         struct cgroup_subsys *ss;
         struct cgroup *cgrp;
         int ssid, count = 0;
 
         if (root == &cgrp_dfl_root && !cgrp_dfl_visible)
             continue;
 
         seq_printf(m, "%d:", root->hierarchy_id);
         if (root != &cgrp_dfl_root)
             for_each_subsys(ss, ssid)
                 if (root->subsys_mask & (1 << ssid))
                     seq_printf(m, "%s%s", count++ ? "," : "",
                            ss->legacy_name);
         if (strlen(root->name))
             seq_printf(m, "%sname=%s", count ? "," : "",
                    root->name);
         seq_putc(m, ':');
 
         cgrp = task_cgroup_from_root(tsk, root);
 
         /*
          * On traditional hierarchies, all zombie tasks show up as
          * belonging to the root cgroup.  On the default hierarchy,
          * while a zombie doesn't show up in "cgroup.procs" and
          * thus can't be migrated, its /proc/PID/cgroup keeps
          * reporting the cgroup it belonged to before exiting.  If
          * the cgroup is removed before the zombie is reaped,
          * " (deleted)" is appended to the cgroup path.
          */
         if (cgroup_on_dfl(cgrp) || !(tsk->flags & PF_EXITING)) {
             retval = cgroup_path_ns_locked(cgrp, buf, PATH_MAX,
                         current->nsproxy->cgroup_ns);
             if (retval >= PATH_MAX)
                 retval = -ENAMETOOLONG;
             if (retval < 0)
                 goto out_unlock;
 
             seq_puts(m, buf);
         } else {
             seq_puts(m, "/");
         }
 
         if (cgroup_on_dfl(cgrp) && cgroup_is_dead(cgrp))
             seq_puts(m, " (deleted)\n");
         else
             seq_putc(m, '\n');
     }
 
     retval = 0;
 out_unlock:
     spin_unlock_irq(&css_set_lock);
     mutex_unlock(&cgroup_mutex);
     kfree(buf);
 out:
     return retval;
 }
 
 /**
  * cgroup_fork - initialize cgroup related fields during copy_process()
  * @child: pointer to task_struct of forking parent process.
  *
  * A task is associated with the init_css_set until cgroup_post_fork()
  * attaches it to the target css_set.
  */
 void cgroup_fork(struct task_struct *child)
 {
     RCU_INIT_POINTER(child->cgroups, &init_css_set);
     INIT_LIST_HEAD(&child->cg_list);
 }
 
 static struct cgroup *cgroup_get_from_file(struct file *f)
 {
     struct cgroup_subsys_state *css;
     struct cgroup *cgrp;
 
     css = css_tryget_online_from_dir(f->f_path.dentry, NULL);
     if (IS_ERR(css))
         return ERR_CAST(css);
 
     cgrp = css->cgroup;
     if (!cgroup_on_dfl(cgrp)) {
         cgroup_put(cgrp);
         return ERR_PTR(-EBADF);
     }
 
     return cgrp;
 }
 
 /**
  * cgroup_css_set_fork - find or create a css_set for a child process
  * @kargs: the arguments passed to create the child process
  *
  * This functions finds or creates a new css_set which the child
  * process will be attached to in cgroup_post_fork(). By default,
  * the child process will be given the same css_set as its parent.
  *
  * If CLONE_INTO_CGROUP is specified this function will try to find an
  * existing css_set which includes the requested cgroup and if not create
  * a new css_set that the child will be attached to later. If this function
  * succeeds it will hold cgroup_threadgroup_rwsem on return. If
  * CLONE_INTO_CGROUP is requested this function will grab cgroup mutex
  * before grabbing cgroup_threadgroup_rwsem and will hold a reference
  * to the target cgroup.
  */
 static int cgroup_css_set_fork(struct kernel_clone_args *kargs)
     __acquires(&cgroup_mutex) __acquires(&cgroup_threadgroup_rwsem)
 {
     int ret;
     struct cgroup *dst_cgrp = NULL;
     struct css_set *cset;
     struct super_block *sb;
     struct file *f;
 
     if (kargs->flags & CLONE_INTO_CGROUP)
         mutex_lock(&cgroup_mutex);
 
     cgroup_threadgroup_change_begin(current);
 
     spin_lock_irq(&css_set_lock);
     cset = task_css_set(current);
     get_css_set(cset);
     spin_unlock_irq(&css_set_lock);
 
     if (!(kargs->flags & CLONE_INTO_CGROUP)) {
         kargs->cset = cset;
         return 0;
     }
 
     f = fget_raw(kargs->cgroup);
     if (!f) {
         ret = -EBADF;
         goto err;
     }
     sb = f->f_path.dentry->d_sb;
 
     dst_cgrp = cgroup_get_from_file(f);
     if (IS_ERR(dst_cgrp)) {
         ret = PTR_ERR(dst_cgrp);
         dst_cgrp = NULL;
         goto err;
     }
 
     if (cgroup_is_dead(dst_cgrp)) {
         ret = -ENODEV;
         goto err;
     }
 
     /*
      * Verify that we the target cgroup is writable for us. This is
      * usually done by the vfs layer but since we're not going through
      * the vfs layer here we need to do it "manually".
      */
     ret = cgroup_may_write(dst_cgrp, sb);
     if (ret)
         goto err;
 
     /*
      * Spawning a task directly into a cgroup works by passing a file
      * descriptor to the target cgroup directory. This can even be an O_PATH
      * file descriptor. But it can never be a cgroup.procs file descriptor.
      * This was done on purpose so spawning into a cgroup could be
      * conceptualized as an atomic
      *
      *   fd = openat(dfd_cgroup, "cgroup.procs", ...);
      *   write(fd, <child-pid>, ...);
      *
      * sequence, i.e. it's a shorthand for the caller opening and writing
      * cgroup.procs of the cgroup indicated by @dfd_cgroup. This allows us
      * to always use the caller's credentials.
      */
     ret = cgroup_attach_permissions(cset->dfl_cgrp, dst_cgrp, sb,
                     !(kargs->flags & CLONE_THREAD),
                     current->nsproxy->cgroup_ns);
     if (ret)
         goto err;
 
     kargs->cset = find_css_set(cset, dst_cgrp);
     if (!kargs->cset) {
         ret = -ENOMEM;
         goto err;
     }
 
     put_css_set(cset);
     fput(f);
     kargs->cgrp = dst_cgrp;
     return ret;
 
 err:
     cgroup_threadgroup_change_end(current);
     mutex_unlock(&cgroup_mutex);
     if (f)
         fput(f);
     if (dst_cgrp)
         cgroup_put(dst_cgrp);
     put_css_set(cset);
     if (kargs->cset)
         put_css_set(kargs->cset);
     return ret;
 }
 
 /**
  * cgroup_css_set_put_fork - drop references we took during fork
  * @kargs: the arguments passed to create the child process
  *
  * Drop references to the prepared css_set and target cgroup if
  * CLONE_INTO_CGROUP was requested.
  */
 static void cgroup_css_set_put_fork(struct kernel_clone_args *kargs)
     __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
 {
     cgroup_threadgroup_change_end(current);
 
     if (kargs->flags & CLONE_INTO_CGROUP) {
         struct cgroup *cgrp = kargs->cgrp;
         struct css_set *cset = kargs->cset;
 
         mutex_unlock(&cgroup_mutex);
 
         if (cset) {
             put_css_set(cset);
             kargs->cset = NULL;
         }
 
         if (cgrp) {
             cgroup_put(cgrp);
             kargs->cgrp = NULL;
         }
     }
 }
 
 /**
  * cgroup_can_fork - called on a new task before the process is exposed
  * @child: the child process
  * @kargs: the arguments passed to create the child process
  *
  * This prepares a new css_set for the child process which the child will
  * be attached to in cgroup_post_fork().
  * This calls the subsystem can_fork() callbacks. If the cgroup_can_fork()
  * callback returns an error, the fork aborts with that error code. This
  * allows for a cgroup subsystem to conditionally allow or deny new forks.
  */
 int cgroup_can_fork(struct task_struct *child, struct kernel_clone_args *kargs)
 {
     struct cgroup_subsys *ss;
     int i, j, ret;
 
     ret = cgroup_css_set_fork(kargs);
     if (ret)
         return ret;
 
     do_each_subsys_mask(ss, i, have_canfork_callback) {
         ret = ss->can_fork(child, kargs->cset);
         if (ret)
             goto out_revert;
     } while_each_subsys_mask();
 
     return 0;
 
 out_revert:
     for_each_subsys(ss, j) {
         if (j >= i)
             break;
         if (ss->cancel_fork)
             ss->cancel_fork(child, kargs->cset);
     }
 
     cgroup_css_set_put_fork(kargs);
 
     return ret;
 }
 
 /**
  * cgroup_cancel_fork - called if a fork failed after cgroup_can_fork()
  * @child: the child process
  * @kargs: the arguments passed to create the child process
  *
  * This calls the cancel_fork() callbacks if a fork failed *after*
  * cgroup_can_fork() succeeded and cleans up references we took to
  * prepare a new css_set for the child process in cgroup_can_fork().
  */
 void cgroup_cancel_fork(struct task_struct *child,
             struct kernel_clone_args *kargs)
 {
     struct cgroup_subsys *ss;
     int i;
 
     for_each_subsys(ss, i)
         if (ss->cancel_fork)
             ss->cancel_fork(child, kargs->cset);
 
     cgroup_css_set_put_fork(kargs);
 }
 
 /**
  * cgroup_post_fork - finalize cgroup setup for the child process
  * @child: the child process
  * @kargs: the arguments passed to create the child process
  *
  * Attach the child process to its css_set calling the subsystem fork()
  * callbacks.
  */
 void cgroup_post_fork(struct task_struct *child,
               struct kernel_clone_args *kargs)
     __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
 {
     unsigned long cgrp_flags = 0;
     bool kill = false;
     struct cgroup_subsys *ss;
     struct css_set *cset;
     int i;
 
     cset = kargs->cset;
     kargs->cset = NULL;
 
     spin_lock_irq(&css_set_lock);
 
     /* init tasks are special, only link regular threads */
     if (likely(child->pid)) {
         if (kargs->cgrp)
             cgrp_flags = kargs->cgrp->flags;
         else
             cgrp_flags = cset->dfl_cgrp->flags;
 
         WARN_ON_ONCE(!list_empty(&child->cg_list));
         cset->nr_tasks++;
         css_set_move_task(child, NULL, cset, false);
     } else {
         put_css_set(cset);
         cset = NULL;
     }
 
     if (!(child->flags & PF_KTHREAD)) {
         if (unlikely(test_bit(CGRP_FREEZE, &cgrp_flags))) {
             /*
              * If the cgroup has to be frozen, the new task has
              * too. Let's set the JOBCTL_TRAP_FREEZE jobctl bit to
              * get the task into the frozen state.
              */
             spin_lock(&child->sighand->siglock);
             WARN_ON_ONCE(child->frozen);
             child->jobctl |= JOBCTL_TRAP_FREEZE;
             spin_unlock(&child->sighand->siglock);
 
             /*
              * Calling cgroup_update_frozen() isn't required here,
              * because it will be called anyway a bit later from
              * do_freezer_trap(). So we avoid cgroup's transient
              * switch from the frozen state and back.
              */
         }
 
         /*
          * If the cgroup is to be killed notice it now and take the
          * child down right after we finished preparing it for
          * userspace.
          */
         kill = test_bit(CGRP_KILL, &cgrp_flags);
     }
 
     spin_unlock_irq(&css_set_lock);
 
     /*
      * Call ss->fork().  This must happen after @child is linked on
      * css_set; otherwise, @child might change state between ->fork()
      * and addition to css_set.
      */
     do_each_subsys_mask(ss, i, have_fork_callback) {
         ss->fork(child);
     } while_each_subsys_mask();
 
     /* Make the new cset the root_cset of the new cgroup namespace. */
     if (kargs->flags & CLONE_NEWCGROUP) {
         struct css_set *rcset = child->nsproxy->cgroup_ns->root_cset;
 
         get_css_set(cset);
         child->nsproxy->cgroup_ns->root_cset = cset;
         put_css_set(rcset);
     }
 
     /* Cgroup has to be killed so take down child immediately. */
     if (unlikely(kill))
         do_send_sig_info(SIGKILL, SEND_SIG_NOINFO, child, PIDTYPE_TGID);
 
     cgroup_css_set_put_fork(kargs);
 }
 
 /**
  * cgroup_exit - detach cgroup from exiting task
  * @tsk: pointer to task_struct of exiting process
  *
  * Description: Detach cgroup from @tsk.
  *
  */
 void cgroup_exit(struct task_struct *tsk)
 {
     struct cgroup_subsys *ss;
     struct css_set *cset;
     int i;
 
     spin_lock_irq(&css_set_lock);
 
     WARN_ON_ONCE(list_empty(&tsk->cg_list));
     cset = task_css_set(tsk);
     css_set_move_task(tsk, cset, NULL, false);
     list_add_tail(&tsk->cg_list, &cset->dying_tasks);
     cset->nr_tasks--;
 
     WARN_ON_ONCE(cgroup_task_frozen(tsk));
     if (unlikely(!(tsk->flags & PF_KTHREAD) &&
              test_bit(CGRP_FREEZE, &task_dfl_cgroup(tsk)->flags)))
         cgroup_update_frozen(task_dfl_cgroup(tsk));
 
     spin_unlock_irq(&css_set_lock);
 
     /* see cgroup_post_fork() for details */
     do_each_subsys_mask(ss, i, have_exit_callback) {
         ss->exit(tsk);
     } while_each_subsys_mask();
 }
 
 void cgroup_release(struct task_struct *task)
 {
     struct cgroup_subsys *ss;
     int ssid;
 
     do_each_subsys_mask(ss, ssid, have_release_callback) {
         ss->release(task);
     } while_each_subsys_mask();
 
     spin_lock_irq(&css_set_lock);
     css_set_skip_task_iters(task_css_set(task), task);
     list_del_init(&task->cg_list);
     spin_unlock_irq(&css_set_lock);
 }
 
 void cgroup_free(struct task_struct *task)
 {
     struct css_set *cset = task_css_set(task);
     put_css_set(cset);
 }
 
 static int __init cgroup_disable(char *str)
 {
     struct cgroup_subsys *ss;
     char *token;
     int i;
 
     while ((token = strsep(&str, ",")) != NULL) {
         if (!*token)
             continue;
 
         for_each_subsys(ss, i) {
             if (strcmp(token, ss->name) &&
                 strcmp(token, ss->legacy_name))
                 continue;
 
             static_branch_disable(cgroup_subsys_enabled_key[i]);
             pr_info("Disabling %s control group subsystem\n",
                 ss->name);
         }
 
         for (i = 0; i < OPT_FEATURE_COUNT; i++) {
             if (strcmp(token, cgroup_opt_feature_names[i]))
                 continue;
             cgroup_feature_disable_mask |= 1 << i;
             pr_info("Disabling %s control group feature\n",
                 cgroup_opt_feature_names[i]);
             break;
         }
     }
     return 1;
 }
 __setup("cgroup_disable=", cgroup_disable);
 
 void __init __weak enable_debug_cgroup(void) { }
 
 static int __init enable_cgroup_debug(char *str)
 {
     cgroup_debug = true;
     enable_debug_cgroup();
     return 1;
 }
 __setup("cgroup_debug", enable_cgroup_debug);
 
 /**
  * css_tryget_online_from_dir - get corresponding css from a cgroup dentry
  * @dentry: directory dentry of interest
  * @ss: subsystem of interest
  *
  * If @dentry is a directory for a cgroup which has @ss enabled on it, try
  * to get the corresponding css and return it.  If such css doesn't exist
  * or can't be pinned, an ERR_PTR value is returned.
  */
 struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
                                struct cgroup_subsys *ss)
 {
     struct kernfs_node *kn = kernfs_node_from_dentry(dentry);
     struct file_system_type *s_type = dentry->d_sb->s_type;
     struct cgroup_subsys_state *css = NULL;
     struct cgroup *cgrp;
 
     /* is @dentry a cgroup dir? */
     if ((s_type != &cgroup_fs_type && s_type != &cgroup2_fs_type) ||
         !kn || kernfs_type(kn) != KERNFS_DIR)
         return ERR_PTR(-EBADF);
 
     rcu_read_lock();
 
     /*
      * This path doesn't originate from kernfs and @kn could already
      * have been or be removed at any point.  @kn->priv is RCU
      * protected for this access.  See css_release_work_fn() for details.
      */
     cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
     if (cgrp)
         css = cgroup_css(cgrp, ss);
 
     if (!css || !css_tryget_online(css))
         css = ERR_PTR(-ENOENT);
 
     rcu_read_unlock();
     return css;
 }
 
 /**
  * css_from_id - lookup css by id
  * @id: the cgroup id
  * @ss: cgroup subsys to be looked into
  *
  * Returns the css if there's valid one with @id, otherwise returns NULL.
  * Should be called under rcu_read_lock().
  */
 struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
 {
     WARN_ON_ONCE(!rcu_read_lock_held());
     return idr_find(&ss->css_idr, id);
 }
 
 /**
  * cgroup_get_from_path - lookup and get a cgroup from its default hierarchy path
  * @path: path on the default hierarchy
  *
  * Find the cgroup at @path on the default hierarchy, increment its
  * reference count and return it.  Returns pointer to the found cgroup on
  * success, ERR_PTR(-ENOENT) if @path doesn't exist or if the cgroup has already
  * been released and ERR_PTR(-ENOTDIR) if @path points to a non-directory.
  */
 struct cgroup *cgroup_get_from_path(const char *path)
 {
     struct kernfs_node *kn;
     struct cgroup *cgrp = ERR_PTR(-ENOENT);
 
     kn = kernfs_walk_and_get(cgrp_dfl_root.cgrp.kn, path);
     if (!kn)
         goto out;
 
     if (kernfs_type(kn) != KERNFS_DIR) {
         cgrp = ERR_PTR(-ENOTDIR);
         goto out_kernfs;
     }
 
     rcu_read_lock();
 
     cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
     if (!cgrp || !cgroup_tryget(cgrp))
         cgrp = ERR_PTR(-ENOENT);
 
     rcu_read_unlock();
 
 out_kernfs:
     kernfs_put(kn);
 out:
     return cgrp;
 }
 EXPORT_SYMBOL_GPL(cgroup_get_from_path);
 
 /**
  * cgroup_get_from_fd - get a cgroup pointer from a fd
  * @fd: fd obtained by open(cgroup2_dir)
  *
  * Find the cgroup from a fd which should be obtained
  * by opening a cgroup directory.  Returns a pointer to the
  * cgroup on success. ERR_PTR is returned if the cgroup
  * cannot be found.
  */
 struct cgroup *cgroup_get_from_fd(int fd)
 {
     struct cgroup *cgrp;
     struct file *f;
 
     f = fget_raw(fd);
     if (!f)
         return ERR_PTR(-EBADF);
 
     cgrp = cgroup_get_from_file(f);
     fput(f);
     return cgrp;
 }
 EXPORT_SYMBOL_GPL(cgroup_get_from_fd);
 
 static u64 power_of_ten(int power)
 {
     u64 v = 1;
     while (power--)
         v *= 10;
     return v;
 }
 
 /**
  * cgroup_parse_float - parse a floating number
  * @input: input string
  * @dec_shift: number of decimal digits to shift
  * @v: output
  *
  * Parse a decimal floating point number in @input and store the result in
  * @v with decimal point right shifted @dec_shift times.  For example, if
  * @input is "12.3456" and @dec_shift is 3, *@v will be set to 12345.
  * Returns 0 on success, -errno otherwise.
  *
  * There's nothing cgroup specific about this function except that it's
  * currently the only user.
  */
 int cgroup_parse_float(const char *input, unsigned dec_shift, s64 *v)
 {
     s64 whole, frac = 0;
     int fstart = 0, fend = 0, flen;
 
     if (!sscanf(input, "%lld.%n%lld%n", &whole, &fstart, &frac, &fend))
         return -EINVAL;
     if (frac < 0)
         return -EINVAL;
 
     flen = fend > fstart ? fend - fstart : 0;
     if (flen < dec_shift)
         frac *= power_of_ten(dec_shift - flen);
     else
         frac = DIV_ROUND_CLOSEST_ULL(frac, power_of_ten(flen - dec_shift));
 
     *v = whole * power_of_ten(dec_shift) + frac;
     return 0;
 }
 
 /*
  * sock->sk_cgrp_data handling.  For more info, see sock_cgroup_data
  * definition in cgroup-defs.h.
  */
 #ifdef CONFIG_SOCK_CGROUP_DATA
 
 void cgroup_sk_alloc(struct sock_cgroup_data *skcd)
 {
     struct cgroup *cgroup;
 
     rcu_read_lock();
     /* Don't associate the sock with unrelated interrupted task's cgroup. */
     if (in_interrupt()) {
         cgroup = &cgrp_dfl_root.cgrp;
         cgroup_get(cgroup);
         goto out;
     }
 
     while (true) {
         struct css_set *cset;
 
         cset = task_css_set(current);
         if (likely(cgroup_tryget(cset->dfl_cgrp))) {
             cgroup = cset->dfl_cgrp;
             break;
         }
         cpu_relax();
     }
 out:
     skcd->cgroup = cgroup;
     cgroup_bpf_get(cgroup);
     rcu_read_unlock();
 }
 
 void cgroup_sk_clone(struct sock_cgroup_data *skcd)
 {
     struct cgroup *cgrp = sock_cgroup_ptr(skcd);
 
     /*
      * We might be cloning a socket which is left in an empty
      * cgroup and the cgroup might have already been rmdir'd.
      * Don't use cgroup_get_live().
      */
     cgroup_get(cgrp);
     cgroup_bpf_get(cgrp);
 }
 
 void cgroup_sk_free(struct sock_cgroup_data *skcd)
 {
     struct cgroup *cgrp = sock_cgroup_ptr(skcd);
 
     cgroup_bpf_put(cgrp);
     cgroup_put(cgrp);
 }
 
 #endif  /* CONFIG_SOCK_CGROUP_DATA */
 
 #ifdef CONFIG_SYSFS
 static ssize_t show_delegatable_files(struct cftype *files, char *buf,
                       ssize_t size, const char *prefix)
 {
     struct cftype *cft;
     ssize_t ret = 0;
 
     for (cft = files; cft && cft->name[0] != '\0'; cft++) {
         if (!(cft->flags & CFTYPE_NS_DELEGATABLE))
             continue;
 
         if ((cft->flags & CFTYPE_PRESSURE) && !cgroup_psi_enabled())
             continue;
 
         if (prefix)
             ret += snprintf(buf + ret, size - ret, "%s.", prefix);
 
         ret += snprintf(buf + ret, size - ret, "%s\n", cft->name);
 
         if (WARN_ON(ret >= size))
             break;
     }
 
     return ret;
 }
 
 static ssize_t delegate_show(struct kobject *kobj, struct kobj_attribute *attr,
                   char *buf)
 {
     struct cgroup_subsys *ss;
     int ssid;
     ssize_t ret = 0;
 
     ret = show_delegatable_files(cgroup_base_files, buf, PAGE_SIZE - ret,
                      NULL);
 
     for_each_subsys(ss, ssid)
         ret += show_delegatable_files(ss->dfl_cftypes, buf + ret,
                           PAGE_SIZE - ret,
                           cgroup_subsys_name[ssid]);
 
     return ret;
 }
 static struct kobj_attribute cgroup_delegate_attr = __ATTR_RO(delegate);
 
 static ssize_t features_show(struct kobject *kobj, struct kobj_attribute *attr,
                  char *buf)
 {
     return snprintf(buf, PAGE_SIZE,
             "nsdelegate\n"
             "favordynmods\n"
             "memory_localevents\n"
             "memory_recursiveprot\n");
 }
 static struct kobj_attribute cgroup_features_attr = __ATTR_RO(features);
 
 static struct attribute *cgroup_sysfs_attrs[] = {
     &cgroup_delegate_attr.attr,
     &cgroup_features_attr.attr,
     NULL,
 };
 
 static const struct attribute_group cgroup_sysfs_attr_group = {
     .attrs = cgroup_sysfs_attrs,
     .name = "cgroup",
 };
 
 static int __init cgroup_sysfs_init(void)
 {
     return sysfs_create_group(kernel_kobj, &cgroup_sysfs_attr_group);
 }
 subsys_initcall(cgroup_sysfs_init);
 
 #endif /* CONFIG_SYSFS */