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 // SPDX-License-Identifier: GPL-2.0+
 /*
  * Sleepable Read-Copy Update mechanism for mutual exclusion.
  *
  * Copyright (C) IBM Corporation, 2006
  * Copyright (C) Fujitsu, 2012
  *
  * Authors: Paul McKenney <paulmck@linux.ibm.com>
  *     Lai Jiangshan <laijs@cn.fujitsu.com>
  *
  * For detailed explanation of Read-Copy Update mechanism see -
  *      Documentation/RCU/ *.txt
  *
  */
 
 #define pr_fmt(fmt) "rcu: " fmt
 
 #include <linux/export.h>
 #include <linux/mutex.h>
 #include <linux/percpu.h>
 #include <linux/preempt.h>
 #include <linux/rcupdate_wait.h>
 #include <linux/sched.h>
 #include <linux/smp.h>
 #include <linux/delay.h>
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/srcu.h>
 
 #include "rcu.h"
 #include "rcu_segcblist.h"
 
 /* Holdoff in nanoseconds for auto-expediting. */
 #define DEFAULT_SRCU_EXP_HOLDOFF (25 * 1000)
 static ulong exp_holdoff = DEFAULT_SRCU_EXP_HOLDOFF;
 module_param(exp_holdoff, ulong, 0444);
 
 /* Overflow-check frequency.  N bits roughly says every 2**N grace periods. */
 static ulong counter_wrap_check = (ULONG_MAX >> 2);
 module_param(counter_wrap_check, ulong, 0444);
 
 /*
  * Control conversion to SRCU_SIZE_BIG:
  *    0: Don't convert at all.
  *    1: Convert at init_srcu_struct() time.
  *    2: Convert when rcutorture invokes srcu_torture_stats_print().
  *    3: Decide at boot time based on system shape (default).
  * 0x1x: Convert when excessive contention encountered.
  */
 #define SRCU_SIZING_NONE    0
 #define SRCU_SIZING_INIT    1
 #define SRCU_SIZING_TORTURE 2
 #define SRCU_SIZING_AUTO    3
 #define SRCU_SIZING_CONTEND 0x10
 #define SRCU_SIZING_IS(x) ((convert_to_big & ~SRCU_SIZING_CONTEND) == x)
 #define SRCU_SIZING_IS_NONE() (SRCU_SIZING_IS(SRCU_SIZING_NONE))
 #define SRCU_SIZING_IS_INIT() (SRCU_SIZING_IS(SRCU_SIZING_INIT))
 #define SRCU_SIZING_IS_TORTURE() (SRCU_SIZING_IS(SRCU_SIZING_TORTURE))
 #define SRCU_SIZING_IS_CONTEND() (convert_to_big & SRCU_SIZING_CONTEND)
 static int convert_to_big = SRCU_SIZING_AUTO;
 module_param(convert_to_big, int, 0444);
 
 /* Number of CPUs to trigger init_srcu_struct()-time transition to big. */
 static int big_cpu_lim __read_mostly = 128;
 module_param(big_cpu_lim, int, 0444);
 
 /* Contention events per jiffy to initiate transition to big. */
 static int small_contention_lim __read_mostly = 100;
 module_param(small_contention_lim, int, 0444);
 
 /* Early-boot callback-management, so early that no lock is required! */
 static LIST_HEAD(srcu_boot_list);
 static bool __read_mostly srcu_init_done;
 
 static void srcu_invoke_callbacks(struct work_struct *work);
 static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay);
 static void process_srcu(struct work_struct *work);
 static void srcu_delay_timer(struct timer_list *t);
 
 /* Wrappers for lock acquisition and release, see raw_spin_lock_rcu_node(). */
 #define spin_lock_rcu_node(p)                           \
 do {                                        \
     spin_lock(&ACCESS_PRIVATE(p, lock));                    \
     smp_mb__after_unlock_lock();                        \
 } while (0)
 
 #define spin_unlock_rcu_node(p) spin_unlock(&ACCESS_PRIVATE(p, lock))
 
 #define spin_lock_irq_rcu_node(p)                       \
 do {                                        \
     spin_lock_irq(&ACCESS_PRIVATE(p, lock));                \
     smp_mb__after_unlock_lock();                        \
 } while (0)
 
 #define spin_unlock_irq_rcu_node(p)                     \
     spin_unlock_irq(&ACCESS_PRIVATE(p, lock))
 
 #define spin_lock_irqsave_rcu_node(p, flags)                    \
 do {                                        \
     spin_lock_irqsave(&ACCESS_PRIVATE(p, lock), flags);         \
     smp_mb__after_unlock_lock();                        \
 } while (0)
 
 #define spin_trylock_irqsave_rcu_node(p, flags)                 \
 ({                                      \
     bool ___locked = spin_trylock_irqsave(&ACCESS_PRIVATE(p, lock), flags); \
                                         \
     if (___locked)                              \
         smp_mb__after_unlock_lock();                    \
     ___locked;                              \
 })
 
 #define spin_unlock_irqrestore_rcu_node(p, flags)               \
     spin_unlock_irqrestore(&ACCESS_PRIVATE(p, lock), flags)         \
 
 /*
  * Initialize SRCU per-CPU data.  Note that statically allocated
  * srcu_struct structures might already have srcu_read_lock() and
  * srcu_read_unlock() running against them.  So if the is_static parameter
  * is set, don't initialize ->srcu_lock_count[] and ->srcu_unlock_count[].
  */
 static void init_srcu_struct_data(struct srcu_struct *ssp)
 {
     int cpu;
     struct srcu_data *sdp;
 
     /*
      * Initialize the per-CPU srcu_data array, which feeds into the
      * leaves of the srcu_node tree.
      */
     WARN_ON_ONCE(ARRAY_SIZE(sdp->srcu_lock_count) !=
              ARRAY_SIZE(sdp->srcu_unlock_count));
     for_each_possible_cpu(cpu) {
         sdp = per_cpu_ptr(ssp->sda, cpu);
         spin_lock_init(&ACCESS_PRIVATE(sdp, lock));
         rcu_segcblist_init(&sdp->srcu_cblist);
         sdp->srcu_cblist_invoking = false;
         sdp->srcu_gp_seq_needed = ssp->srcu_gp_seq;
         sdp->srcu_gp_seq_needed_exp = ssp->srcu_gp_seq;
         sdp->mynode = NULL;
         sdp->cpu = cpu;
         INIT_WORK(&sdp->work, srcu_invoke_callbacks);
         timer_setup(&sdp->delay_work, srcu_delay_timer, 0);
         sdp->ssp = ssp;
     }
 }
 
 /* Invalid seq state, used during snp node initialization */
 #define SRCU_SNP_INIT_SEQ       0x2
 
 /*
  * Check whether sequence number corresponding to snp node,
  * is invalid.
  */
 static inline bool srcu_invl_snp_seq(unsigned long s)
 {
     return rcu_seq_state(s) == SRCU_SNP_INIT_SEQ;
 }
 
 /*
  * Allocated and initialize SRCU combining tree.  Returns @true if
  * allocation succeeded and @false otherwise.
  */
 static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
 {
     int cpu;
     int i;
     int level = 0;
     int levelspread[RCU_NUM_LVLS];
     struct srcu_data *sdp;
     struct srcu_node *snp;
     struct srcu_node *snp_first;
 
     /* Initialize geometry if it has not already been initialized. */
     rcu_init_geometry();
     ssp->node = kcalloc(rcu_num_nodes, sizeof(*ssp->node), gfp_flags);
     if (!ssp->node)
         return false;
 
     /* Work out the overall tree geometry. */
     ssp->level[0] = &ssp->node[0];
     for (i = 1; i < rcu_num_lvls; i++)
         ssp->level[i] = ssp->level[i - 1] + num_rcu_lvl[i - 1];
     rcu_init_levelspread(levelspread, num_rcu_lvl);
 
     /* Each pass through this loop initializes one srcu_node structure. */
     srcu_for_each_node_breadth_first(ssp, snp) {
         spin_lock_init(&ACCESS_PRIVATE(snp, lock));
         WARN_ON_ONCE(ARRAY_SIZE(snp->srcu_have_cbs) !=
                  ARRAY_SIZE(snp->srcu_data_have_cbs));
         for (i = 0; i < ARRAY_SIZE(snp->srcu_have_cbs); i++) {
             snp->srcu_have_cbs[i] = SRCU_SNP_INIT_SEQ;
             snp->srcu_data_have_cbs[i] = 0;
         }
         snp->srcu_gp_seq_needed_exp = SRCU_SNP_INIT_SEQ;
         snp->grplo = -1;
         snp->grphi = -1;
         if (snp == &ssp->node[0]) {
             /* Root node, special case. */
             snp->srcu_parent = NULL;
             continue;
         }
 
         /* Non-root node. */
         if (snp == ssp->level[level + 1])
             level++;
         snp->srcu_parent = ssp->level[level - 1] +
                    (snp - ssp->level[level]) /
                    levelspread[level - 1];
     }
 
     /*
      * Initialize the per-CPU srcu_data array, which feeds into the
      * leaves of the srcu_node tree.
      */
     level = rcu_num_lvls - 1;
     snp_first = ssp->level[level];
     for_each_possible_cpu(cpu) {
         sdp = per_cpu_ptr(ssp->sda, cpu);
         sdp->mynode = &snp_first[cpu / levelspread[level]];
         for (snp = sdp->mynode; snp != NULL; snp = snp->srcu_parent) {
             if (snp->grplo < 0)
                 snp->grplo = cpu;
             snp->grphi = cpu;
         }
         sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
     }
     smp_store_release(&ssp->srcu_size_state, SRCU_SIZE_WAIT_BARRIER);
     return true;
 }
 
 /*
  * Initialize non-compile-time initialized fields, including the
  * associated srcu_node and srcu_data structures.  The is_static parameter
  * tells us that ->sda has already been wired up to srcu_data.
  */
 static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 {
     ssp->srcu_size_state = SRCU_SIZE_SMALL;
     ssp->node = NULL;
     mutex_init(&ssp->srcu_cb_mutex);
     mutex_init(&ssp->srcu_gp_mutex);
     ssp->srcu_idx = 0;
     ssp->srcu_gp_seq = 0;
     ssp->srcu_barrier_seq = 0;
     mutex_init(&ssp->srcu_barrier_mutex);
     atomic_set(&ssp->srcu_barrier_cpu_cnt, 0);
     INIT_DELAYED_WORK(&ssp->work, process_srcu);
     ssp->sda_is_static = is_static;
     if (!is_static)
         ssp->sda = alloc_percpu(struct srcu_data);
     if (!ssp->sda)
         return -ENOMEM;
     init_srcu_struct_data(ssp);
     ssp->srcu_gp_seq_needed_exp = 0;
     ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
     if (READ_ONCE(ssp->srcu_size_state) == SRCU_SIZE_SMALL && SRCU_SIZING_IS_INIT()) {
         if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC)) {
             if (!ssp->sda_is_static) {
                 free_percpu(ssp->sda);
                 ssp->sda = NULL;
                 return -ENOMEM;
             }
         } else {
             WRITE_ONCE(ssp->srcu_size_state, SRCU_SIZE_BIG);
         }
     }
     smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */
     return 0;
 }
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
 int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
                struct lock_class_key *key)
 {
     /* Don't re-initialize a lock while it is held. */
     debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
     lockdep_init_map(&ssp->dep_map, name, key, 0);
     spin_lock_init(&ACCESS_PRIVATE(ssp, lock));
     return init_srcu_struct_fields(ssp, false);
 }
 EXPORT_SYMBOL_GPL(__init_srcu_struct);
 
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /**
  * init_srcu_struct - initialize a sleep-RCU structure
  * @ssp: structure to initialize.
  *
  * Must invoke this on a given srcu_struct before passing that srcu_struct
  * to any other function.  Each srcu_struct represents a separate domain
  * of SRCU protection.
  */
 int init_srcu_struct(struct srcu_struct *ssp)
 {
     spin_lock_init(&ACCESS_PRIVATE(ssp, lock));
     return init_srcu_struct_fields(ssp, false);
 }
 EXPORT_SYMBOL_GPL(init_srcu_struct);
 
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /*
  * Initiate a transition to SRCU_SIZE_BIG with lock held.
  */
 static void __srcu_transition_to_big(struct srcu_struct *ssp)
 {
     lockdep_assert_held(&ACCESS_PRIVATE(ssp, lock));
     smp_store_release(&ssp->srcu_size_state, SRCU_SIZE_ALLOC);
 }
 
 /*
  * Initiate an idempotent transition to SRCU_SIZE_BIG.
  */
 static void srcu_transition_to_big(struct srcu_struct *ssp)
 {
     unsigned long flags;
 
     /* Double-checked locking on ->srcu_size-state. */
     if (smp_load_acquire(&ssp->srcu_size_state) != SRCU_SIZE_SMALL)
         return;
     spin_lock_irqsave_rcu_node(ssp, flags);
     if (smp_load_acquire(&ssp->srcu_size_state) != SRCU_SIZE_SMALL) {
         spin_unlock_irqrestore_rcu_node(ssp, flags);
         return;
     }
     __srcu_transition_to_big(ssp);
     spin_unlock_irqrestore_rcu_node(ssp, flags);
 }
 
 /*
  * Check to see if the just-encountered contention event justifies
  * a transition to SRCU_SIZE_BIG.
  */
 static void spin_lock_irqsave_check_contention(struct srcu_struct *ssp)
 {
     unsigned long j;
 
     if (!SRCU_SIZING_IS_CONTEND() || ssp->srcu_size_state)
         return;
     j = jiffies;
     if (ssp->srcu_size_jiffies != j) {
         ssp->srcu_size_jiffies = j;
         ssp->srcu_n_lock_retries = 0;
     }
     if (++ssp->srcu_n_lock_retries <= small_contention_lim)
         return;
     __srcu_transition_to_big(ssp);
 }
 
 /*
  * Acquire the specified srcu_data structure's ->lock, but check for
  * excessive contention, which results in initiation of a transition
  * to SRCU_SIZE_BIG.  But only if the srcutree.convert_to_big module
  * parameter permits this.
  */
 static void spin_lock_irqsave_sdp_contention(struct srcu_data *sdp, unsigned long *flags)
 {
     struct srcu_struct *ssp = sdp->ssp;
 
     if (spin_trylock_irqsave_rcu_node(sdp, *flags))
         return;
     spin_lock_irqsave_rcu_node(ssp, *flags);
     spin_lock_irqsave_check_contention(ssp);
     spin_unlock_irqrestore_rcu_node(ssp, *flags);
     spin_lock_irqsave_rcu_node(sdp, *flags);
 }
 
 /*
  * Acquire the specified srcu_struct structure's ->lock, but check for
  * excessive contention, which results in initiation of a transition
  * to SRCU_SIZE_BIG.  But only if the srcutree.convert_to_big module
  * parameter permits this.
  */
 static void spin_lock_irqsave_ssp_contention(struct srcu_struct *ssp, unsigned long *flags)
 {
     if (spin_trylock_irqsave_rcu_node(ssp, *flags))
         return;
     spin_lock_irqsave_rcu_node(ssp, *flags);
     spin_lock_irqsave_check_contention(ssp);
 }
 
 /*
  * First-use initialization of statically allocated srcu_struct
  * structure.  Wiring up the combining tree is more than can be
  * done with compile-time initialization, so this check is added
  * to each update-side SRCU primitive.  Use ssp->lock, which -is-
  * compile-time initialized, to resolve races involving multiple
  * CPUs trying to garner first-use privileges.
  */
 static void check_init_srcu_struct(struct srcu_struct *ssp)
 {
     unsigned long flags;
 
     /* The smp_load_acquire() pairs with the smp_store_release(). */
     if (!rcu_seq_state(smp_load_acquire(&ssp->srcu_gp_seq_needed))) /*^^^*/
         return; /* Already initialized. */
     spin_lock_irqsave_rcu_node(ssp, flags);
     if (!rcu_seq_state(ssp->srcu_gp_seq_needed)) {
         spin_unlock_irqrestore_rcu_node(ssp, flags);
         return;
     }
     init_srcu_struct_fields(ssp, true);
     spin_unlock_irqrestore_rcu_node(ssp, flags);
 }
 
 /*
  * Returns approximate total of the readers' ->srcu_lock_count[] values
  * for the rank of per-CPU counters specified by idx.
  */
 static unsigned long srcu_readers_lock_idx(struct srcu_struct *ssp, int idx)
 {
     int cpu;
     unsigned long sum = 0;
 
     for_each_possible_cpu(cpu) {
         struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
 
         sum += READ_ONCE(cpuc->srcu_lock_count[idx]);
     }
     return sum;
 }
 
 /*
  * Returns approximate total of the readers' ->srcu_unlock_count[] values
  * for the rank of per-CPU counters specified by idx.
  */
 static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
 {
     int cpu;
     unsigned long sum = 0;
 
     for_each_possible_cpu(cpu) {
         struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
 
         sum += READ_ONCE(cpuc->srcu_unlock_count[idx]);
     }
     return sum;
 }
 
 /*
  * Return true if the number of pre-existing readers is determined to
  * be zero.
  */
 static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
 {
     unsigned long unlocks;
 
     unlocks = srcu_readers_unlock_idx(ssp, idx);
 
     /*
      * Make sure that a lock is always counted if the corresponding
      * unlock is counted. Needs to be a smp_mb() as the read side may
      * contain a read from a variable that is written to before the
      * synchronize_srcu() in the write side. In this case smp_mb()s
      * A and B act like the store buffering pattern.
      *
      * This smp_mb() also pairs with smp_mb() C to prevent accesses
      * after the synchronize_srcu() from being executed before the
      * grace period ends.
      */
     smp_mb(); /* A */
 
     /*
      * If the locks are the same as the unlocks, then there must have
      * been no readers on this index at some time in between. This does
      * not mean that there are no more readers, as one could have read
      * the current index but not have incremented the lock counter yet.
      *
      * So suppose that the updater is preempted here for so long
      * that more than ULONG_MAX non-nested readers come and go in
      * the meantime.  It turns out that this cannot result in overflow
      * because if a reader modifies its unlock count after we read it
      * above, then that reader's next load of ->srcu_idx is guaranteed
      * to get the new value, which will cause it to operate on the
      * other bank of counters, where it cannot contribute to the
      * overflow of these counters.  This means that there is a maximum
      * of 2*NR_CPUS increments, which cannot overflow given current
      * systems, especially not on 64-bit systems.
      *
      * OK, how about nesting?  This does impose a limit on nesting
      * of floor(ULONG_MAX/NR_CPUS/2), which should be sufficient,
      * especially on 64-bit systems.
      */
     return srcu_readers_lock_idx(ssp, idx) == unlocks;
 }
 
 /**
  * srcu_readers_active - returns true if there are readers. and false
  *                       otherwise
  * @ssp: which srcu_struct to count active readers (holding srcu_read_lock).
  *
  * Note that this is not an atomic primitive, and can therefore suffer
  * severe errors when invoked on an active srcu_struct.  That said, it
  * can be useful as an error check at cleanup time.
  */
 static bool srcu_readers_active(struct srcu_struct *ssp)
 {
     int cpu;
     unsigned long sum = 0;
 
     for_each_possible_cpu(cpu) {
         struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
 
         sum += READ_ONCE(cpuc->srcu_lock_count[0]);
         sum += READ_ONCE(cpuc->srcu_lock_count[1]);
         sum -= READ_ONCE(cpuc->srcu_unlock_count[0]);
         sum -= READ_ONCE(cpuc->srcu_unlock_count[1]);
     }
     return sum;
 }
 
 /*
  * We use an adaptive strategy for synchronize_srcu() and especially for
  * synchronize_srcu_expedited().  We spin for a fixed time period
  * (defined below, boot time configurable) to allow SRCU readers to exit
  * their read-side critical sections.  If there are still some readers
  * after one jiffy, we repeatedly block for one jiffy time periods.
  * The blocking time is increased as the grace-period age increases,
  * with max blocking time capped at 10 jiffies.
  */
 #define SRCU_DEFAULT_RETRY_CHECK_DELAY      5
 
 static ulong srcu_retry_check_delay = SRCU_DEFAULT_RETRY_CHECK_DELAY;
 module_param(srcu_retry_check_delay, ulong, 0444);
 
 #define SRCU_INTERVAL       1       // Base delay if no expedited GPs pending.
 #define SRCU_MAX_INTERVAL   10      // Maximum incremental delay from slow readers.
 
 #define SRCU_DEFAULT_MAX_NODELAY_PHASE_LO   3UL // Lowmark on default per-GP-phase
                             // no-delay instances.
 #define SRCU_DEFAULT_MAX_NODELAY_PHASE_HI   1000UL  // Highmark on default per-GP-phase
                             // no-delay instances.
 
 #define SRCU_UL_CLAMP_LO(val, low)  ((val) > (low) ? (val) : (low))
 #define SRCU_UL_CLAMP_HI(val, high) ((val) < (high) ? (val) : (high))
 #define SRCU_UL_CLAMP(val, low, high)   SRCU_UL_CLAMP_HI(SRCU_UL_CLAMP_LO((val), (low)), (high))
 // per-GP-phase no-delay instances adjusted to allow non-sleeping poll upto
 // one jiffies time duration. Mult by 2 is done to factor in the srcu_get_delay()
 // called from process_srcu().
 #define SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED \
     (2UL * USEC_PER_SEC / HZ / SRCU_DEFAULT_RETRY_CHECK_DELAY)
 
 // Maximum per-GP-phase consecutive no-delay instances.
 #define SRCU_DEFAULT_MAX_NODELAY_PHASE  \
     SRCU_UL_CLAMP(SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED,  \
               SRCU_DEFAULT_MAX_NODELAY_PHASE_LO,    \
               SRCU_DEFAULT_MAX_NODELAY_PHASE_HI)
 
 static ulong srcu_max_nodelay_phase = SRCU_DEFAULT_MAX_NODELAY_PHASE;
 module_param(srcu_max_nodelay_phase, ulong, 0444);
 
 // Maximum consecutive no-delay instances.
 #define SRCU_DEFAULT_MAX_NODELAY    (SRCU_DEFAULT_MAX_NODELAY_PHASE > 100 ? \
                      SRCU_DEFAULT_MAX_NODELAY_PHASE : 100)
 
 static ulong srcu_max_nodelay = SRCU_DEFAULT_MAX_NODELAY;
 module_param(srcu_max_nodelay, ulong, 0444);
 
 /*
  * Return grace-period delay, zero if there are expedited grace
  * periods pending, SRCU_INTERVAL otherwise.
  */
 static unsigned long srcu_get_delay(struct srcu_struct *ssp)
 {
     unsigned long gpstart;
     unsigned long j;
     unsigned long jbase = SRCU_INTERVAL;
 
     if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq), READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
         jbase = 0;
     if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq))) {
         j = jiffies - 1;
         gpstart = READ_ONCE(ssp->srcu_gp_start);
         if (time_after(j, gpstart))
             jbase += j - gpstart;
         if (!jbase) {
             WRITE_ONCE(ssp->srcu_n_exp_nodelay, READ_ONCE(ssp->srcu_n_exp_nodelay) + 1);
             if (READ_ONCE(ssp->srcu_n_exp_nodelay) > srcu_max_nodelay_phase)
                 jbase = 1;
         }
     }
     return jbase > SRCU_MAX_INTERVAL ? SRCU_MAX_INTERVAL : jbase;
 }
 
 /**
  * cleanup_srcu_struct - deconstruct a sleep-RCU structure
  * @ssp: structure to clean up.
  *
  * Must invoke this after you are finished using a given srcu_struct that
  * was initialized via init_srcu_struct(), else you leak memory.
  */
 void cleanup_srcu_struct(struct srcu_struct *ssp)
 {
     int cpu;
 
     if (WARN_ON(!srcu_get_delay(ssp)))
         return; /* Just leak it! */
     if (WARN_ON(srcu_readers_active(ssp)))
         return; /* Just leak it! */
     flush_delayed_work(&ssp->work);
     for_each_possible_cpu(cpu) {
         struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
 
         del_timer_sync(&sdp->delay_work);
         flush_work(&sdp->work);
         if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
             return; /* Forgot srcu_barrier(), so just leak it! */
     }
     if (WARN_ON(rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) != SRCU_STATE_IDLE) ||
         WARN_ON(rcu_seq_current(&ssp->srcu_gp_seq) != ssp->srcu_gp_seq_needed) ||
         WARN_ON(srcu_readers_active(ssp))) {
         pr_info("%s: Active srcu_struct %p read state: %d gp state: %lu/%lu\n",
             __func__, ssp, rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)),
             rcu_seq_current(&ssp->srcu_gp_seq), ssp->srcu_gp_seq_needed);
         return; /* Caller forgot to stop doing call_srcu()? */
     }
     if (!ssp->sda_is_static) {
         free_percpu(ssp->sda);
         ssp->sda = NULL;
     }
     kfree(ssp->node);
     ssp->node = NULL;
     ssp->srcu_size_state = SRCU_SIZE_SMALL;
 }
 EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
 
 /*
  * Counts the new reader in the appropriate per-CPU element of the
  * srcu_struct.
  * Returns an index that must be passed to the matching srcu_read_unlock().
  */
 int __srcu_read_lock(struct srcu_struct *ssp)
 {
     int idx;
 
     idx = READ_ONCE(ssp->srcu_idx) & 0x1;
     this_cpu_inc(ssp->sda->srcu_lock_count[idx]);
     smp_mb(); /* B */  /* Avoid leaking the critical section. */
     return idx;
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock);
 
 /*
  * Removes the count for the old reader from the appropriate per-CPU
  * element of the srcu_struct.  Note that this may well be a different
  * CPU than that which was incremented by the corresponding srcu_read_lock().
  */
 void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
 {
     smp_mb(); /* C */  /* Avoid leaking the critical section. */
     this_cpu_inc(ssp->sda->srcu_unlock_count[idx]);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 
 /*
  * Start an SRCU grace period.
  */
 static void srcu_gp_start(struct srcu_struct *ssp)
 {
     struct srcu_data *sdp;
     int state;
 
     if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
         sdp = per_cpu_ptr(ssp->sda, 0);
     else
         sdp = this_cpu_ptr(ssp->sda);
     lockdep_assert_held(&ACCESS_PRIVATE(ssp, lock));
     WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
     spin_lock_rcu_node(sdp);  /* Interrupts already disabled. */
     rcu_segcblist_advance(&sdp->srcu_cblist,
                   rcu_seq_current(&ssp->srcu_gp_seq));
     (void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
                        rcu_seq_snap(&ssp->srcu_gp_seq));
     spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
     WRITE_ONCE(ssp->srcu_gp_start, jiffies);
     WRITE_ONCE(ssp->srcu_n_exp_nodelay, 0);
     smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
     rcu_seq_start(&ssp->srcu_gp_seq);
     state = rcu_seq_state(ssp->srcu_gp_seq);
     WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
 }
 
 
 static void srcu_delay_timer(struct timer_list *t)
 {
     struct srcu_data *sdp = container_of(t, struct srcu_data, delay_work);
 
     queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
 }
 
 static void srcu_queue_delayed_work_on(struct srcu_data *sdp,
                        unsigned long delay)
 {
     if (!delay) {
         queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
         return;
     }
 
     timer_reduce(&sdp->delay_work, jiffies + delay);
 }
 
 /*
  * Schedule callback invocation for the specified srcu_data structure,
  * if possible, on the corresponding CPU.
  */
 static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)
 {
     srcu_queue_delayed_work_on(sdp, delay);
 }
 
 /*
  * Schedule callback invocation for all srcu_data structures associated
  * with the specified srcu_node structure that have callbacks for the
  * just-completed grace period, the one corresponding to idx.  If possible,
  * schedule this invocation on the corresponding CPUs.
  */
 static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp,
                   unsigned long mask, unsigned long delay)
 {
     int cpu;
 
     for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
         if (!(mask & (1 << (cpu - snp->grplo))))
             continue;
         srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, cpu), delay);
     }
 }
 
 /*
  * Note the end of an SRCU grace period.  Initiates callback invocation
  * and starts a new grace period if needed.
  *
  * The ->srcu_cb_mutex acquisition does not protect any data, but
  * instead prevents more than one grace period from starting while we
  * are initiating callback invocation.  This allows the ->srcu_have_cbs[]
  * array to have a finite number of elements.
  */
 static void srcu_gp_end(struct srcu_struct *ssp)
 {
     unsigned long cbdelay = 1;
     bool cbs;
     bool last_lvl;
     int cpu;
     unsigned long flags;
     unsigned long gpseq;
     int idx;
     unsigned long mask;
     struct srcu_data *sdp;
     unsigned long sgsne;
     struct srcu_node *snp;
     int ss_state;
 
     /* Prevent more than one additional grace period. */
     mutex_lock(&ssp->srcu_cb_mutex);
 
     /* End the current grace period. */
     spin_lock_irq_rcu_node(ssp);
     idx = rcu_seq_state(ssp->srcu_gp_seq);
     WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
     if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq), READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
         cbdelay = 0;
 
     WRITE_ONCE(ssp->srcu_last_gp_end, ktime_get_mono_fast_ns());
     rcu_seq_end(&ssp->srcu_gp_seq);
     gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
     if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, gpseq))
         WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, gpseq);
     spin_unlock_irq_rcu_node(ssp);
     mutex_unlock(&ssp->srcu_gp_mutex);
     /* A new grace period can start at this point.  But only one. */
 
     /* Initiate callback invocation as needed. */
     ss_state = smp_load_acquire(&ssp->srcu_size_state);
     if (ss_state < SRCU_SIZE_WAIT_BARRIER) {
         srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, 0), cbdelay);
     } else {
         idx = rcu_seq_ctr(gpseq) % ARRAY_SIZE(snp->srcu_have_cbs);
         srcu_for_each_node_breadth_first(ssp, snp) {
             spin_lock_irq_rcu_node(snp);
             cbs = false;
             last_lvl = snp >= ssp->level[rcu_num_lvls - 1];
             if (last_lvl)
                 cbs = ss_state < SRCU_SIZE_BIG || snp->srcu_have_cbs[idx] == gpseq;
             snp->srcu_have_cbs[idx] = gpseq;
             rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
             sgsne = snp->srcu_gp_seq_needed_exp;
             if (srcu_invl_snp_seq(sgsne) || ULONG_CMP_LT(sgsne, gpseq))
                 WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
             if (ss_state < SRCU_SIZE_BIG)
                 mask = ~0;
             else
                 mask = snp->srcu_data_have_cbs[idx];
             snp->srcu_data_have_cbs[idx] = 0;
             spin_unlock_irq_rcu_node(snp);
             if (cbs)
                 srcu_schedule_cbs_snp(ssp, snp, mask, cbdelay);
         }
     }
 
     /* Occasionally prevent srcu_data counter wrap. */
     if (!(gpseq & counter_wrap_check))
         for_each_possible_cpu(cpu) {
             sdp = per_cpu_ptr(ssp->sda, cpu);
             spin_lock_irqsave_rcu_node(sdp, flags);
             if (ULONG_CMP_GE(gpseq, sdp->srcu_gp_seq_needed + 100))
                 sdp->srcu_gp_seq_needed = gpseq;
             if (ULONG_CMP_GE(gpseq, sdp->srcu_gp_seq_needed_exp + 100))
                 sdp->srcu_gp_seq_needed_exp = gpseq;
             spin_unlock_irqrestore_rcu_node(sdp, flags);
         }
 
     /* Callback initiation done, allow grace periods after next. */
     mutex_unlock(&ssp->srcu_cb_mutex);
 
     /* Start a new grace period if needed. */
     spin_lock_irq_rcu_node(ssp);
     gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
     if (!rcu_seq_state(gpseq) &&
         ULONG_CMP_LT(gpseq, ssp->srcu_gp_seq_needed)) {
         srcu_gp_start(ssp);
         spin_unlock_irq_rcu_node(ssp);
         srcu_reschedule(ssp, 0);
     } else {
         spin_unlock_irq_rcu_node(ssp);
     }
 
     /* Transition to big if needed. */
     if (ss_state != SRCU_SIZE_SMALL && ss_state != SRCU_SIZE_BIG) {
         if (ss_state == SRCU_SIZE_ALLOC)
             init_srcu_struct_nodes(ssp, GFP_KERNEL);
         else
             smp_store_release(&ssp->srcu_size_state, ss_state + 1);
     }
 }
 
 /*
  * Funnel-locking scheme to scalably mediate many concurrent expedited
  * grace-period requests.  This function is invoked for the first known
  * expedited request for a grace period that has already been requested,
  * but without expediting.  To start a completely new grace period,
  * whether expedited or not, use srcu_funnel_gp_start() instead.
  */
 static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp,
                   unsigned long s)
 {
     unsigned long flags;
     unsigned long sgsne;
 
     if (snp)
         for (; snp != NULL; snp = snp->srcu_parent) {
             sgsne = READ_ONCE(snp->srcu_gp_seq_needed_exp);
             if (rcu_seq_done(&ssp->srcu_gp_seq, s) ||
                 (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)))
                 return;
             spin_lock_irqsave_rcu_node(snp, flags);
             sgsne = snp->srcu_gp_seq_needed_exp;
             if (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)) {
                 spin_unlock_irqrestore_rcu_node(snp, flags);
                 return;
             }
             WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
             spin_unlock_irqrestore_rcu_node(snp, flags);
         }
     spin_lock_irqsave_ssp_contention(ssp, &flags);
     if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
         WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
     spin_unlock_irqrestore_rcu_node(ssp, flags);
 }
 
 /*
  * Funnel-locking scheme to scalably mediate many concurrent grace-period
  * requests.  The winner has to do the work of actually starting grace
  * period s.  Losers must either ensure that their desired grace-period
  * number is recorded on at least their leaf srcu_node structure, or they
  * must take steps to invoke their own callbacks.
  *
  * Note that this function also does the work of srcu_funnel_exp_start(),
  * in some cases by directly invoking it.
  */
 static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
                  unsigned long s, bool do_norm)
 {
     unsigned long flags;
     int idx = rcu_seq_ctr(s) % ARRAY_SIZE(sdp->mynode->srcu_have_cbs);
     unsigned long sgsne;
     struct srcu_node *snp;
     struct srcu_node *snp_leaf;
     unsigned long snp_seq;
 
     /* Ensure that snp node tree is fully initialized before traversing it */
     if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
         snp_leaf = NULL;
     else
         snp_leaf = sdp->mynode;
 
     if (snp_leaf)
         /* Each pass through the loop does one level of the srcu_node tree. */
         for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
             if (rcu_seq_done(&ssp->srcu_gp_seq, s) && snp != snp_leaf)
                 return; /* GP already done and CBs recorded. */
             spin_lock_irqsave_rcu_node(snp, flags);
             snp_seq = snp->srcu_have_cbs[idx];
             if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) {
                 if (snp == snp_leaf && snp_seq == s)
                     snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
                 spin_unlock_irqrestore_rcu_node(snp, flags);
                 if (snp == snp_leaf && snp_seq != s) {
                     srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0);
                     return;
                 }
                 if (!do_norm)
                     srcu_funnel_exp_start(ssp, snp, s);
                 return;
             }
             snp->srcu_have_cbs[idx] = s;
             if (snp == snp_leaf)
                 snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
             sgsne = snp->srcu_gp_seq_needed_exp;
             if (!do_norm && (srcu_invl_snp_seq(sgsne) || ULONG_CMP_LT(sgsne, s)))
                 WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
             spin_unlock_irqrestore_rcu_node(snp, flags);
         }
 
     /* Top of tree, must ensure the grace period will be started. */
     spin_lock_irqsave_ssp_contention(ssp, &flags);
     if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed, s)) {
         /*
          * Record need for grace period s.  Pair with load
          * acquire setting up for initialization.
          */
         smp_store_release(&ssp->srcu_gp_seq_needed, s); /*^^^*/
     }
     if (!do_norm && ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
         WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
 
     /* If grace period not already done and none in progress, start it. */
     if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&
         rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
         WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
         srcu_gp_start(ssp);
 
         // And how can that list_add() in the "else" clause
         // possibly be safe for concurrent execution?  Well,
         // it isn't.  And it does not have to be.  After all, it
         // can only be executed during early boot when there is only
         // the one boot CPU running with interrupts still disabled.
         if (likely(srcu_init_done))
             queue_delayed_work(rcu_gp_wq, &ssp->work,
                        !!srcu_get_delay(ssp));
         else if (list_empty(&ssp->work.work.entry))
             list_add(&ssp->work.work.entry, &srcu_boot_list);
     }
     spin_unlock_irqrestore_rcu_node(ssp, flags);
 }
 
 /*
  * Wait until all readers counted by array index idx complete, but
  * loop an additional time if there is an expedited grace period pending.
  * The caller must ensure that ->srcu_idx is not changed while checking.
  */
 static bool try_check_zero(struct srcu_struct *ssp, int idx, int trycount)
 {
     unsigned long curdelay;
 
     curdelay = !srcu_get_delay(ssp);
 
     for (;;) {
         if (srcu_readers_active_idx_check(ssp, idx))
             return true;
         if ((--trycount + curdelay) <= 0)
             return false;
         udelay(srcu_retry_check_delay);
     }
 }
 
 /*
  * Increment the ->srcu_idx counter so that future SRCU readers will
  * use the other rank of the ->srcu_(un)lock_count[] arrays.  This allows
  * us to wait for pre-existing readers in a starvation-free manner.
  */
 static void srcu_flip(struct srcu_struct *ssp)
 {
     /*
      * Ensure that if this updater saw a given reader's increment
      * from __srcu_read_lock(), that reader was using an old value
      * of ->srcu_idx.  Also ensure that if a given reader sees the
      * new value of ->srcu_idx, this updater's earlier scans cannot
      * have seen that reader's increments (which is OK, because this
      * grace period need not wait on that reader).
      */
     smp_mb(); /* E */  /* Pairs with B and C. */
 
     WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
 
     /*
      * Ensure that if the updater misses an __srcu_read_unlock()
      * increment, that task's next __srcu_read_lock() will see the
      * above counter update.  Note that both this memory barrier
      * and the one in srcu_readers_active_idx_check() provide the
      * guarantee for __srcu_read_lock().
      */
     smp_mb(); /* D */  /* Pairs with C. */
 }
 
 /*
  * If SRCU is likely idle, return true, otherwise return false.
  *
  * Note that it is OK for several current from-idle requests for a new
  * grace period from idle to specify expediting because they will all end
  * up requesting the same grace period anyhow.  So no loss.
  *
  * Note also that if any CPU (including the current one) is still invoking
  * callbacks, this function will nevertheless say "idle".  This is not
  * ideal, but the overhead of checking all CPUs' callback lists is even
  * less ideal, especially on large systems.  Furthermore, the wakeup
  * can happen before the callback is fully removed, so we have no choice
  * but to accept this type of error.
  *
  * This function is also subject to counter-wrap errors, but let's face
  * it, if this function was preempted for enough time for the counters
  * to wrap, it really doesn't matter whether or not we expedite the grace
  * period.  The extra overhead of a needlessly expedited grace period is
  * negligible when amortized over that time period, and the extra latency
  * of a needlessly non-expedited grace period is similarly negligible.
  */
 static bool srcu_might_be_idle(struct srcu_struct *ssp)
 {
     unsigned long curseq;
     unsigned long flags;
     struct srcu_data *sdp;
     unsigned long t;
     unsigned long tlast;
 
     check_init_srcu_struct(ssp);
     /* If the local srcu_data structure has callbacks, not idle.  */
     sdp = raw_cpu_ptr(ssp->sda);
     spin_lock_irqsave_rcu_node(sdp, flags);
     if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
         spin_unlock_irqrestore_rcu_node(sdp, flags);
         return false; /* Callbacks already present, so not idle. */
     }
     spin_unlock_irqrestore_rcu_node(sdp, flags);
 
     /*
      * No local callbacks, so probabilistically probe global state.
      * Exact information would require acquiring locks, which would
      * kill scalability, hence the probabilistic nature of the probe.
      */
 
     /* First, see if enough time has passed since the last GP. */
     t = ktime_get_mono_fast_ns();
     tlast = READ_ONCE(ssp->srcu_last_gp_end);
     if (exp_holdoff == 0 ||
         time_in_range_open(t, tlast, tlast + exp_holdoff))
         return false; /* Too soon after last GP. */
 
     /* Next, check for probable idleness. */
     curseq = rcu_seq_current(&ssp->srcu_gp_seq);
     smp_mb(); /* Order ->srcu_gp_seq with ->srcu_gp_seq_needed. */
     if (ULONG_CMP_LT(curseq, READ_ONCE(ssp->srcu_gp_seq_needed)))
         return false; /* Grace period in progress, so not idle. */
     smp_mb(); /* Order ->srcu_gp_seq with prior access. */
     if (curseq != rcu_seq_current(&ssp->srcu_gp_seq))
         return false; /* GP # changed, so not idle. */
     return true; /* With reasonable probability, idle! */
 }
 
 /*
  * SRCU callback function to leak a callback.
  */
 static void srcu_leak_callback(struct rcu_head *rhp)
 {
 }
 
 /*
  * Start an SRCU grace period, and also queue the callback if non-NULL.
  */
 static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
                          struct rcu_head *rhp, bool do_norm)
 {
     unsigned long flags;
     int idx;
     bool needexp = false;
     bool needgp = false;
     unsigned long s;
     struct srcu_data *sdp;
     struct srcu_node *sdp_mynode;
     int ss_state;
 
     check_init_srcu_struct(ssp);
     idx = srcu_read_lock(ssp);
     ss_state = smp_load_acquire(&ssp->srcu_size_state);
     if (ss_state < SRCU_SIZE_WAIT_CALL)
         sdp = per_cpu_ptr(ssp->sda, 0);
     else
         sdp = raw_cpu_ptr(ssp->sda);
     spin_lock_irqsave_sdp_contention(sdp, &flags);
     if (rhp)
         rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
     rcu_segcblist_advance(&sdp->srcu_cblist,
                   rcu_seq_current(&ssp->srcu_gp_seq));
     s = rcu_seq_snap(&ssp->srcu_gp_seq);
     (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
     if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
         sdp->srcu_gp_seq_needed = s;
         needgp = true;
     }
     if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
         sdp->srcu_gp_seq_needed_exp = s;
         needexp = true;
     }
     spin_unlock_irqrestore_rcu_node(sdp, flags);
 
     /* Ensure that snp node tree is fully initialized before traversing it */
     if (ss_state < SRCU_SIZE_WAIT_BARRIER)
         sdp_mynode = NULL;
     else
         sdp_mynode = sdp->mynode;
 
     if (needgp)
         srcu_funnel_gp_start(ssp, sdp, s, do_norm);
     else if (needexp)
         srcu_funnel_exp_start(ssp, sdp_mynode, s);
     srcu_read_unlock(ssp, idx);
     return s;
 }
 
 /*
  * Enqueue an SRCU callback on the srcu_data structure associated with
  * the current CPU and the specified srcu_struct structure, initiating
  * grace-period processing if it is not already running.
  *
  * Note that all CPUs must agree that the grace period extended beyond
  * all pre-existing SRCU read-side critical section.  On systems with
  * more than one CPU, this means that when "func()" is invoked, each CPU
  * is guaranteed to have executed a full memory barrier since the end of
  * its last corresponding SRCU read-side critical section whose beginning
  * preceded the call to call_srcu().  It also means that each CPU executing
  * an SRCU read-side critical section that continues beyond the start of
  * "func()" must have executed a memory barrier after the call_srcu()
  * but before the beginning of that SRCU read-side critical section.
  * Note that these guarantees include CPUs that are offline, idle, or
  * executing in user mode, as well as CPUs that are executing in the kernel.
  *
  * Furthermore, if CPU A invoked call_srcu() and CPU B invoked the
  * resulting SRCU callback function "func()", then both CPU A and CPU
  * B are guaranteed to execute a full memory barrier during the time
  * interval between the call to call_srcu() and the invocation of "func()".
  * This guarantee applies even if CPU A and CPU B are the same CPU (but
  * again only if the system has more than one CPU).
  *
  * Of course, these guarantees apply only for invocations of call_srcu(),
  * srcu_read_lock(), and srcu_read_unlock() that are all passed the same
  * srcu_struct structure.
  */
 static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
             rcu_callback_t func, bool do_norm)
 {
     if (debug_rcu_head_queue(rhp)) {
         /* Probable double call_srcu(), so leak the callback. */
         WRITE_ONCE(rhp->func, srcu_leak_callback);
         WARN_ONCE(1, "call_srcu(): Leaked duplicate callback\n");
         return;
     }
     rhp->func = func;
     (void)srcu_gp_start_if_needed(ssp, rhp, do_norm);
 }
 
 /**
  * call_srcu() - Queue a callback for invocation after an SRCU grace period
  * @ssp: srcu_struct in queue the callback
  * @rhp: structure to be used for queueing the SRCU callback.
  * @func: function to be invoked after the SRCU grace period
  *
  * The callback function will be invoked some time after a full SRCU
  * grace period elapses, in other words after all pre-existing SRCU
  * read-side critical sections have completed.  However, the callback
  * function might well execute concurrently with other SRCU read-side
  * critical sections that started after call_srcu() was invoked.  SRCU
  * read-side critical sections are delimited by srcu_read_lock() and
  * srcu_read_unlock(), and may be nested.
  *
  * The callback will be invoked from process context, but must nevertheless
  * be fast and must not block.
  */
 void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
            rcu_callback_t func)
 {
     __call_srcu(ssp, rhp, func, true);
 }
 EXPORT_SYMBOL_GPL(call_srcu);
 
 /*
  * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
  */
 static void __synchronize_srcu(struct srcu_struct *ssp, bool do_norm)
 {
     struct rcu_synchronize rcu;
 
     RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
              lock_is_held(&rcu_bh_lock_map) ||
              lock_is_held(&rcu_lock_map) ||
              lock_is_held(&rcu_sched_lock_map),
              "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
 
     if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
         return;
     might_sleep();
     check_init_srcu_struct(ssp);
     init_completion(&rcu.completion);
     init_rcu_head_on_stack(&rcu.head);
     __call_srcu(ssp, &rcu.head, wakeme_after_rcu, do_norm);
     wait_for_completion(&rcu.completion);
     destroy_rcu_head_on_stack(&rcu.head);
 
     /*
      * Make sure that later code is ordered after the SRCU grace
      * period.  This pairs with the spin_lock_irq_rcu_node()
      * in srcu_invoke_callbacks().  Unlike Tree RCU, this is needed
      * because the current CPU might have been totally uninvolved with
      * (and thus unordered against) that grace period.
      */
     smp_mb();
 }
 
 /**
  * synchronize_srcu_expedited - Brute-force SRCU grace period
  * @ssp: srcu_struct with which to synchronize.
  *
  * Wait for an SRCU grace period to elapse, but be more aggressive about
  * spinning rather than blocking when waiting.
  *
  * Note that synchronize_srcu_expedited() has the same deadlock and
  * memory-ordering properties as does synchronize_srcu().
  */
 void synchronize_srcu_expedited(struct srcu_struct *ssp)
 {
     __synchronize_srcu(ssp, rcu_gp_is_normal());
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
 
 /**
  * synchronize_srcu - wait for prior SRCU read-side critical-section completion
  * @ssp: srcu_struct with which to synchronize.
  *
  * Wait for the count to drain to zero of both indexes. To avoid the
  * possible starvation of synchronize_srcu(), it waits for the count of
  * the index=((->srcu_idx & 1) ^ 1) to drain to zero at first,
  * and then flip the srcu_idx and wait for the count of the other index.
  *
  * Can block; must be called from process context.
  *
  * Note that it is illegal to call synchronize_srcu() from the corresponding
  * SRCU read-side critical section; doing so will result in deadlock.
  * However, it is perfectly legal to call synchronize_srcu() on one
  * srcu_struct from some other srcu_struct's read-side critical section,
  * as long as the resulting graph of srcu_structs is acyclic.
  *
  * There are memory-ordering constraints implied by synchronize_srcu().
  * On systems with more than one CPU, when synchronize_srcu() returns,
  * each CPU is guaranteed to have executed a full memory barrier since
  * the end of its last corresponding SRCU read-side critical section
  * whose beginning preceded the call to synchronize_srcu().  In addition,
  * each CPU having an SRCU read-side critical section that extends beyond
  * the return from synchronize_srcu() is guaranteed to have executed a
  * full memory barrier after the beginning of synchronize_srcu() and before
  * the beginning of that SRCU read-side critical section.  Note that these
  * guarantees include CPUs that are offline, idle, or executing in user mode,
  * as well as CPUs that are executing in the kernel.
  *
  * Furthermore, if CPU A invoked synchronize_srcu(), which returned
  * to its caller on CPU B, then both CPU A and CPU B are guaranteed
  * to have executed a full memory barrier during the execution of
  * synchronize_srcu().  This guarantee applies even if CPU A and CPU B
  * are the same CPU, but again only if the system has more than one CPU.
  *
  * Of course, these memory-ordering guarantees apply only when
  * synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
  * passed the same srcu_struct structure.
  *
  * Implementation of these memory-ordering guarantees is similar to
  * that of synchronize_rcu().
  *
  * If SRCU is likely idle, expedite the first request.  This semantic
  * was provided by Classic SRCU, and is relied upon by its users, so TREE
  * SRCU must also provide it.  Note that detecting idleness is heuristic
  * and subject to both false positives and negatives.
  */
 void synchronize_srcu(struct srcu_struct *ssp)
 {
     if (srcu_might_be_idle(ssp) || rcu_gp_is_expedited())
         synchronize_srcu_expedited(ssp);
     else
         __synchronize_srcu(ssp, true);
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);
 
 /**
  * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
  * @ssp: srcu_struct to provide cookie for.
  *
  * This function returns a cookie that can be passed to
  * poll_state_synchronize_srcu(), which will return true if a full grace
  * period has elapsed in the meantime.  It is the caller's responsibility
  * to make sure that grace period happens, for example, by invoking
  * call_srcu() after return from get_state_synchronize_srcu().
  */
 unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
 {
     // Any prior manipulation of SRCU-protected data must happen
     // before the load from ->srcu_gp_seq.
     smp_mb();
     return rcu_seq_snap(&ssp->srcu_gp_seq);
 }
 EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
 
 /**
  * start_poll_synchronize_srcu - Provide cookie and start grace period
  * @ssp: srcu_struct to provide cookie for.
  *
  * This function returns a cookie that can be passed to
  * poll_state_synchronize_srcu(), which will return true if a full grace
  * period has elapsed in the meantime.  Unlike get_state_synchronize_srcu(),
  * this function also ensures that any needed SRCU grace period will be
  * started.  This convenience does come at a cost in terms of CPU overhead.
  */
 unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
 {
     return srcu_gp_start_if_needed(ssp, NULL, true);
 }
 EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
 
 /**
  * poll_state_synchronize_srcu - Has cookie's grace period ended?
  * @ssp: srcu_struct to provide cookie for.
  * @cookie: Return value from get_state_synchronize_srcu() or start_poll_synchronize_srcu().
  *
  * This function takes the cookie that was returned from either
  * get_state_synchronize_srcu() or start_poll_synchronize_srcu(), and
  * returns @true if an SRCU grace period elapsed since the time that the
  * cookie was created.
  *
  * Because cookies are finite in size, wrapping/overflow is possible.
  * This is more pronounced on 32-bit systems where cookies are 32 bits,
  * where in theory wrapping could happen in about 14 hours assuming
  * 25-microsecond expedited SRCU grace periods.  However, a more likely
  * overflow lower bound is on the order of 24 days in the case of
  * one-millisecond SRCU grace periods.  Of course, wrapping in a 64-bit
  * system requires geologic timespans, as in more than seven million years
  * even for expedited SRCU grace periods.
  *
  * Wrapping/overflow is much more of an issue for CONFIG_SMP=n systems
  * that also have CONFIG_PREEMPTION=n, which selects Tiny SRCU.  This uses
  * a 16-bit cookie, which rcutorture routinely wraps in a matter of a
  * few minutes.  If this proves to be a problem, this counter will be
  * expanded to the same size as for Tree SRCU.
  */
 bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
 {
     if (!rcu_seq_done(&ssp->srcu_gp_seq, cookie))
         return false;
     // Ensure that the end of the SRCU grace period happens before
     // any subsequent code that the caller might execute.
     smp_mb(); // ^^^
     return true;
 }
 EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
 
 /*
  * Callback function for srcu_barrier() use.
  */
 static void srcu_barrier_cb(struct rcu_head *rhp)
 {
     struct srcu_data *sdp;
     struct srcu_struct *ssp;
 
     sdp = container_of(rhp, struct srcu_data, srcu_barrier_head);
     ssp = sdp->ssp;
     if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt))
         complete(&ssp->srcu_barrier_completion);
 }
 
 /*
  * Enqueue an srcu_barrier() callback on the specified srcu_data
  * structure's ->cblist.  but only if that ->cblist already has at least one
  * callback enqueued.  Note that if a CPU already has callbacks enqueue,
  * it must have already registered the need for a future grace period,
  * so all we need do is enqueue a callback that will use the same grace
  * period as the last callback already in the queue.
  */
 static void srcu_barrier_one_cpu(struct srcu_struct *ssp, struct srcu_data *sdp)
 {
     spin_lock_irq_rcu_node(sdp);
     atomic_inc(&ssp->srcu_barrier_cpu_cnt);
     sdp->srcu_barrier_head.func = srcu_barrier_cb;
     debug_rcu_head_queue(&sdp->srcu_barrier_head);
     if (!rcu_segcblist_entrain(&sdp->srcu_cblist,
                    &sdp->srcu_barrier_head)) {
         debug_rcu_head_unqueue(&sdp->srcu_barrier_head);
         atomic_dec(&ssp->srcu_barrier_cpu_cnt);
     }
     spin_unlock_irq_rcu_node(sdp);
 }
 
 /**
  * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
  * @ssp: srcu_struct on which to wait for in-flight callbacks.
  */
 void srcu_barrier(struct srcu_struct *ssp)
 {
     int cpu;
     int idx;
     unsigned long s = rcu_seq_snap(&ssp->srcu_barrier_seq);
 
     check_init_srcu_struct(ssp);
     mutex_lock(&ssp->srcu_barrier_mutex);
     if (rcu_seq_done(&ssp->srcu_barrier_seq, s)) {
         smp_mb(); /* Force ordering following return. */
         mutex_unlock(&ssp->srcu_barrier_mutex);
         return; /* Someone else did our work for us. */
     }
     rcu_seq_start(&ssp->srcu_barrier_seq);
     init_completion(&ssp->srcu_barrier_completion);
 
     /* Initial count prevents reaching zero until all CBs are posted. */
     atomic_set(&ssp->srcu_barrier_cpu_cnt, 1);
 
     idx = srcu_read_lock(ssp);
     if (smp_load_acquire(&ssp->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
         srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, 0));
     else
         for_each_possible_cpu(cpu)
             srcu_barrier_one_cpu(ssp, per_cpu_ptr(ssp->sda, cpu));
     srcu_read_unlock(ssp, idx);
 
     /* Remove the initial count, at which point reaching zero can happen. */
     if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt))
         complete(&ssp->srcu_barrier_completion);
     wait_for_completion(&ssp->srcu_barrier_completion);
 
     rcu_seq_end(&ssp->srcu_barrier_seq);
     mutex_unlock(&ssp->srcu_barrier_mutex);
 }
 EXPORT_SYMBOL_GPL(srcu_barrier);
 
 /**
  * srcu_batches_completed - return batches completed.
  * @ssp: srcu_struct on which to report batch completion.
  *
  * Report the number of batches, correlated with, but not necessarily
  * precisely the same as, the number of grace periods that have elapsed.
  */
 unsigned long srcu_batches_completed(struct srcu_struct *ssp)
 {
     return READ_ONCE(ssp->srcu_idx);
 }
 EXPORT_SYMBOL_GPL(srcu_batches_completed);
 
 /*
  * Core SRCU state machine.  Push state bits of ->srcu_gp_seq
  * to SRCU_STATE_SCAN2, and invoke srcu_gp_end() when scan has
  * completed in that state.
  */
 static void srcu_advance_state(struct srcu_struct *ssp)
 {
     int idx;
 
     mutex_lock(&ssp->srcu_gp_mutex);
 
     /*
      * Because readers might be delayed for an extended period after
      * fetching ->srcu_idx for their index, at any point in time there
      * might well be readers using both idx=0 and idx=1.  We therefore
      * need to wait for readers to clear from both index values before
      * invoking a callback.
      *
      * The load-acquire ensures that we see the accesses performed
      * by the prior grace period.
      */
     idx = rcu_seq_state(smp_load_acquire(&ssp->srcu_gp_seq)); /* ^^^ */
     if (idx == SRCU_STATE_IDLE) {
         spin_lock_irq_rcu_node(ssp);
         if (ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed)) {
             WARN_ON_ONCE(rcu_seq_state(ssp->srcu_gp_seq));
             spin_unlock_irq_rcu_node(ssp);
             mutex_unlock(&ssp->srcu_gp_mutex);
             return;
         }
         idx = rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq));
         if (idx == SRCU_STATE_IDLE)
             srcu_gp_start(ssp);
         spin_unlock_irq_rcu_node(ssp);
         if (idx != SRCU_STATE_IDLE) {
             mutex_unlock(&ssp->srcu_gp_mutex);
             return; /* Someone else started the grace period. */
         }
     }
 
     if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
         idx = 1 ^ (ssp->srcu_idx & 1);
         if (!try_check_zero(ssp, idx, 1)) {
             mutex_unlock(&ssp->srcu_gp_mutex);
             return; /* readers present, retry later. */
         }
         srcu_flip(ssp);
         spin_lock_irq_rcu_node(ssp);
         rcu_seq_set_state(&ssp->srcu_gp_seq, SRCU_STATE_SCAN2);
         ssp->srcu_n_exp_nodelay = 0;
         spin_unlock_irq_rcu_node(ssp);
     }
 
     if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {
 
         /*
          * SRCU read-side critical sections are normally short,
          * so check at least twice in quick succession after a flip.
          */
         idx = 1 ^ (ssp->srcu_idx & 1);
         if (!try_check_zero(ssp, idx, 2)) {
             mutex_unlock(&ssp->srcu_gp_mutex);
             return; /* readers present, retry later. */
         }
         ssp->srcu_n_exp_nodelay = 0;
         srcu_gp_end(ssp);  /* Releases ->srcu_gp_mutex. */
     }
 }
 
 /*
  * Invoke a limited number of SRCU callbacks that have passed through
  * their grace period.  If there are more to do, SRCU will reschedule
  * the workqueue.  Note that needed memory barriers have been executed
  * in this task's context by srcu_readers_active_idx_check().
  */
 static void srcu_invoke_callbacks(struct work_struct *work)
 {
     long len;
     bool more;
     struct rcu_cblist ready_cbs;
     struct rcu_head *rhp;
     struct srcu_data *sdp;
     struct srcu_struct *ssp;
 
     sdp = container_of(work, struct srcu_data, work);
 
     ssp = sdp->ssp;
     rcu_cblist_init(&ready_cbs);
     spin_lock_irq_rcu_node(sdp);
     rcu_segcblist_advance(&sdp->srcu_cblist,
                   rcu_seq_current(&ssp->srcu_gp_seq));
     if (sdp->srcu_cblist_invoking ||
         !rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
         spin_unlock_irq_rcu_node(sdp);
         return;  /* Someone else on the job or nothing to do. */
     }
 
     /* We are on the job!  Extract and invoke ready callbacks. */
     sdp->srcu_cblist_invoking = true;
     rcu_segcblist_extract_done_cbs(&sdp->srcu_cblist, &ready_cbs);
     len = ready_cbs.len;
     spin_unlock_irq_rcu_node(sdp);
     rhp = rcu_cblist_dequeue(&ready_cbs);
     for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
         debug_rcu_head_unqueue(rhp);
         local_bh_disable();
         rhp->func(rhp);
         local_bh_enable();
     }
     WARN_ON_ONCE(ready_cbs.len);
 
     /*
      * Update counts, accelerate new callbacks, and if needed,
      * schedule another round of callback invocation.
      */
     spin_lock_irq_rcu_node(sdp);
     rcu_segcblist_add_len(&sdp->srcu_cblist, -len);
     (void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
                        rcu_seq_snap(&ssp->srcu_gp_seq));
     sdp->srcu_cblist_invoking = false;
     more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
     spin_unlock_irq_rcu_node(sdp);
     if (more)
         srcu_schedule_cbs_sdp(sdp, 0);
 }
 
 /*
  * Finished one round of SRCU grace period.  Start another if there are
  * more SRCU callbacks queued, otherwise put SRCU into not-running state.
  */
 static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay)
 {
     bool pushgp = true;
 
     spin_lock_irq_rcu_node(ssp);
     if (ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed)) {
         if (!WARN_ON_ONCE(rcu_seq_state(ssp->srcu_gp_seq))) {
             /* All requests fulfilled, time to go idle. */
             pushgp = false;
         }
     } else if (!rcu_seq_state(ssp->srcu_gp_seq)) {
         /* Outstanding request and no GP.  Start one. */
         srcu_gp_start(ssp);
     }
     spin_unlock_irq_rcu_node(ssp);
 
     if (pushgp)
         queue_delayed_work(rcu_gp_wq, &ssp->work, delay);
 }
 
 /*
  * This is the work-queue function that handles SRCU grace periods.
  */
 static void process_srcu(struct work_struct *work)
 {
     unsigned long curdelay;
     unsigned long j;
     struct srcu_struct *ssp;
 
     ssp = container_of(work, struct srcu_struct, work.work);
 
     srcu_advance_state(ssp);
     curdelay = srcu_get_delay(ssp);
     if (curdelay) {
         WRITE_ONCE(ssp->reschedule_count, 0);
     } else {
         j = jiffies;
         if (READ_ONCE(ssp->reschedule_jiffies) == j) {
             WRITE_ONCE(ssp->reschedule_count, READ_ONCE(ssp->reschedule_count) + 1);
             if (READ_ONCE(ssp->reschedule_count) > srcu_max_nodelay)
                 curdelay = 1;
         } else {
             WRITE_ONCE(ssp->reschedule_count, 1);
             WRITE_ONCE(ssp->reschedule_jiffies, j);
         }
     }
     srcu_reschedule(ssp, curdelay);
 }
 
 void srcutorture_get_gp_data(enum rcutorture_type test_type,
                  struct srcu_struct *ssp, int *flags,
                  unsigned long *gp_seq)
 {
     if (test_type != SRCU_FLAVOR)
         return;
     *flags = 0;
     *gp_seq = rcu_seq_current(&ssp->srcu_gp_seq);
 }
 EXPORT_SYMBOL_GPL(srcutorture_get_gp_data);
 
 static const char * const srcu_size_state_name[] = {
     "SRCU_SIZE_SMALL",
     "SRCU_SIZE_ALLOC",
     "SRCU_SIZE_WAIT_BARRIER",
     "SRCU_SIZE_WAIT_CALL",
     "SRCU_SIZE_WAIT_CBS1",
     "SRCU_SIZE_WAIT_CBS2",
     "SRCU_SIZE_WAIT_CBS3",
     "SRCU_SIZE_WAIT_CBS4",
     "SRCU_SIZE_BIG",
     "SRCU_SIZE_???",
 };
 
 void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
 {
     int cpu;
     int idx;
     unsigned long s0 = 0, s1 = 0;
     int ss_state = READ_ONCE(ssp->srcu_size_state);
     int ss_state_idx = ss_state;
 
     idx = ssp->srcu_idx & 0x1;
     if (ss_state < 0 || ss_state >= ARRAY_SIZE(srcu_size_state_name))
         ss_state_idx = ARRAY_SIZE(srcu_size_state_name) - 1;
     pr_alert("%s%s Tree SRCU g%ld state %d (%s)",
          tt, tf, rcu_seq_current(&ssp->srcu_gp_seq), ss_state,
          srcu_size_state_name[ss_state_idx]);
     if (!ssp->sda) {
         // Called after cleanup_srcu_struct(), perhaps.
         pr_cont(" No per-CPU srcu_data structures (->sda == NULL).\n");
     } else {
         pr_cont(" per-CPU(idx=%d):", idx);
         for_each_possible_cpu(cpu) {
             unsigned long l0, l1;
             unsigned long u0, u1;
             long c0, c1;
             struct srcu_data *sdp;
 
             sdp = per_cpu_ptr(ssp->sda, cpu);
             u0 = data_race(sdp->srcu_unlock_count[!idx]);
             u1 = data_race(sdp->srcu_unlock_count[idx]);
 
             /*
              * Make sure that a lock is always counted if the corresponding
              * unlock is counted.
              */
             smp_rmb();
 
             l0 = data_race(sdp->srcu_lock_count[!idx]);
             l1 = data_race(sdp->srcu_lock_count[idx]);
 
             c0 = l0 - u0;
             c1 = l1 - u1;
             pr_cont(" %d(%ld,%ld %c)",
                 cpu, c0, c1,
                 "C."[rcu_segcblist_empty(&sdp->srcu_cblist)]);
             s0 += c0;
             s1 += c1;
         }
         pr_cont(" T(%ld,%ld)\n", s0, s1);
     }
     if (SRCU_SIZING_IS_TORTURE())
         srcu_transition_to_big(ssp);
 }
 EXPORT_SYMBOL_GPL(srcu_torture_stats_print);
 
 static int __init srcu_bootup_announce(void)
 {
     pr_info("Hierarchical SRCU implementation.\n");
     if (exp_holdoff != DEFAULT_SRCU_EXP_HOLDOFF)
         pr_info("\tNon-default auto-expedite holdoff of %lu ns.\n", exp_holdoff);
     if (srcu_retry_check_delay != SRCU_DEFAULT_RETRY_CHECK_DELAY)
         pr_info("\tNon-default retry check delay of %lu us.\n", srcu_retry_check_delay);
     if (srcu_max_nodelay != SRCU_DEFAULT_MAX_NODELAY)
         pr_info("\tNon-default max no-delay of %lu.\n", srcu_max_nodelay);
     pr_info("\tMax phase no-delay instances is %lu.\n", srcu_max_nodelay_phase);
     return 0;
 }
 early_initcall(srcu_bootup_announce);
 
 void __init srcu_init(void)
 {
     struct srcu_struct *ssp;
 
     /* Decide on srcu_struct-size strategy. */
     if (SRCU_SIZING_IS(SRCU_SIZING_AUTO)) {
         if (nr_cpu_ids >= big_cpu_lim) {
             convert_to_big = SRCU_SIZING_INIT; // Don't bother waiting for contention.
             pr_info("%s: Setting srcu_struct sizes to big.\n", __func__);
         } else {
             convert_to_big = SRCU_SIZING_NONE | SRCU_SIZING_CONTEND;
             pr_info("%s: Setting srcu_struct sizes based on contention.\n", __func__);
         }
     }
 
     /*
      * Once that is set, call_srcu() can follow the normal path and
      * queue delayed work. This must follow RCU workqueues creation
      * and timers initialization.
      */
     srcu_init_done = true;
     while (!list_empty(&srcu_boot_list)) {
         ssp = list_first_entry(&srcu_boot_list, struct srcu_struct,
                       work.work.entry);
         list_del_init(&ssp->work.work.entry);
         if (SRCU_SIZING_IS(SRCU_SIZING_INIT) && ssp->srcu_size_state == SRCU_SIZE_SMALL)
             ssp->srcu_size_state = SRCU_SIZE_ALLOC;
         queue_work(rcu_gp_wq, &ssp->work.work);
     }
 }
 
 #ifdef CONFIG_MODULES
 
 /* Initialize any global-scope srcu_struct structures used by this module. */
 static int srcu_module_coming(struct module *mod)
 {
     int i;
     struct srcu_struct **sspp = mod->srcu_struct_ptrs;
     int ret;
 
     for (i = 0; i < mod->num_srcu_structs; i++) {
         ret = init_srcu_struct(*(sspp++));
         if (WARN_ON_ONCE(ret))
             return ret;
     }
     return 0;
 }
 
 /* Clean up any global-scope srcu_struct structures used by this module. */
 static void srcu_module_going(struct module *mod)
 {
     int i;
     struct srcu_struct **sspp = mod->srcu_struct_ptrs;
 
     for (i = 0; i < mod->num_srcu_structs; i++)
         cleanup_srcu_struct(*(sspp++));
 }
 
 /* Handle one module, either coming or going. */
 static int srcu_module_notify(struct notifier_block *self,
                   unsigned long val, void *data)
 {
     struct module *mod = data;
     int ret = 0;
 
     switch (val) {
     case MODULE_STATE_COMING:
         ret = srcu_module_coming(mod);
         break;
     case MODULE_STATE_GOING:
         srcu_module_going(mod);
         break;
     default:
         break;
     }
     return ret;
 }
 
 static struct notifier_block srcu_module_nb = {
     .notifier_call = srcu_module_notify,
     .priority = 0,
 };
 
 static __init int init_srcu_module_notifier(void)
 {
     int ret;
 
     ret = register_module_notifier(&srcu_module_nb);
     if (ret)
         pr_warn("Failed to register srcu module notifier\n");
     return ret;
 }
 late_initcall(init_srcu_module_notifier);
 
 #endif /* #ifdef CONFIG_MODULES */

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