OSCL-LXR linux-6.0.1/​kernel/​futex/​core.c	Source navigation Diff markup Identifier search General search
	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 // SPDX-License-Identifier: GPL-2.0-or-later
 /*
  *  Fast Userspace Mutexes (which I call "Futexes!").
  *  (C) Rusty Russell, IBM 2002
  *
  *  Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
  *  (C) Copyright 2003 Red Hat Inc, All Rights Reserved
  *
  *  Removed page pinning, fix privately mapped COW pages and other cleanups
  *  (C) Copyright 2003, 2004 Jamie Lokier
  *
  *  Robust futex support started by Ingo Molnar
  *  (C) Copyright 2006 Red Hat Inc, All Rights Reserved
  *  Thanks to Thomas Gleixner for suggestions, analysis and fixes.
  *
  *  PI-futex support started by Ingo Molnar and Thomas Gleixner
  *  Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
  *  Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
  *
  *  PRIVATE futexes by Eric Dumazet
  *  Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.com>
  *
  *  Requeue-PI support by Darren Hart <dvhltc@us.ibm.com>
  *  Copyright (C) IBM Corporation, 2009
  *  Thanks to Thomas Gleixner for conceptual design and careful reviews.
  *
  *  Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
  *  enough at me, Linus for the original (flawed) idea, Matthew
  *  Kirkwood for proof-of-concept implementation.
  *
  *  "The futexes are also cursed."
  *  "But they come in a choice of three flavours!"
  */
 #include <linux/compat.h>
 #include <linux/jhash.h>
 #include <linux/pagemap.h>
 #include <linux/memblock.h>
 #include <linux/fault-inject.h>
 #include <linux/slab.h>
 
 #include "futex.h"
 #include "../locking/rtmutex_common.h"
 
 /*
  * The base of the bucket array and its size are always used together
  * (after initialization only in futex_hash()), so ensure that they
  * reside in the same cacheline.
  */
 static struct {
     struct futex_hash_bucket *queues;
     unsigned long            hashsize;
 } __futex_data __read_mostly __aligned(2*sizeof(long));
 #define futex_queues   (__futex_data.queues)
 #define futex_hashsize (__futex_data.hashsize)
 
 
 /*
  * Fault injections for futexes.
  */
 #ifdef CONFIG_FAIL_FUTEX
 
 static struct {
     struct fault_attr attr;
 
     bool ignore_private;
 } fail_futex = {
     .attr = FAULT_ATTR_INITIALIZER,
     .ignore_private = false,
 };
 
 static int __init setup_fail_futex(char *str)
 {
     return setup_fault_attr(&fail_futex.attr, str);
 }
 __setup("fail_futex=", setup_fail_futex);
 
 bool should_fail_futex(bool fshared)
 {
     if (fail_futex.ignore_private && !fshared)
         return false;
 
     return should_fail(&fail_futex.attr, 1);
 }
 
 #ifdef CONFIG_FAULT_INJECTION_DEBUG_FS
 
 static int __init fail_futex_debugfs(void)
 {
     umode_t mode = S_IFREG | S_IRUSR | S_IWUSR;
     struct dentry *dir;
 
     dir = fault_create_debugfs_attr("fail_futex", NULL,
                     &fail_futex.attr);
     if (IS_ERR(dir))
         return PTR_ERR(dir);
 
     debugfs_create_bool("ignore-private", mode, dir,
                 &fail_futex.ignore_private);
     return 0;
 }
 
 late_initcall(fail_futex_debugfs);
 
 #endif /* CONFIG_FAULT_INJECTION_DEBUG_FS */
 
 #endif /* CONFIG_FAIL_FUTEX */
 
 /**
  * futex_hash - Return the hash bucket in the global hash
  * @key:    Pointer to the futex key for which the hash is calculated
  *
  * We hash on the keys returned from get_futex_key (see below) and return the
  * corresponding hash bucket in the global hash.
  */
 struct futex_hash_bucket *futex_hash(union futex_key *key)
 {
     u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
               key->both.offset);
 
     return &futex_queues[hash & (futex_hashsize - 1)];
 }
 
 
 /**
  * futex_setup_timer - set up the sleeping hrtimer.
  * @time:   ptr to the given timeout value
  * @timeout:    the hrtimer_sleeper structure to be set up
  * @flags:  futex flags
  * @range_ns:   optional range in ns
  *
  * Return: Initialized hrtimer_sleeper structure or NULL if no timeout
  *     value given
  */
 struct hrtimer_sleeper *
 futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout,
           int flags, u64 range_ns)
 {
     if (!time)
         return NULL;
 
     hrtimer_init_sleeper_on_stack(timeout, (flags & FLAGS_CLOCKRT) ?
                       CLOCK_REALTIME : CLOCK_MONOTONIC,
                       HRTIMER_MODE_ABS);
     /*
      * If range_ns is 0, calling hrtimer_set_expires_range_ns() is
      * effectively the same as calling hrtimer_set_expires().
      */
     hrtimer_set_expires_range_ns(&timeout->timer, *time, range_ns);
 
     return timeout;
 }
 
 /*
  * Generate a machine wide unique identifier for this inode.
  *
  * This relies on u64 not wrapping in the life-time of the machine; which with
  * 1ns resolution means almost 585 years.
  *
  * This further relies on the fact that a well formed program will not unmap
  * the file while it has a (shared) futex waiting on it. This mapping will have
  * a file reference which pins the mount and inode.
  *
  * If for some reason an inode gets evicted and read back in again, it will get
  * a new sequence number and will _NOT_ match, even though it is the exact same
  * file.
  *
  * It is important that futex_match() will never have a false-positive, esp.
  * for PI futexes that can mess up the state. The above argues that false-negatives
  * are only possible for malformed programs.
  */
 static u64 get_inode_sequence_number(struct inode *inode)
 {
     static atomic64_t i_seq;
     u64 old;
 
     /* Does the inode already have a sequence number? */
     old = atomic64_read(&inode->i_sequence);
     if (likely(old))
         return old;
 
     for (;;) {
         u64 new = atomic64_add_return(1, &i_seq);
         if (WARN_ON_ONCE(!new))
             continue;
 
         old = atomic64_cmpxchg_relaxed(&inode->i_sequence, 0, new);
         if (old)
             return old;
         return new;
     }
 }
 
 /**
  * get_futex_key() - Get parameters which are the keys for a futex
  * @uaddr:  virtual address of the futex
  * @fshared:    false for a PROCESS_PRIVATE futex, true for PROCESS_SHARED
  * @key:    address where result is stored.
  * @rw:     mapping needs to be read/write (values: FUTEX_READ,
  *              FUTEX_WRITE)
  *
  * Return: a negative error code or 0
  *
  * The key words are stored in @key on success.
  *
  * For shared mappings (when @fshared), the key is:
  *
  *   ( inode->i_sequence, page->index, offset_within_page )
  *
  * [ also see get_inode_sequence_number() ]
  *
  * For private mappings (or when !@fshared), the key is:
  *
  *   ( current->mm, address, 0 )
  *
  * This allows (cross process, where applicable) identification of the futex
  * without keeping the page pinned for the duration of the FUTEX_WAIT.
  *
  * lock_page() might sleep, the caller should not hold a spinlock.
  */
 int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
           enum futex_access rw)
 {
     unsigned long address = (unsigned long)uaddr;
     struct mm_struct *mm = current->mm;
     struct page *page, *tail;
     struct address_space *mapping;
     int err, ro = 0;
 
     /*
      * The futex address must be "naturally" aligned.
      */
     key->both.offset = address % PAGE_SIZE;
     if (unlikely((address % sizeof(u32)) != 0))
         return -EINVAL;
     address -= key->both.offset;
 
     if (unlikely(!access_ok(uaddr, sizeof(u32))))
         return -EFAULT;
 
     if (unlikely(should_fail_futex(fshared)))
         return -EFAULT;
 
     /*
      * PROCESS_PRIVATE futexes are fast.
      * As the mm cannot disappear under us and the 'key' only needs
      * virtual address, we dont even have to find the underlying vma.
      * Note : We do have to check 'uaddr' is a valid user address,
      *        but access_ok() should be faster than find_vma()
      */
     if (!fshared) {
         key->private.mm = mm;
         key->private.address = address;
         return 0;
     }
 
 again:
     /* Ignore any VERIFY_READ mapping (futex common case) */
     if (unlikely(should_fail_futex(true)))
         return -EFAULT;
 
     err = get_user_pages_fast(address, 1, FOLL_WRITE, &page);
     /*
      * If write access is not required (eg. FUTEX_WAIT), try
      * and get read-only access.
      */
     if (err == -EFAULT && rw == FUTEX_READ) {
         err = get_user_pages_fast(address, 1, 0, &page);
         ro = 1;
     }
     if (err < 0)
         return err;
     else
         err = 0;
 
     /*
      * The treatment of mapping from this point on is critical. The page
      * lock protects many things but in this context the page lock
      * stabilizes mapping, prevents inode freeing in the shared
      * file-backed region case and guards against movement to swap cache.
      *
      * Strictly speaking the page lock is not needed in all cases being
      * considered here and page lock forces unnecessarily serialization
      * From this point on, mapping will be re-verified if necessary and
      * page lock will be acquired only if it is unavoidable
      *
      * Mapping checks require the head page for any compound page so the
      * head page and mapping is looked up now. For anonymous pages, it
      * does not matter if the page splits in the future as the key is
      * based on the address. For filesystem-backed pages, the tail is
      * required as the index of the page determines the key. For
      * base pages, there is no tail page and tail == page.
      */
     tail = page;
     page = compound_head(page);
     mapping = READ_ONCE(page->mapping);
 
     /*
      * If page->mapping is NULL, then it cannot be a PageAnon
      * page; but it might be the ZERO_PAGE or in the gate area or
      * in a special mapping (all cases which we are happy to fail);
      * or it may have been a good file page when get_user_pages_fast
      * found it, but truncated or holepunched or subjected to
      * invalidate_complete_page2 before we got the page lock (also
      * cases which we are happy to fail).  And we hold a reference,
      * so refcount care in invalidate_inode_page's remove_mapping
      * prevents drop_caches from setting mapping to NULL beneath us.
      *
      * The case we do have to guard against is when memory pressure made
      * shmem_writepage move it from filecache to swapcache beneath us:
      * an unlikely race, but we do need to retry for page->mapping.
      */
     if (unlikely(!mapping)) {
         int shmem_swizzled;
 
         /*
          * Page lock is required to identify which special case above
          * applies. If this is really a shmem page then the page lock
          * will prevent unexpected transitions.
          */
         lock_page(page);
         shmem_swizzled = PageSwapCache(page) || page->mapping;
         unlock_page(page);
         put_page(page);
 
         if (shmem_swizzled)
             goto again;
 
         return -EFAULT;
     }
 
     /*
      * Private mappings are handled in a simple way.
      *
      * If the futex key is stored on an anonymous page, then the associated
      * object is the mm which is implicitly pinned by the calling process.
      *
      * NOTE: When userspace waits on a MAP_SHARED mapping, even if
      * it's a read-only handle, it's expected that futexes attach to
      * the object not the particular process.
      */
     if (PageAnon(page)) {
         /*
          * A RO anonymous page will never change and thus doesn't make
          * sense for futex operations.
          */
         if (unlikely(should_fail_futex(true)) || ro) {
             err = -EFAULT;
             goto out;
         }
 
         key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
         key->private.mm = mm;
         key->private.address = address;
 
     } else {
         struct inode *inode;
 
         /*
          * The associated futex object in this case is the inode and
          * the page->mapping must be traversed. Ordinarily this should
          * be stabilised under page lock but it's not strictly
          * necessary in this case as we just want to pin the inode, not
          * update the radix tree or anything like that.
          *
          * The RCU read lock is taken as the inode is finally freed
          * under RCU. If the mapping still matches expectations then the
          * mapping->host can be safely accessed as being a valid inode.
          */
         rcu_read_lock();
 
         if (READ_ONCE(page->mapping) != mapping) {
             rcu_read_unlock();
             put_page(page);
 
             goto again;
         }
 
         inode = READ_ONCE(mapping->host);
         if (!inode) {
             rcu_read_unlock();
             put_page(page);
 
             goto again;
         }
 
         key->both.offset |= FUT_OFF_INODE; /* inode-based key */
         key->shared.i_seq = get_inode_sequence_number(inode);
         key->shared.pgoff = page_to_pgoff(tail);
         rcu_read_unlock();
     }
 
 out:
     put_page(page);
     return err;
 }
 
 /**
  * fault_in_user_writeable() - Fault in user address and verify RW access
  * @uaddr:  pointer to faulting user space address
  *
  * Slow path to fixup the fault we just took in the atomic write
  * access to @uaddr.
  *
  * We have no generic implementation of a non-destructive write to the
  * user address. We know that we faulted in the atomic pagefault
  * disabled section so we can as well avoid the #PF overhead by
  * calling get_user_pages() right away.
  */
 int fault_in_user_writeable(u32 __user *uaddr)
 {
     struct mm_struct *mm = current->mm;
     int ret;
 
     mmap_read_lock(mm);
     ret = fixup_user_fault(mm, (unsigned long)uaddr,
                    FAULT_FLAG_WRITE, NULL);
     mmap_read_unlock(mm);
 
     return ret < 0 ? ret : 0;
 }
 
 /**
  * futex_top_waiter() - Return the highest priority waiter on a futex
  * @hb:     the hash bucket the futex_q's reside in
  * @key:    the futex key (to distinguish it from other futex futex_q's)
  *
  * Must be called with the hb lock held.
  */
 struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb, union futex_key *key)
 {
     struct futex_q *this;
 
     plist_for_each_entry(this, &hb->chain, list) {
         if (futex_match(&this->key, key))
             return this;
     }
     return NULL;
 }
 
 int futex_cmpxchg_value_locked(u32 *curval, u32 __user *uaddr, u32 uval, u32 newval)
 {
     int ret;
 
     pagefault_disable();
     ret = futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval);
     pagefault_enable();
 
     return ret;
 }
 
 int futex_get_value_locked(u32 *dest, u32 __user *from)
 {
     int ret;
 
     pagefault_disable();
     ret = __get_user(*dest, from);
     pagefault_enable();
 
     return ret ? -EFAULT : 0;
 }
 
 /**
  * wait_for_owner_exiting - Block until the owner has exited
  * @ret: owner's current futex lock status
  * @exiting:    Pointer to the exiting task
  *
  * Caller must hold a refcount on @exiting.
  */
 void wait_for_owner_exiting(int ret, struct task_struct *exiting)
 {
     if (ret != -EBUSY) {
         WARN_ON_ONCE(exiting);
         return;
     }
 
     if (WARN_ON_ONCE(ret == -EBUSY && !exiting))
         return;
 
     mutex_lock(&exiting->futex_exit_mutex);
     /*
      * No point in doing state checking here. If the waiter got here
      * while the task was in exec()->exec_futex_release() then it can
      * have any FUTEX_STATE_* value when the waiter has acquired the
      * mutex. OK, if running, EXITING or DEAD if it reached exit()
      * already. Highly unlikely and not a problem. Just one more round
      * through the futex maze.
      */
     mutex_unlock(&exiting->futex_exit_mutex);
 
     put_task_struct(exiting);
 }
 
 /**
  * __futex_unqueue() - Remove the futex_q from its futex_hash_bucket
  * @q:  The futex_q to unqueue
  *
  * The q->lock_ptr must not be NULL and must be held by the caller.
  */
 void __futex_unqueue(struct futex_q *q)
 {
     struct futex_hash_bucket *hb;
 
     if (WARN_ON_SMP(!q->lock_ptr) || WARN_ON(plist_node_empty(&q->list)))
         return;
     lockdep_assert_held(q->lock_ptr);
 
     hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock);
     plist_del(&q->list, &hb->chain);
     futex_hb_waiters_dec(hb);
 }
 
 /* The key must be already stored in q->key. */
 struct futex_hash_bucket *futex_q_lock(struct futex_q *q)
     __acquires(&hb->lock)
 {
     struct futex_hash_bucket *hb;
 
     hb = futex_hash(&q->key);
 
     /*
      * Increment the counter before taking the lock so that
      * a potential waker won't miss a to-be-slept task that is
      * waiting for the spinlock. This is safe as all futex_q_lock()
      * users end up calling futex_queue(). Similarly, for housekeeping,
      * decrement the counter at futex_q_unlock() when some error has
      * occurred and we don't end up adding the task to the list.
      */
     futex_hb_waiters_inc(hb); /* implies smp_mb(); (A) */
 
     q->lock_ptr = &hb->lock;
 
     spin_lock(&hb->lock);
     return hb;
 }
 
 void futex_q_unlock(struct futex_hash_bucket *hb)
     __releases(&hb->lock)
 {
     spin_unlock(&hb->lock);
     futex_hb_waiters_dec(hb);
 }
 
 void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)
 {
     int prio;
 
     /*
      * The priority used to register this element is
      * - either the real thread-priority for the real-time threads
      * (i.e. threads with a priority lower than MAX_RT_PRIO)
      * - or MAX_RT_PRIO for non-RT threads.
      * Thus, all RT-threads are woken first in priority order, and
      * the others are woken last, in FIFO order.
      */
     prio = min(current->normal_prio, MAX_RT_PRIO);
 
     plist_node_init(&q->list, prio);
     plist_add(&q->list, &hb->chain);
     q->task = current;
 }
 
 /**
  * futex_unqueue() - Remove the futex_q from its futex_hash_bucket
  * @q:  The futex_q to unqueue
  *
  * The q->lock_ptr must not be held by the caller. A call to futex_unqueue() must
  * be paired with exactly one earlier call to futex_queue().
  *
  * Return:
  *  - 1 - if the futex_q was still queued (and we removed unqueued it);
  *  - 0 - if the futex_q was already removed by the waking thread
  */
 int futex_unqueue(struct futex_q *q)
 {
     spinlock_t *lock_ptr;
     int ret = 0;
 
     /* In the common case we don't take the spinlock, which is nice. */
 retry:
     /*
      * q->lock_ptr can change between this read and the following spin_lock.
      * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
      * optimizing lock_ptr out of the logic below.
      */
     lock_ptr = READ_ONCE(q->lock_ptr);
     if (lock_ptr != NULL) {
         spin_lock(lock_ptr);
         /*
          * q->lock_ptr can change between reading it and
          * spin_lock(), causing us to take the wrong lock.  This
          * corrects the race condition.
          *
          * Reasoning goes like this: if we have the wrong lock,
          * q->lock_ptr must have changed (maybe several times)
          * between reading it and the spin_lock().  It can
          * change again after the spin_lock() but only if it was
          * already changed before the spin_lock().  It cannot,
          * however, change back to the original value.  Therefore
          * we can detect whether we acquired the correct lock.
          */
         if (unlikely(lock_ptr != q->lock_ptr)) {
             spin_unlock(lock_ptr);
             goto retry;
         }
         __futex_unqueue(q);
 
         BUG_ON(q->pi_state);
 
         spin_unlock(lock_ptr);
         ret = 1;
     }
 
     return ret;
 }
 
 /*
  * PI futexes can not be requeued and must remove themselves from the
  * hash bucket. The hash bucket lock (i.e. lock_ptr) is held.
  */
 void futex_unqueue_pi(struct futex_q *q)
 {
     __futex_unqueue(q);
 
     BUG_ON(!q->pi_state);
     put_pi_state(q->pi_state);
     q->pi_state = NULL;
 }
 
 /* Constants for the pending_op argument of handle_futex_death */
 #define HANDLE_DEATH_PENDING    true
 #define HANDLE_DEATH_LIST   false
 
 /*
  * Process a futex-list entry, check whether it's owned by the
  * dying task, and do notification if so:
  */
 static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
                   bool pi, bool pending_op)
 {
     u32 uval, nval, mval;
     int err;
 
     /* Futex address must be 32bit aligned */
     if ((((unsigned long)uaddr) % sizeof(*uaddr)) != 0)
         return -1;
 
 retry:
     if (get_user(uval, uaddr))
         return -1;
 
     /*
      * Special case for regular (non PI) futexes. The unlock path in
      * user space has two race scenarios:
      *
      * 1. The unlock path releases the user space futex value and
      *    before it can execute the futex() syscall to wake up
      *    waiters it is killed.
      *
      * 2. A woken up waiter is killed before it can acquire the
      *    futex in user space.
      *
      * In both cases the TID validation below prevents a wakeup of
      * potential waiters which can cause these waiters to block
      * forever.
      *
      * In both cases the following conditions are met:
      *
      *  1) task->robust_list->list_op_pending != NULL
      *     @pending_op == true
      *  2) User space futex value == 0
      *  3) Regular futex: @pi == false
      *
      * If these conditions are met, it is safe to attempt waking up a
      * potential waiter without touching the user space futex value and
      * trying to set the OWNER_DIED bit. The user space futex value is
      * uncontended and the rest of the user space mutex state is
      * consistent, so a woken waiter will just take over the
      * uncontended futex. Setting the OWNER_DIED bit would create
      * inconsistent state and malfunction of the user space owner died
      * handling.
      */
     if (pending_op && !pi && !uval) {
         futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
         return 0;
     }
 
     if ((uval & FUTEX_TID_MASK) != task_pid_vnr(curr))
         return 0;
 
     /*
      * Ok, this dying thread is truly holding a futex
      * of interest. Set the OWNER_DIED bit atomically
      * via cmpxchg, and if the value had FUTEX_WAITERS
      * set, wake up a waiter (if any). (We have to do a
      * futex_wake() even if OWNER_DIED is already set -
      * to handle the rare but possible case of recursive
      * thread-death.) The rest of the cleanup is done in
      * userspace.
      */
     mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
 
     /*
      * We are not holding a lock here, but we want to have
      * the pagefault_disable/enable() protection because
      * we want to handle the fault gracefully. If the
      * access fails we try to fault in the futex with R/W
      * verification via get_user_pages. get_user() above
      * does not guarantee R/W access. If that fails we
      * give up and leave the futex locked.
      */
     if ((err = futex_cmpxchg_value_locked(&nval, uaddr, uval, mval))) {
         switch (err) {
         case -EFAULT:
             if (fault_in_user_writeable(uaddr))
                 return -1;
             goto retry;
 
         case -EAGAIN:
             cond_resched();
             goto retry;
 
         default:
             WARN_ON_ONCE(1);
             return err;
         }
     }
 
     if (nval != uval)
         goto retry;
 
     /*
      * Wake robust non-PI futexes here. The wakeup of
      * PI futexes happens in exit_pi_state():
      */
     if (!pi && (uval & FUTEX_WAITERS))
         futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
 
     return 0;
 }
 
 /*
  * Fetch a robust-list pointer. Bit 0 signals PI futexes:
  */
 static inline int fetch_robust_entry(struct robust_list __user **entry,
                      struct robust_list __user * __user *head,
                      unsigned int *pi)
 {
     unsigned long uentry;
 
     if (get_user(uentry, (unsigned long __user *)head))
         return -EFAULT;
 
     *entry = (void __user *)(uentry & ~1UL);
     *pi = uentry & 1;
 
     return 0;
 }
 
 /*
  * Walk curr->robust_list (very carefully, it's a userspace list!)
  * and mark any locks found there dead, and notify any waiters.
  *
  * We silently return on any sign of list-walking problem.
  */
 static void exit_robust_list(struct task_struct *curr)
 {
     struct robust_list_head __user *head = curr->robust_list;
     struct robust_list __user *entry, *next_entry, *pending;
     unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
     unsigned int next_pi;
     unsigned long futex_offset;
     int rc;
 
     /*
      * Fetch the list head (which was registered earlier, via
      * sys_set_robust_list()):
      */
     if (fetch_robust_entry(&entry, &head->list.next, &pi))
         return;
     /*
      * Fetch the relative futex offset:
      */
     if (get_user(futex_offset, &head->futex_offset))
         return;
     /*
      * Fetch any possibly pending lock-add first, and handle it
      * if it exists:
      */
     if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
         return;
 
     next_entry = NULL;  /* avoid warning with gcc */
     while (entry != &head->list) {
         /*
          * Fetch the next entry in the list before calling
          * handle_futex_death:
          */
         rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
         /*
          * A pending lock might already be on the list, so
          * don't process it twice:
          */
         if (entry != pending) {
             if (handle_futex_death((void __user *)entry + futex_offset,
                         curr, pi, HANDLE_DEATH_LIST))
                 return;
         }
         if (rc)
             return;
         entry = next_entry;
         pi = next_pi;
         /*
          * Avoid excessively long or circular lists:
          */
         if (!--limit)
             break;
 
         cond_resched();
     }
 
     if (pending) {
         handle_futex_death((void __user *)pending + futex_offset,
                    curr, pip, HANDLE_DEATH_PENDING);
     }
 }
 
 #ifdef CONFIG_COMPAT
 static void __user *futex_uaddr(struct robust_list __user *entry,
                 compat_long_t futex_offset)
 {
     compat_uptr_t base = ptr_to_compat(entry);
     void __user *uaddr = compat_ptr(base + futex_offset);
 
     return uaddr;
 }
 
 /*
  * Fetch a robust-list pointer. Bit 0 signals PI futexes:
  */
 static inline int
 compat_fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
            compat_uptr_t __user *head, unsigned int *pi)
 {
     if (get_user(*uentry, head))
         return -EFAULT;
 
     *entry = compat_ptr((*uentry) & ~1);
     *pi = (unsigned int)(*uentry) & 1;
 
     return 0;
 }
 
 /*
  * Walk curr->robust_list (very carefully, it's a userspace list!)
  * and mark any locks found there dead, and notify any waiters.
  *
  * We silently return on any sign of list-walking problem.
  */
 static void compat_exit_robust_list(struct task_struct *curr)
 {
     struct compat_robust_list_head __user *head = curr->compat_robust_list;
     struct robust_list __user *entry, *next_entry, *pending;
     unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
     unsigned int next_pi;
     compat_uptr_t uentry, next_uentry, upending;
     compat_long_t futex_offset;
     int rc;
 
     /*
      * Fetch the list head (which was registered earlier, via
      * sys_set_robust_list()):
      */
     if (compat_fetch_robust_entry(&uentry, &entry, &head->list.next, &pi))
         return;
     /*
      * Fetch the relative futex offset:
      */
     if (get_user(futex_offset, &head->futex_offset))
         return;
     /*
      * Fetch any possibly pending lock-add first, and handle it
      * if it exists:
      */
     if (compat_fetch_robust_entry(&upending, &pending,
                    &head->list_op_pending, &pip))
         return;
 
     next_entry = NULL;  /* avoid warning with gcc */
     while (entry != (struct robust_list __user *) &head->list) {
         /*
          * Fetch the next entry in the list before calling
          * handle_futex_death:
          */
         rc = compat_fetch_robust_entry(&next_uentry, &next_entry,
             (compat_uptr_t __user *)&entry->next, &next_pi);
         /*
          * A pending lock might already be on the list, so
          * dont process it twice:
          */
         if (entry != pending) {
             void __user *uaddr = futex_uaddr(entry, futex_offset);
 
             if (handle_futex_death(uaddr, curr, pi,
                            HANDLE_DEATH_LIST))
                 return;
         }
         if (rc)
             return;
         uentry = next_uentry;
         entry = next_entry;
         pi = next_pi;
         /*
          * Avoid excessively long or circular lists:
          */
         if (!--limit)
             break;
 
         cond_resched();
     }
     if (pending) {
         void __user *uaddr = futex_uaddr(pending, futex_offset);
 
         handle_futex_death(uaddr, curr, pip, HANDLE_DEATH_PENDING);
     }
 }
 #endif
 
 #ifdef CONFIG_FUTEX_PI
 
 /*
  * This task is holding PI mutexes at exit time => bad.
  * Kernel cleans up PI-state, but userspace is likely hosed.
  * (Robust-futex cleanup is separate and might save the day for userspace.)
  */
 static void exit_pi_state_list(struct task_struct *curr)
 {
     struct list_head *next, *head = &curr->pi_state_list;
     struct futex_pi_state *pi_state;
     struct futex_hash_bucket *hb;
     union futex_key key = FUTEX_KEY_INIT;
 
     /*
      * We are a ZOMBIE and nobody can enqueue itself on
      * pi_state_list anymore, but we have to be careful
      * versus waiters unqueueing themselves:
      */
     raw_spin_lock_irq(&curr->pi_lock);
     while (!list_empty(head)) {
         next = head->next;
         pi_state = list_entry(next, struct futex_pi_state, list);
         key = pi_state->key;
         hb = futex_hash(&key);
 
         /*
          * We can race against put_pi_state() removing itself from the
          * list (a waiter going away). put_pi_state() will first
          * decrement the reference count and then modify the list, so
          * its possible to see the list entry but fail this reference
          * acquire.
          *
          * In that case; drop the locks to let put_pi_state() make
          * progress and retry the loop.
          */
         if (!refcount_inc_not_zero(&pi_state->refcount)) {
             raw_spin_unlock_irq(&curr->pi_lock);
             cpu_relax();
             raw_spin_lock_irq(&curr->pi_lock);
             continue;
         }
         raw_spin_unlock_irq(&curr->pi_lock);
 
         spin_lock(&hb->lock);
         raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
         raw_spin_lock(&curr->pi_lock);
         /*
          * We dropped the pi-lock, so re-check whether this
          * task still owns the PI-state:
          */
         if (head->next != next) {
             /* retain curr->pi_lock for the loop invariant */
             raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
             spin_unlock(&hb->lock);
             put_pi_state(pi_state);
             continue;
         }
 
         WARN_ON(pi_state->owner != curr);
         WARN_ON(list_empty(&pi_state->list));
         list_del_init(&pi_state->list);
         pi_state->owner = NULL;
 
         raw_spin_unlock(&curr->pi_lock);
         raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
         spin_unlock(&hb->lock);
 
         rt_mutex_futex_unlock(&pi_state->pi_mutex);
         put_pi_state(pi_state);
 
         raw_spin_lock_irq(&curr->pi_lock);
     }
     raw_spin_unlock_irq(&curr->pi_lock);
 }
 #else
 static inline void exit_pi_state_list(struct task_struct *curr) { }
 #endif
 
 static void futex_cleanup(struct task_struct *tsk)
 {
     if (unlikely(tsk->robust_list)) {
         exit_robust_list(tsk);
         tsk->robust_list = NULL;
     }
 
 #ifdef CONFIG_COMPAT
     if (unlikely(tsk->compat_robust_list)) {
         compat_exit_robust_list(tsk);
         tsk->compat_robust_list = NULL;
     }
 #endif
 
     if (unlikely(!list_empty(&tsk->pi_state_list)))
         exit_pi_state_list(tsk);
 }
 
 /**
  * futex_exit_recursive - Set the tasks futex state to FUTEX_STATE_DEAD
  * @tsk:    task to set the state on
  *
  * Set the futex exit state of the task lockless. The futex waiter code
  * observes that state when a task is exiting and loops until the task has
  * actually finished the futex cleanup. The worst case for this is that the
  * waiter runs through the wait loop until the state becomes visible.
  *
  * This is called from the recursive fault handling path in make_task_dead().
  *
  * This is best effort. Either the futex exit code has run already or
  * not. If the OWNER_DIED bit has been set on the futex then the waiter can
  * take it over. If not, the problem is pushed back to user space. If the
  * futex exit code did not run yet, then an already queued waiter might
  * block forever, but there is nothing which can be done about that.
  */
 void futex_exit_recursive(struct task_struct *tsk)
 {
     /* If the state is FUTEX_STATE_EXITING then futex_exit_mutex is held */
     if (tsk->futex_state == FUTEX_STATE_EXITING)
         mutex_unlock(&tsk->futex_exit_mutex);
     tsk->futex_state = FUTEX_STATE_DEAD;
 }
 
 static void futex_cleanup_begin(struct task_struct *tsk)
 {
     /*
      * Prevent various race issues against a concurrent incoming waiter
      * including live locks by forcing the waiter to block on
      * tsk->futex_exit_mutex when it observes FUTEX_STATE_EXITING in
      * attach_to_pi_owner().
      */
     mutex_lock(&tsk->futex_exit_mutex);
 
     /*
      * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
      *
      * This ensures that all subsequent checks of tsk->futex_state in
      * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
      * tsk->pi_lock held.
      *
      * It guarantees also that a pi_state which was queued right before
      * the state change under tsk->pi_lock by a concurrent waiter must
      * be observed in exit_pi_state_list().
      */
     raw_spin_lock_irq(&tsk->pi_lock);
     tsk->futex_state = FUTEX_STATE_EXITING;
     raw_spin_unlock_irq(&tsk->pi_lock);
 }
 
 static void futex_cleanup_end(struct task_struct *tsk, int state)
 {
     /*
      * Lockless store. The only side effect is that an observer might
      * take another loop until it becomes visible.
      */
     tsk->futex_state = state;
     /*
      * Drop the exit protection. This unblocks waiters which observed
      * FUTEX_STATE_EXITING to reevaluate the state.
      */
     mutex_unlock(&tsk->futex_exit_mutex);
 }
 
 void futex_exec_release(struct task_struct *tsk)
 {
     /*
      * The state handling is done for consistency, but in the case of
      * exec() there is no way to prevent further damage as the PID stays
      * the same. But for the unlikely and arguably buggy case that a
      * futex is held on exec(), this provides at least as much state
      * consistency protection which is possible.
      */
     futex_cleanup_begin(tsk);
     futex_cleanup(tsk);
     /*
      * Reset the state to FUTEX_STATE_OK. The task is alive and about
      * exec a new binary.
      */
     futex_cleanup_end(tsk, FUTEX_STATE_OK);
 }
 
 void futex_exit_release(struct task_struct *tsk)
 {
     futex_cleanup_begin(tsk);
     futex_cleanup(tsk);
     futex_cleanup_end(tsk, FUTEX_STATE_DEAD);
 }
 
 static int __init futex_init(void)
 {
     unsigned int futex_shift;
     unsigned long i;
 
 #if CONFIG_BASE_SMALL
     futex_hashsize = 16;
 #else
     futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
 #endif
 
     futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
                            futex_hashsize, 0,
                            futex_hashsize < 256 ? HASH_SMALL : 0,
                            &futex_shift, NULL,
                            futex_hashsize, futex_hashsize);
     futex_hashsize = 1UL << futex_shift;
 
     for (i = 0; i < futex_hashsize; i++) {
         atomic_set(&futex_queues[i].waiters, 0);
         plist_head_init(&futex_queues[i].chain);
         spin_lock_init(&futex_queues[i].lock);
     }
 
     return 0;
 }
 core_initcall(futex_init);

This page was automatically generated by the 2.1.0 LXR engine. The LXR team