/* * 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 * * 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!" * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */#include <linux/slab.h>#include <linux/poll.h>#include <linux/fs.h>#include <linux/file.h>#include <linux/jhash.h>#include <linux/init.h>#include <linux/futex.h>#include <linux/mount.h>#include <linux/pagemap.h>#include <linux/syscalls.h>#include <linux/signal.h> #define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8) /* * Futexes are matched on equal values of this key. * The key type depends on whether it's a shared or private mapping. * Don't rearrange members without looking at hash_futex(). * * offset is aligned to a multiple of sizeof(u32) (== 4) by definition. * We set bit 0 to indicate if it's an inode-based key. */union futex_key { struct { unsigned long pgoff; struct inode *inode; int offset; } shared; struct { unsigned long uaddr; struct mm_struct *mm; int offset; } private; struct { unsigned long word; void *ptr; int offset; } both;}; /* * We use this hashed waitqueue instead of a normal wait_queue_t, so * we can wake only the relevant ones (hashed queues may be shared). * * A futex_q has a woken state, just like tasks have TASK_RUNNING. * It is considered woken when list_empty(&q->list) || q->lock_ptr == 0. * The order of wakup is always to make the first condition true, then * wake up q->waiters, then make the second condition true. */struct futex_q { struct list_head list; wait_queue_head_t waiters; /* Which hash list lock to use. */ spinlock_t *lock_ptr; /* Key which the futex is hashed on. */ union futex_key key; /* For fd, sigio sent using these. */ int fd; struct file *filp;}; /* * Split the global futex_lock into every hash list lock. */struct futex_hash_bucket { spinlock_t lock; struct list_head chain;}; static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS]; /* Futex-fs vfsmount entry: */static struct vfsmount *futex_mnt; /* * We hash on the keys returned from get_futex_key (see below). */static struct futex_hash_bucket *hash_futex(union futex_key *key){ u32 hash = jhash2((u32*)&key->both.word, (sizeof(key->both.word)+sizeof(key->both.ptr))/4, key->both.offset); return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)];} /* * Return 1 if two futex_keys are equal, 0 otherwise. */static inline int match_futex(union futex_key *key1, union futex_key *key2){ return (key1->both.word == key2->both.word && key1->both.ptr == key2->both.ptr && key1->both.offset == key2->both.offset);} /* * Get parameters which are the keys for a futex. * * For shared mappings, it's (page->index, vma->vm_file->f_dentry->d_inode, * offset_within_page). For private mappings, it's (uaddr, current->mm). * We can usually work out the index without swapping in the page. * * Returns: 0, or negative error code. * The key words are stored in *key on success. * * Should be called with ¤t->mm->mmap_sem but NOT any spinlocks. */static int get_futex_key(unsigned long uaddr, union futex_key *key){ struct mm_struct *mm = current->mm; struct vm_area_struct *vma; struct page *page; int err; /* * The futex address must be "naturally" aligned. */ key->both.offset = uaddr % PAGE_SIZE; if (unlikely((key->both.offset % sizeof(u32)) != 0)) return -EINVAL; uaddr -= key->both.offset; /* * The futex is hashed differently depending on whether * it's in a shared or private mapping. So check vma first. */ vma = find_extend_vma(mm, uaddr); if (unlikely(!vma)) return -EFAULT; /* * Permissions. */ if (unlikely((vma->vm_flags & (VM_IO|VM_READ)) != VM_READ)) return (vma->vm_flags & VM_IO) ? -EPERM : -EACCES; /* * Private mappings are handled in a simple way. * * 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. Therefore we use * VM_MAYSHARE here, not VM_SHARED which is restricted to shared * mappings of _writable_ handles. */ if (likely(!(vma->vm_flags & VM_MAYSHARE))) { key->private.mm = mm; key->private.uaddr = uaddr; return 0; } /* * Linear file mappings are also simple. */ key->shared.inode = vma->vm_file->f_dentry->d_inode; key->both.offset++; /* Bit 0 of offset indicates inode-based key. */ if (likely(!(vma->vm_flags & VM_NONLINEAR))) { key->shared.pgoff = (((uaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff); return 0; } /* * We could walk the page table to read the non-linear * pte, and get the page index without fetching the page * from swap. But that's a lot of code to duplicate here * for a rare case, so we simply fetch the page. */ /* * Do a quick atomic lookup first - this is the fastpath. */ spin_lock(¤t->mm->page_table_lock); page = follow_page(mm, uaddr, 0); if (likely(page != NULL)) { key->shared.pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT); spin_unlock(¤t->mm->page_table_lock); return 0; } spin_unlock(¤t->mm->page_table_lock); /* * Do it the general way. */ err = get_user_pages(current, mm, uaddr, 1, 0, 0, &page, NULL); if (err >= 0) { key->shared.pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT); put_page(page); return 0; } return err;} /* * Take a reference to the resource addressed by a key. * Can be called while holding spinlocks. * * NOTE: mmap_sem MUST be held between get_futex_key() and calling this * function, if it is called at all. mmap_sem keeps key->shared.inode valid. */static inline void get_key_refs(union futex_key *key){ if (key->both.ptr != 0) { if (key->both.offset & 1) atomic_inc(&key->shared.inode->i_count); else atomic_inc(&key->private.mm->mm_count); }} /* * Drop a reference to the resource addressed by a key. * The hash bucket spinlock must not be held. */static void drop_key_refs(union futex_key *key){ if (key->both.ptr != 0) { if (key->both.offset & 1) iput(key->shared.inode); else mmdrop(key->private.mm); }} static inline int get_futex_value_locked(int *dest, int __user *from){ int ret; inc_preempt_count(); ret = __copy_from_user_inatomic(dest, from, sizeof(int)); dec_preempt_count(); return ret ? -EFAULT : 0;} /* * The hash bucket lock must be held when this is called. * Afterwards, the futex_q must not be accessed. */static void wake_futex(struct futex_q *q){ list_del_init(&q->list); if (q->filp) send_sigio(&q->filp->f_owner, q->fd, POLL_IN); /* * The lock in wake_up_all() is a crucial memory barrier after the * list_del_init() and also before assigning to q->lock_ptr. */ wake_up_all(&q->waiters); /* * The waiting task can free the futex_q as soon as this is written, * without taking any locks. This must come last. */ q->lock_ptr = NULL;} /* * Wake up all waiters hashed on the physical page that is mapped * to this virtual address: */static int futex_wake(unsigned long uaddr, int nr_wake){ union futex_key key; struct futex_hash_bucket *bh; struct list_head *head; struct futex_q *this, *next; int ret; down_read(¤t->mm->mmap_sem); ret = get_futex_key(uaddr, &key); if (unlikely(ret != 0)) goto out; bh = hash_futex(&key); spin_lock(&bh->lock); head = &bh->chain; list_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key)) { wake_futex(this); if (++ret >= nr_wake) break; } } spin_unlock(&bh->lock);out: up_read(¤t->mm->mmap_sem); return ret;} /* * Requeue all waiters hashed on one physical page to another * physical page. */static int futex_requeue(unsigned long uaddr1, unsigned long uaddr2, int nr_wake, int nr_requeue, int *valp){ union futex_key key1, key2; struct futex_hash_bucket *bh1, *bh2; struct list_head *head1; struct futex_q *this, *next; int ret, drop_count = 0; retry: down_read(¤t->mm->mmap_sem); ret = get_futex_key(uaddr1, &key1); if (unlikely(ret != 0)) goto out; ret = get_futex_key(uaddr2, &key2); if (unlikely(ret != 0)) goto out; bh1 = hash_futex(&key1); bh2 = hash_futex(&key2); if (bh1 < bh2) spin_lock(&bh1->lock); spin_lock(&bh2->lock); if (bh1 > bh2) spin_lock(&bh1->lock); if (likely(valp != NULL)) { int curval; ret = get_futex_value_locked(&curval, (int __user *)uaddr1); if (unlikely(ret)) { spin_unlock(&bh1->lock); if (bh1 != bh2) spin_unlock(&bh2->lock); /* If we would have faulted, release mmap_sem, fault * it in and start all over again. */ up_read(¤t->mm->mmap_sem); ret = get_user(curval, (int __user *)uaddr1); if (!ret) goto retry; return ret; } if (curval != *valp) { ret = -EAGAIN; goto out_unlock; } } head1 = &bh1->chain; list_for_each_entry_safe(this, next, head1, list) { if (!match_futex (&this->key, &key1)) continue; if (++ret <= nr_wake) { wake_futex(this); } else { list_move_tail(&this->list, &bh2->chain); this->lock_ptr = &bh2->lock; this->key = key2; get_key_refs(&key2); drop_count++; if (ret - nr_wake >= nr_requeue) break; /* Make sure to stop if key1 == key2 */ if (head1 == &bh2->chain && head1 != &next->list) head1 = &this->list; } } out_unlock: spin_unlock(&bh1->lock); if (bh1 != bh2) spin_unlock(&bh2->lock); /* drop_key_refs() must be called outside the spinlocks. */ while (--drop_count >= 0) drop_key_refs(&key1); out: up_read(¤t->mm->mmap_sem); return ret;} /* The key must be already stored in q->key. */static inline struct futex_hash_bucket *queue_lock(struct futex_q *q, int fd, struct file *filp){ struct futex_hash_bucket *bh; q->fd = fd; q->filp = filp; init_waitqueue_head(&q->waiters); get_key_refs(&q->key); bh = hash_futex(&q->key); q->lock_ptr = &bh->lock; spin_lock(&bh->lock); return bh;} static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *bh){ list_add_tail(&q->list, &bh->chain); spin_unlock(&bh->lock);} static inline voidqueue_unlock(struct futex_q *q, struct futex_hash_bucket *bh){ spin_unlock(&bh->lock); drop_key_refs(&q->key);} /* * queue_me and unqueue_me must be called as a pair, each * exactly once. They are called with the hashed spinlock held. */ /* The key must be already stored in q->key. */static void queue_me(struct futex_q *q, int fd, struct file *filp){ struct futex_hash_bucket *bh; bh = queue_lock(q, fd, filp); __queue_me(q, bh);} /* Return 1 if we were still queued (ie. 0 means we were woken) */static int unqueue_me(struct futex_q *q){ int ret = 0; spinlock_t *lock_ptr; /* In the common case we don't take the spinlock, which is nice. */ retry: lock_ptr = q->lock_ptr; if (lock_ptr != 0) { 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; } WARN_ON(list_empty(&q->list)); list_del(&q->list); spin_unlock(lock_ptr); ret = 1;Lines 1–500 of 799. Load more
/* * 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 * * 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!" * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ #include <linux/slab.h> #include <linux/poll.h> #include <linux/fs.h> #include <linux/file.h> #include <linux/jhash.h> #include <linux/init.h> #include <linux/futex.h> #include <linux/mount.h> #include <linux/pagemap.h> #include <linux/syscalls.h> #include <linux/signal.h> #define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8) /* * Futexes are matched on equal values of this key. * The key type depends on whether it's a shared or private mapping. * Don't rearrange members without looking at hash_futex(). * * offset is aligned to a multiple of sizeof(u32) (== 4) by definition. * We set bit 0 to indicate if it's an inode-based key. */ union futex_key { struct { unsigned long pgoff; struct inode *inode; int offset; } shared; struct { unsigned long uaddr; struct mm_struct *mm; int offset; } private; struct { unsigned long word; void *ptr; int offset; } both; }; /* * We use this hashed waitqueue instead of a normal wait_queue_t, so * we can wake only the relevant ones (hashed queues may be shared). * * A futex_q has a woken state, just like tasks have TASK_RUNNING. * It is considered woken when list_empty(&q->list) || q->lock_ptr == 0. * The order of wakup is always to make the first condition true, then * wake up q->waiters, then make the second condition true. */ struct futex_q { struct list_head list; wait_queue_head_t waiters; /* Which hash list lock to use. */ spinlock_t *lock_ptr; /* Key which the futex is hashed on. */ union futex_key key; /* For fd, sigio sent using these. */ int fd; struct file *filp; }; /* * Split the global futex_lock into every hash list lock. */ struct futex_hash_bucket { spinlock_t lock; struct list_head chain; }; static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS]; /* Futex-fs vfsmount entry: */ static struct vfsmount *futex_mnt; /* * We hash on the keys returned from get_futex_key (see below). */ static struct futex_hash_bucket *hash_futex(union futex_key *key) { u32 hash = jhash2((u32*)&key->both.word, (sizeof(key->both.word)+sizeof(key->both.ptr))/4, key->both.offset); return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)]; } /* * Return 1 if two futex_keys are equal, 0 otherwise. */ static inline int match_futex(union futex_key *key1, union futex_key *key2) { return (key1->both.word == key2->both.word && key1->both.ptr == key2->both.ptr && key1->both.offset == key2->both.offset); } /* * Get parameters which are the keys for a futex. * * For shared mappings, it's (page->index, vma->vm_file->f_dentry->d_inode, * offset_within_page). For private mappings, it's (uaddr, current->mm). * We can usually work out the index without swapping in the page. * * Returns: 0, or negative error code. * The key words are stored in *key on success. * * Should be called with ¤t->mm->mmap_sem but NOT any spinlocks. */ static int get_futex_key(unsigned long uaddr, union futex_key *key) { struct mm_struct *mm = current->mm; struct vm_area_struct *vma; struct page *page; int err; /* * The futex address must be "naturally" aligned. */ key->both.offset = uaddr % PAGE_SIZE; if (unlikely((key->both.offset % sizeof(u32)) != 0)) return -EINVAL; uaddr -= key->both.offset; /* * The futex is hashed differently depending on whether * it's in a shared or private mapping. So check vma first. */ vma = find_extend_vma(mm, uaddr); if (unlikely(!vma)) return -EFAULT; /* * Permissions. */ if (unlikely((vma->vm_flags & (VM_IO|VM_READ)) != VM_READ)) return (vma->vm_flags & VM_IO) ? -EPERM : -EACCES; /* * Private mappings are handled in a simple way. * * 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. Therefore we use * VM_MAYSHARE here, not VM_SHARED which is restricted to shared * mappings of _writable_ handles. */ if (likely(!(vma->vm_flags & VM_MAYSHARE))) { key->private.mm = mm; key->private.uaddr = uaddr; return 0; } /* * Linear file mappings are also simple. */ key->shared.inode = vma->vm_file->f_dentry->d_inode; key->both.offset++; /* Bit 0 of offset indicates inode-based key. */ if (likely(!(vma->vm_flags & VM_NONLINEAR))) { key->shared.pgoff = (((uaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff); return 0; } /* * We could walk the page table to read the non-linear * pte, and get the page index without fetching the page * from swap. But that's a lot of code to duplicate here * for a rare case, so we simply fetch the page. */ /* * Do a quick atomic lookup first - this is the fastpath. */ spin_lock(¤t->mm->page_table_lock); page = follow_page(mm, uaddr, 0); if (likely(page != NULL)) { key->shared.pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT); spin_unlock(¤t->mm->page_table_lock); return 0; } spin_unlock(¤t->mm->page_table_lock); /* * Do it the general way. */ err = get_user_pages(current, mm, uaddr, 1, 0, 0, &page, NULL); if (err >= 0) { key->shared.pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT); put_page(page); return 0; } return err; } /* * Take a reference to the resource addressed by a key. * Can be called while holding spinlocks. * * NOTE: mmap_sem MUST be held between get_futex_key() and calling this * function, if it is called at all. mmap_sem keeps key->shared.inode valid. */ static inline void get_key_refs(union futex_key *key) { if (key->both.ptr != 0) { if (key->both.offset & 1) atomic_inc(&key->shared.inode->i_count); else atomic_inc(&key->private.mm->mm_count); } } /* * Drop a reference to the resource addressed by a key. * The hash bucket spinlock must not be held. */ static void drop_key_refs(union futex_key *key) { if (key->both.ptr != 0) { if (key->both.offset & 1) iput(key->shared.inode); else mmdrop(key->private.mm); } } static inline int get_futex_value_locked(int *dest, int __user *from) { int ret; inc_preempt_count(); ret = __copy_from_user_inatomic(dest, from, sizeof(int)); dec_preempt_count(); return ret ? -EFAULT : 0; } /* * The hash bucket lock must be held when this is called. * Afterwards, the futex_q must not be accessed. */ static void wake_futex(struct futex_q *q) { list_del_init(&q->list); if (q->filp) send_sigio(&q->filp->f_owner, q->fd, POLL_IN); /* * The lock in wake_up_all() is a crucial memory barrier after the * list_del_init() and also before assigning to q->lock_ptr. */ wake_up_all(&q->waiters); /* * The waiting task can free the futex_q as soon as this is written, * without taking any locks. This must come last. */ q->lock_ptr = NULL; } /* * Wake up all waiters hashed on the physical page that is mapped * to this virtual address: */ static int futex_wake(unsigned long uaddr, int nr_wake) { union futex_key key; struct futex_hash_bucket *bh; struct list_head *head; struct futex_q *this, *next; int ret; down_read(¤t->mm->mmap_sem); ret = get_futex_key(uaddr, &key); if (unlikely(ret != 0)) goto out; bh = hash_futex(&key); spin_lock(&bh->lock); head = &bh->chain; list_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key)) { wake_futex(this); if (++ret >= nr_wake) break; } } spin_unlock(&bh->lock); out: up_read(¤t->mm->mmap_sem); return ret; } /* * Requeue all waiters hashed on one physical page to another * physical page. */ static int futex_requeue(unsigned long uaddr1, unsigned long uaddr2, int nr_wake, int nr_requeue, int *valp) { union futex_key key1, key2; struct futex_hash_bucket *bh1, *bh2; struct list_head *head1; struct futex_q *this, *next; int ret, drop_count = 0; retry: down_read(¤t->mm->mmap_sem); ret = get_futex_key(uaddr1, &key1); if (unlikely(ret != 0)) goto out; ret = get_futex_key(uaddr2, &key2); if (unlikely(ret != 0)) goto out; bh1 = hash_futex(&key1); bh2 = hash_futex(&key2); if (bh1 < bh2) spin_lock(&bh1->lock); spin_lock(&bh2->lock); if (bh1 > bh2) spin_lock(&bh1->lock); if (likely(valp != NULL)) { int curval; ret = get_futex_value_locked(&curval, (int __user *)uaddr1); if (unlikely(ret)) { spin_unlock(&bh1->lock); if (bh1 != bh2) spin_unlock(&bh2->lock); /* If we would have faulted, release mmap_sem, fault * it in and start all over again. */ up_read(¤t->mm->mmap_sem); ret = get_user(curval, (int __user *)uaddr1); if (!ret) goto retry; return ret; } if (curval != *valp) { ret = -EAGAIN; goto out_unlock; } } head1 = &bh1->chain; list_for_each_entry_safe(this, next, head1, list) { if (!match_futex (&this->key, &key1)) continue; if (++ret <= nr_wake) { wake_futex(this); } else { list_move_tail(&this->list, &bh2->chain); this->lock_ptr = &bh2->lock; this->key = key2; get_key_refs(&key2); drop_count++; if (ret - nr_wake >= nr_requeue) break; /* Make sure to stop if key1 == key2 */ if (head1 == &bh2->chain && head1 != &next->list) head1 = &this->list; } } out_unlock: spin_unlock(&bh1->lock); if (bh1 != bh2) spin_unlock(&bh2->lock); /* drop_key_refs() must be called outside the spinlocks. */ while (--drop_count >= 0) drop_key_refs(&key1); out: up_read(¤t->mm->mmap_sem); return ret; } /* The key must be already stored in q->key. */ static inline struct futex_hash_bucket * queue_lock(struct futex_q *q, int fd, struct file *filp) { struct futex_hash_bucket *bh; q->fd = fd; q->filp = filp; init_waitqueue_head(&q->waiters); get_key_refs(&q->key); bh = hash_futex(&q->key); q->lock_ptr = &bh->lock; spin_lock(&bh->lock); return bh; } static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *bh) { list_add_tail(&q->list, &bh->chain); spin_unlock(&bh->lock); } static inline void queue_unlock(struct futex_q *q, struct futex_hash_bucket *bh) { spin_unlock(&bh->lock); drop_key_refs(&q->key); } /* * queue_me and unqueue_me must be called as a pair, each * exactly once. They are called with the hashed spinlock held. */ /* The key must be already stored in q->key. */ static void queue_me(struct futex_q *q, int fd, struct file *filp) { struct futex_hash_bucket *bh; bh = queue_lock(q, fd, filp); __queue_me(q, bh); } /* Return 1 if we were still queued (ie. 0 means we were woken) */ static int unqueue_me(struct futex_q *q) { int ret = 0; spinlock_t *lock_ptr; /* In the common case we don't take the spinlock, which is nice. */ retry: lock_ptr = q->lock_ptr; if (lock_ptr != 0) { 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; } WARN_ON(list_empty(&q->list)); list_del(&q->list); spin_unlock(lock_ptr); ret = 1;