2 * Fast Userspace Mutexes (which I call "Futexes!").
3 * (C) Rusty Russell, IBM 2002
5 * Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
6 * (C) Copyright 2003 Red Hat Inc, All Rights Reserved
8 * Removed page pinning, fix privately mapped COW pages and other cleanups
9 * (C) Copyright 2003, 2004 Jamie Lokier
11 * Robust futex support started by Ingo Molnar
12 * (C) Copyright 2006 Red Hat Inc, All Rights Reserved
13 * Thanks to Thomas Gleixner for suggestions, analysis and fixes.
15 * PI-futex support started by Ingo Molnar and Thomas Gleixner
16 * Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
17 * Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
19 * PRIVATE futexes by Eric Dumazet
20 * Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.com>
22 * Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
23 * enough at me, Linus for the original (flawed) idea, Matthew
24 * Kirkwood for proof-of-concept implementation.
26 * "The futexes are also cursed."
27 * "But they come in a choice of three flavours!"
29 * This program is free software; you can redistribute it and/or modify
30 * it under the terms of the GNU General Public License as published by
31 * the Free Software Foundation; either version 2 of the License, or
32 * (at your option) any later version.
34 * This program is distributed in the hope that it will be useful,
35 * but WITHOUT ANY WARRANTY; without even the implied warranty of
36 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
37 * GNU General Public License for more details.
39 * You should have received a copy of the GNU General Public License
40 * along with this program; if not, write to the Free Software
41 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
43 #include <linux/slab.h>
44 #include <linux/poll.h>
46 #include <linux/file.h>
47 #include <linux/jhash.h>
48 #include <linux/init.h>
49 #include <linux/futex.h>
50 #include <linux/mount.h>
51 #include <linux/pagemap.h>
52 #include <linux/syscalls.h>
53 #include <linux/signal.h>
54 #include <linux/module.h>
55 #include <linux/magic.h>
56 #include <linux/pid.h>
57 #include <linux/nsproxy.h>
59 #include <asm/futex.h>
61 #include "rtmutex_common.h"
63 int __read_mostly futex_cmpxchg_enabled;
65 #define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)
68 * Priority Inheritance state:
70 struct futex_pi_state {
72 * list of 'owned' pi_state instances - these have to be
73 * cleaned up in do_exit() if the task exits prematurely:
75 struct list_head list;
80 struct rt_mutex pi_mutex;
82 struct task_struct *owner;
89 * We use this hashed waitqueue instead of a normal wait_queue_t, so
90 * we can wake only the relevant ones (hashed queues may be shared).
92 * A futex_q has a woken state, just like tasks have TASK_RUNNING.
93 * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
94 * The order of wakup is always to make the first condition true, then
95 * wake up q->waiters, then make the second condition true.
98 struct plist_node list;
99 wait_queue_head_t waiters;
101 /* Which hash list lock to use: */
102 spinlock_t *lock_ptr;
104 /* Key which the futex is hashed on: */
107 /* Optional priority inheritance state: */
108 struct futex_pi_state *pi_state;
109 struct task_struct *task;
111 /* Bitset for the optional bitmasked wakeup */
116 * Split the global futex_lock into every hash list lock.
118 struct futex_hash_bucket {
120 struct plist_head chain;
123 static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];
126 * We hash on the keys returned from get_futex_key (see below).
128 static struct futex_hash_bucket *hash_futex(union futex_key *key)
130 u32 hash = jhash2((u32*)&key->both.word,
131 (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
133 return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)];
137 * Return 1 if two futex_keys are equal, 0 otherwise.
139 static inline int match_futex(union futex_key *key1, union futex_key *key2)
141 return (key1->both.word == key2->both.word
142 && key1->both.ptr == key2->both.ptr
143 && key1->both.offset == key2->both.offset);
147 * Take a reference to the resource addressed by a key.
148 * Can be called while holding spinlocks.
151 static void get_futex_key_refs(union futex_key *key)
156 switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
158 atomic_inc(&key->shared.inode->i_count);
160 case FUT_OFF_MMSHARED:
161 atomic_inc(&key->private.mm->mm_count);
167 * Drop a reference to the resource addressed by a key.
168 * The hash bucket spinlock must not be held.
170 static void drop_futex_key_refs(union futex_key *key)
175 switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
177 iput(key->shared.inode);
179 case FUT_OFF_MMSHARED:
180 mmdrop(key->private.mm);
186 * get_futex_key - Get parameters which are the keys for a futex.
187 * @uaddr: virtual address of the futex
188 * @shared: NULL for a PROCESS_PRIVATE futex,
189 * ¤t->mm->mmap_sem for a PROCESS_SHARED futex
190 * @key: address where result is stored.
192 * Returns a negative error code or 0
193 * The key words are stored in *key on success.
195 * For shared mappings, it's (page->index, vma->vm_file->f_path.dentry->d_inode,
196 * offset_within_page). For private mappings, it's (uaddr, current->mm).
197 * We can usually work out the index without swapping in the page.
199 * fshared is NULL for PROCESS_PRIVATE futexes
200 * For other futexes, it points to ¤t->mm->mmap_sem and
201 * caller must have taken the reader lock. but NOT any spinlocks.
203 static int get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
205 unsigned long address = (unsigned long)uaddr;
206 struct mm_struct *mm = current->mm;
211 * The futex address must be "naturally" aligned.
213 key->both.offset = address % PAGE_SIZE;
214 if (unlikely((address % sizeof(u32)) != 0))
216 address -= key->both.offset;
219 * PROCESS_PRIVATE futexes are fast.
220 * As the mm cannot disappear under us and the 'key' only needs
221 * virtual address, we dont even have to find the underlying vma.
222 * Note : We do have to check 'uaddr' is a valid user address,
223 * but access_ok() should be faster than find_vma()
226 if (unlikely(!access_ok(VERIFY_WRITE, uaddr, sizeof(u32))))
228 key->private.mm = mm;
229 key->private.address = address;
230 get_futex_key_refs(key);
235 err = get_user_pages_fast(address, 1, 0, &page);
240 if (!page->mapping) {
247 * Private mappings are handled in a simple way.
249 * NOTE: When userspace waits on a MAP_SHARED mapping, even if
250 * it's a read-only handle, it's expected that futexes attach to
251 * the object not the particular process.
253 if (PageAnon(page)) {
254 key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
255 key->private.mm = mm;
256 key->private.address = address;
258 key->both.offset |= FUT_OFF_INODE; /* inode-based key */
259 key->shared.inode = page->mapping->host;
260 key->shared.pgoff = page->index;
263 get_futex_key_refs(key);
271 void put_futex_key(int fshared, union futex_key *key)
273 drop_futex_key_refs(key);
276 static u32 cmpxchg_futex_value_locked(u32 __user *uaddr, u32 uval, u32 newval)
281 curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval);
287 static int get_futex_value_locked(u32 *dest, u32 __user *from)
292 ret = __copy_from_user_inatomic(dest, from, sizeof(u32));
295 return ret ? -EFAULT : 0;
301 static int futex_handle_fault(unsigned long address, int attempt)
303 struct vm_area_struct * vma;
304 struct mm_struct *mm = current->mm;
310 down_read(&mm->mmap_sem);
311 vma = find_vma(mm, address);
312 if (vma && address >= vma->vm_start &&
313 (vma->vm_flags & VM_WRITE)) {
315 fault = handle_mm_fault(mm, vma, address, 1);
316 if (unlikely((fault & VM_FAULT_ERROR))) {
318 /* XXX: let's do this when we verify it is OK */
319 if (ret & VM_FAULT_OOM)
324 if (fault & VM_FAULT_MAJOR)
330 up_read(&mm->mmap_sem);
337 static int refill_pi_state_cache(void)
339 struct futex_pi_state *pi_state;
341 if (likely(current->pi_state_cache))
344 pi_state = kzalloc(sizeof(*pi_state), GFP_KERNEL);
349 INIT_LIST_HEAD(&pi_state->list);
350 /* pi_mutex gets initialized later */
351 pi_state->owner = NULL;
352 atomic_set(&pi_state->refcount, 1);
353 pi_state->key = FUTEX_KEY_INIT;
355 current->pi_state_cache = pi_state;
360 static struct futex_pi_state * alloc_pi_state(void)
362 struct futex_pi_state *pi_state = current->pi_state_cache;
365 current->pi_state_cache = NULL;
370 static void free_pi_state(struct futex_pi_state *pi_state)
372 if (!atomic_dec_and_test(&pi_state->refcount))
376 * If pi_state->owner is NULL, the owner is most probably dying
377 * and has cleaned up the pi_state already
379 if (pi_state->owner) {
380 spin_lock_irq(&pi_state->owner->pi_lock);
381 list_del_init(&pi_state->list);
382 spin_unlock_irq(&pi_state->owner->pi_lock);
384 rt_mutex_proxy_unlock(&pi_state->pi_mutex, pi_state->owner);
387 if (current->pi_state_cache)
391 * pi_state->list is already empty.
392 * clear pi_state->owner.
393 * refcount is at 0 - put it back to 1.
395 pi_state->owner = NULL;
396 atomic_set(&pi_state->refcount, 1);
397 current->pi_state_cache = pi_state;
402 * Look up the task based on what TID userspace gave us.
405 static struct task_struct * futex_find_get_task(pid_t pid)
407 struct task_struct *p;
410 p = find_task_by_vpid(pid);
411 if (!p || ((current->euid != p->euid) && (current->euid != p->uid)))
422 * This task is holding PI mutexes at exit time => bad.
423 * Kernel cleans up PI-state, but userspace is likely hosed.
424 * (Robust-futex cleanup is separate and might save the day for userspace.)
426 void exit_pi_state_list(struct task_struct *curr)
428 struct list_head *next, *head = &curr->pi_state_list;
429 struct futex_pi_state *pi_state;
430 struct futex_hash_bucket *hb;
431 union futex_key key = FUTEX_KEY_INIT;
433 if (!futex_cmpxchg_enabled)
436 * We are a ZOMBIE and nobody can enqueue itself on
437 * pi_state_list anymore, but we have to be careful
438 * versus waiters unqueueing themselves:
440 spin_lock_irq(&curr->pi_lock);
441 while (!list_empty(head)) {
444 pi_state = list_entry(next, struct futex_pi_state, list);
446 hb = hash_futex(&key);
447 spin_unlock_irq(&curr->pi_lock);
449 spin_lock(&hb->lock);
451 spin_lock_irq(&curr->pi_lock);
453 * We dropped the pi-lock, so re-check whether this
454 * task still owns the PI-state:
456 if (head->next != next) {
457 spin_unlock(&hb->lock);
461 WARN_ON(pi_state->owner != curr);
462 WARN_ON(list_empty(&pi_state->list));
463 list_del_init(&pi_state->list);
464 pi_state->owner = NULL;
465 spin_unlock_irq(&curr->pi_lock);
467 rt_mutex_unlock(&pi_state->pi_mutex);
469 spin_unlock(&hb->lock);
471 spin_lock_irq(&curr->pi_lock);
473 spin_unlock_irq(&curr->pi_lock);
477 lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
478 union futex_key *key, struct futex_pi_state **ps)
480 struct futex_pi_state *pi_state = NULL;
481 struct futex_q *this, *next;
482 struct plist_head *head;
483 struct task_struct *p;
484 pid_t pid = uval & FUTEX_TID_MASK;
488 plist_for_each_entry_safe(this, next, head, list) {
489 if (match_futex(&this->key, key)) {
491 * Another waiter already exists - bump up
492 * the refcount and return its pi_state:
494 pi_state = this->pi_state;
496 * Userspace might have messed up non PI and PI futexes
498 if (unlikely(!pi_state))
501 WARN_ON(!atomic_read(&pi_state->refcount));
502 WARN_ON(pid && pi_state->owner &&
503 pi_state->owner->pid != pid);
505 atomic_inc(&pi_state->refcount);
513 * We are the first waiter - try to look up the real owner and attach
514 * the new pi_state to it, but bail out when TID = 0
518 p = futex_find_get_task(pid);
523 * We need to look at the task state flags to figure out,
524 * whether the task is exiting. To protect against the do_exit
525 * change of the task flags, we do this protected by
528 spin_lock_irq(&p->pi_lock);
529 if (unlikely(p->flags & PF_EXITING)) {
531 * The task is on the way out. When PF_EXITPIDONE is
532 * set, we know that the task has finished the
535 int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN;
537 spin_unlock_irq(&p->pi_lock);
542 pi_state = alloc_pi_state();
545 * Initialize the pi_mutex in locked state and make 'p'
548 rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);
550 /* Store the key for possible exit cleanups: */
551 pi_state->key = *key;
553 WARN_ON(!list_empty(&pi_state->list));
554 list_add(&pi_state->list, &p->pi_state_list);
556 spin_unlock_irq(&p->pi_lock);
566 * The hash bucket lock must be held when this is called.
567 * Afterwards, the futex_q must not be accessed.
569 static void wake_futex(struct futex_q *q)
571 plist_del(&q->list, &q->list.plist);
573 * The lock in wake_up_all() is a crucial memory barrier after the
574 * plist_del() and also before assigning to q->lock_ptr.
576 wake_up_all(&q->waiters);
578 * The waiting task can free the futex_q as soon as this is written,
579 * without taking any locks. This must come last.
581 * A memory barrier is required here to prevent the following store
582 * to lock_ptr from getting ahead of the wakeup. Clearing the lock
583 * at the end of wake_up_all() does not prevent this store from
590 static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this)
592 struct task_struct *new_owner;
593 struct futex_pi_state *pi_state = this->pi_state;
599 spin_lock(&pi_state->pi_mutex.wait_lock);
600 new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
603 * This happens when we have stolen the lock and the original
604 * pending owner did not enqueue itself back on the rt_mutex.
605 * Thats not a tragedy. We know that way, that a lock waiter
606 * is on the fly. We make the futex_q waiter the pending owner.
609 new_owner = this->task;
612 * We pass it to the next owner. (The WAITERS bit is always
613 * kept enabled while there is PI state around. We must also
614 * preserve the owner died bit.)
616 if (!(uval & FUTEX_OWNER_DIED)) {
619 newval = FUTEX_WAITERS | task_pid_vnr(new_owner);
621 curval = cmpxchg_futex_value_locked(uaddr, uval, newval);
623 if (curval == -EFAULT)
625 else if (curval != uval)
628 spin_unlock(&pi_state->pi_mutex.wait_lock);
633 spin_lock_irq(&pi_state->owner->pi_lock);
634 WARN_ON(list_empty(&pi_state->list));
635 list_del_init(&pi_state->list);
636 spin_unlock_irq(&pi_state->owner->pi_lock);
638 spin_lock_irq(&new_owner->pi_lock);
639 WARN_ON(!list_empty(&pi_state->list));
640 list_add(&pi_state->list, &new_owner->pi_state_list);
641 pi_state->owner = new_owner;
642 spin_unlock_irq(&new_owner->pi_lock);
644 spin_unlock(&pi_state->pi_mutex.wait_lock);
645 rt_mutex_unlock(&pi_state->pi_mutex);
650 static int unlock_futex_pi(u32 __user *uaddr, u32 uval)
655 * There is no waiter, so we unlock the futex. The owner died
656 * bit has not to be preserved here. We are the owner:
658 oldval = cmpxchg_futex_value_locked(uaddr, uval, 0);
660 if (oldval == -EFAULT)
669 * Express the locking dependencies for lockdep:
672 double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
675 spin_lock(&hb1->lock);
677 spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
678 } else { /* hb1 > hb2 */
679 spin_lock(&hb2->lock);
680 spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
685 * Wake up all waiters hashed on the physical page that is mapped
686 * to this virtual address:
688 static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)
690 struct futex_hash_bucket *hb;
691 struct futex_q *this, *next;
692 struct plist_head *head;
693 union futex_key key = FUTEX_KEY_INIT;
699 ret = get_futex_key(uaddr, fshared, &key);
700 if (unlikely(ret != 0))
703 hb = hash_futex(&key);
704 spin_lock(&hb->lock);
707 plist_for_each_entry_safe(this, next, head, list) {
708 if (match_futex (&this->key, &key)) {
709 if (this->pi_state) {
714 /* Check if one of the bits is set in both bitsets */
715 if (!(this->bitset & bitset))
719 if (++ret >= nr_wake)
724 spin_unlock(&hb->lock);
726 put_futex_key(fshared, &key);
731 * Wake up all waiters hashed on the physical page that is mapped
732 * to this virtual address:
735 futex_wake_op(u32 __user *uaddr1, int fshared, u32 __user *uaddr2,
736 int nr_wake, int nr_wake2, int op)
738 union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
739 struct futex_hash_bucket *hb1, *hb2;
740 struct plist_head *head;
741 struct futex_q *this, *next;
742 int ret, op_ret, attempt = 0;
745 ret = get_futex_key(uaddr1, fshared, &key1);
746 if (unlikely(ret != 0))
748 ret = get_futex_key(uaddr2, fshared, &key2);
749 if (unlikely(ret != 0))
752 hb1 = hash_futex(&key1);
753 hb2 = hash_futex(&key2);
756 double_lock_hb(hb1, hb2);
758 op_ret = futex_atomic_op_inuser(op, uaddr2);
759 if (unlikely(op_ret < 0)) {
762 spin_unlock(&hb1->lock);
764 spin_unlock(&hb2->lock);
768 * we don't get EFAULT from MMU faults if we don't have an MMU,
769 * but we might get them from range checking
775 if (unlikely(op_ret != -EFAULT)) {
781 * futex_atomic_op_inuser needs to both read and write
782 * *(int __user *)uaddr2, but we can't modify it
783 * non-atomically. Therefore, if get_user below is not
784 * enough, we need to handle the fault ourselves, while
785 * still holding the mmap_sem.
788 ret = futex_handle_fault((unsigned long)uaddr2,
795 ret = get_user(dummy, uaddr2);
804 plist_for_each_entry_safe(this, next, head, list) {
805 if (match_futex (&this->key, &key1)) {
807 if (++ret >= nr_wake)
816 plist_for_each_entry_safe(this, next, head, list) {
817 if (match_futex (&this->key, &key2)) {
819 if (++op_ret >= nr_wake2)
826 spin_unlock(&hb1->lock);
828 spin_unlock(&hb2->lock);
830 put_futex_key(fshared, &key2);
831 put_futex_key(fshared, &key1);
837 * Requeue all waiters hashed on one physical page to another
840 static int futex_requeue(u32 __user *uaddr1, int fshared, u32 __user *uaddr2,
841 int nr_wake, int nr_requeue, u32 *cmpval)
843 union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
844 struct futex_hash_bucket *hb1, *hb2;
845 struct plist_head *head1;
846 struct futex_q *this, *next;
847 int ret, drop_count = 0;
850 ret = get_futex_key(uaddr1, fshared, &key1);
851 if (unlikely(ret != 0))
853 ret = get_futex_key(uaddr2, fshared, &key2);
854 if (unlikely(ret != 0))
857 hb1 = hash_futex(&key1);
858 hb2 = hash_futex(&key2);
860 double_lock_hb(hb1, hb2);
862 if (likely(cmpval != NULL)) {
865 ret = get_futex_value_locked(&curval, uaddr1);
868 spin_unlock(&hb1->lock);
870 spin_unlock(&hb2->lock);
872 ret = get_user(curval, uaddr1);
879 if (curval != *cmpval) {
886 plist_for_each_entry_safe(this, next, head1, list) {
887 if (!match_futex (&this->key, &key1))
889 if (++ret <= nr_wake) {
893 * If key1 and key2 hash to the same bucket, no need to
896 if (likely(head1 != &hb2->chain)) {
897 plist_del(&this->list, &hb1->chain);
898 plist_add(&this->list, &hb2->chain);
899 this->lock_ptr = &hb2->lock;
900 #ifdef CONFIG_DEBUG_PI_LIST
901 this->list.plist.lock = &hb2->lock;
905 get_futex_key_refs(&key2);
908 if (ret - nr_wake >= nr_requeue)
914 spin_unlock(&hb1->lock);
916 spin_unlock(&hb2->lock);
918 /* drop_futex_key_refs() must be called outside the spinlocks. */
919 while (--drop_count >= 0)
920 drop_futex_key_refs(&key1);
923 put_futex_key(fshared, &key2);
924 put_futex_key(fshared, &key1);
928 /* The key must be already stored in q->key. */
929 static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
931 struct futex_hash_bucket *hb;
933 init_waitqueue_head(&q->waiters);
935 get_futex_key_refs(&q->key);
936 hb = hash_futex(&q->key);
937 q->lock_ptr = &hb->lock;
939 spin_lock(&hb->lock);
943 static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
948 * The priority used to register this element is
949 * - either the real thread-priority for the real-time threads
950 * (i.e. threads with a priority lower than MAX_RT_PRIO)
951 * - or MAX_RT_PRIO for non-RT threads.
952 * Thus, all RT-threads are woken first in priority order, and
953 * the others are woken last, in FIFO order.
955 prio = min(current->normal_prio, MAX_RT_PRIO);
957 plist_node_init(&q->list, prio);
958 #ifdef CONFIG_DEBUG_PI_LIST
959 q->list.plist.lock = &hb->lock;
961 plist_add(&q->list, &hb->chain);
963 spin_unlock(&hb->lock);
967 queue_unlock(struct futex_q *q, struct futex_hash_bucket *hb)
969 spin_unlock(&hb->lock);
970 drop_futex_key_refs(&q->key);
974 * queue_me and unqueue_me must be called as a pair, each
975 * exactly once. They are called with the hashed spinlock held.
978 /* Return 1 if we were still queued (ie. 0 means we were woken) */
979 static int unqueue_me(struct futex_q *q)
981 spinlock_t *lock_ptr;
984 /* In the common case we don't take the spinlock, which is nice. */
986 lock_ptr = q->lock_ptr;
988 if (lock_ptr != NULL) {
991 * q->lock_ptr can change between reading it and
992 * spin_lock(), causing us to take the wrong lock. This
993 * corrects the race condition.
995 * Reasoning goes like this: if we have the wrong lock,
996 * q->lock_ptr must have changed (maybe several times)
997 * between reading it and the spin_lock(). It can
998 * change again after the spin_lock() but only if it was
999 * already changed before the spin_lock(). It cannot,
1000 * however, change back to the original value. Therefore
1001 * we can detect whether we acquired the correct lock.
1003 if (unlikely(lock_ptr != q->lock_ptr)) {
1004 spin_unlock(lock_ptr);
1007 WARN_ON(plist_node_empty(&q->list));
1008 plist_del(&q->list, &q->list.plist);
1010 BUG_ON(q->pi_state);
1012 spin_unlock(lock_ptr);
1016 drop_futex_key_refs(&q->key);
1021 * PI futexes can not be requeued and must remove themself from the
1022 * hash bucket. The hash bucket lock (i.e. lock_ptr) is held on entry
1025 static void unqueue_me_pi(struct futex_q *q)
1027 WARN_ON(plist_node_empty(&q->list));
1028 plist_del(&q->list, &q->list.plist);
1030 BUG_ON(!q->pi_state);
1031 free_pi_state(q->pi_state);
1034 spin_unlock(q->lock_ptr);
1036 drop_futex_key_refs(&q->key);
1040 * Fixup the pi_state owner with the new owner.
1042 * Must be called with hash bucket lock held and mm->sem held for non
1045 static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
1046 struct task_struct *newowner, int fshared)
1048 u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
1049 struct futex_pi_state *pi_state = q->pi_state;
1050 struct task_struct *oldowner = pi_state->owner;
1051 u32 uval, curval, newval;
1052 int ret, attempt = 0;
1055 if (!pi_state->owner)
1056 newtid |= FUTEX_OWNER_DIED;
1059 * We are here either because we stole the rtmutex from the
1060 * pending owner or we are the pending owner which failed to
1061 * get the rtmutex. We have to replace the pending owner TID
1062 * in the user space variable. This must be atomic as we have
1063 * to preserve the owner died bit here.
1065 * Note: We write the user space value _before_ changing the
1066 * pi_state because we can fault here. Imagine swapped out
1067 * pages or a fork, which was running right before we acquired
1068 * mmap_sem, that marked all the anonymous memory readonly for
1071 * Modifying pi_state _before_ the user space value would
1072 * leave the pi_state in an inconsistent state when we fault
1073 * here, because we need to drop the hash bucket lock to
1074 * handle the fault. This might be observed in the PID check
1075 * in lookup_pi_state.
1078 if (get_futex_value_locked(&uval, uaddr))
1082 newval = (uval & FUTEX_OWNER_DIED) | newtid;
1084 curval = cmpxchg_futex_value_locked(uaddr, uval, newval);
1086 if (curval == -EFAULT)
1094 * We fixed up user space. Now we need to fix the pi_state
1097 if (pi_state->owner != NULL) {
1098 spin_lock_irq(&pi_state->owner->pi_lock);
1099 WARN_ON(list_empty(&pi_state->list));
1100 list_del_init(&pi_state->list);
1101 spin_unlock_irq(&pi_state->owner->pi_lock);
1104 pi_state->owner = newowner;
1106 spin_lock_irq(&newowner->pi_lock);
1107 WARN_ON(!list_empty(&pi_state->list));
1108 list_add(&pi_state->list, &newowner->pi_state_list);
1109 spin_unlock_irq(&newowner->pi_lock);
1113 * To handle the page fault we need to drop the hash bucket
1114 * lock here. That gives the other task (either the pending
1115 * owner itself or the task which stole the rtmutex) the
1116 * chance to try the fixup of the pi_state. So once we are
1117 * back from handling the fault we need to check the pi_state
1118 * after reacquiring the hash bucket lock and before trying to
1119 * do another fixup. When the fixup has been done already we
1123 spin_unlock(q->lock_ptr);
1125 ret = futex_handle_fault((unsigned long)uaddr, attempt++);
1127 spin_lock(q->lock_ptr);
1130 * Check if someone else fixed it for us:
1132 if (pi_state->owner != oldowner)
1142 * In case we must use restart_block to restart a futex_wait,
1143 * we encode in the 'flags' shared capability
1145 #define FLAGS_SHARED 1
1147 static long futex_wait_restart(struct restart_block *restart);
1149 static int futex_wait(u32 __user *uaddr, int fshared,
1150 u32 val, ktime_t *abs_time, u32 bitset)
1152 struct task_struct *curr = current;
1153 DECLARE_WAITQUEUE(wait, curr);
1154 struct futex_hash_bucket *hb;
1158 struct hrtimer_sleeper t;
1167 q.key = FUTEX_KEY_INIT;
1168 ret = get_futex_key(uaddr, fshared, &q.key);
1169 if (unlikely(ret != 0))
1170 goto out_release_sem;
1172 hb = queue_lock(&q);
1175 * Access the page AFTER the futex is queued.
1176 * Order is important:
1178 * Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
1179 * Userspace waker: if (cond(var)) { var = new; futex_wake(&var); }
1181 * The basic logical guarantee of a futex is that it blocks ONLY
1182 * if cond(var) is known to be true at the time of blocking, for
1183 * any cond. If we queued after testing *uaddr, that would open
1184 * a race condition where we could block indefinitely with
1185 * cond(var) false, which would violate the guarantee.
1187 * A consequence is that futex_wait() can return zero and absorb
1188 * a wakeup when *uaddr != val on entry to the syscall. This is
1191 * for shared futexes, we hold the mmap semaphore, so the mapping
1192 * cannot have changed since we looked it up in get_futex_key.
1194 ret = get_futex_value_locked(&uval, uaddr);
1196 if (unlikely(ret)) {
1197 queue_unlock(&q, hb);
1199 ret = get_user(uval, uaddr);
1207 goto out_unlock_release_sem;
1209 /* Only actually queue if *uaddr contained val. */
1213 * There might have been scheduling since the queue_me(), as we
1214 * cannot hold a spinlock across the get_user() in case it
1215 * faults, and we cannot just set TASK_INTERRUPTIBLE state when
1216 * queueing ourselves into the futex hash. This code thus has to
1217 * rely on the futex_wake() code removing us from hash when it
1221 /* add_wait_queue is the barrier after __set_current_state. */
1222 __set_current_state(TASK_INTERRUPTIBLE);
1223 add_wait_queue(&q.waiters, &wait);
1225 * !plist_node_empty() is safe here without any lock.
1226 * q.lock_ptr != 0 is not safe, because of ordering against wakeup.
1228 if (likely(!plist_node_empty(&q.list))) {
1232 hrtimer_init_on_stack(&t.timer, CLOCK_MONOTONIC,
1234 hrtimer_init_sleeper(&t, current);
1235 t.timer.expires = *abs_time;
1237 hrtimer_start(&t.timer, t.timer.expires,
1239 if (!hrtimer_active(&t.timer))
1243 * the timer could have already expired, in which
1244 * case current would be flagged for rescheduling.
1245 * Don't bother calling schedule.
1250 hrtimer_cancel(&t.timer);
1252 /* Flag if a timeout occured */
1253 rem = (t.task == NULL);
1255 destroy_hrtimer_on_stack(&t.timer);
1258 __set_current_state(TASK_RUNNING);
1261 * NOTE: we don't remove ourselves from the waitqueue because
1262 * we are the only user of it.
1265 /* If we were woken (and unqueued), we succeeded, whatever. */
1266 if (!unqueue_me(&q))
1272 * We expect signal_pending(current), but another thread may
1273 * have handled it for us already.
1276 return -ERESTARTSYS;
1278 struct restart_block *restart;
1279 restart = ¤t_thread_info()->restart_block;
1280 restart->fn = futex_wait_restart;
1281 restart->futex.uaddr = (u32 *)uaddr;
1282 restart->futex.val = val;
1283 restart->futex.time = abs_time->tv64;
1284 restart->futex.bitset = bitset;
1285 restart->futex.flags = 0;
1288 restart->futex.flags |= FLAGS_SHARED;
1289 return -ERESTART_RESTARTBLOCK;
1292 out_unlock_release_sem:
1293 queue_unlock(&q, hb);
1296 put_futex_key(fshared, &q.key);
1301 static long futex_wait_restart(struct restart_block *restart)
1303 u32 __user *uaddr = (u32 __user *)restart->futex.uaddr;
1307 t.tv64 = restart->futex.time;
1308 restart->fn = do_no_restart_syscall;
1309 if (restart->futex.flags & FLAGS_SHARED)
1311 return (long)futex_wait(uaddr, fshared, restart->futex.val, &t,
1312 restart->futex.bitset);
1317 * Userspace tried a 0 -> TID atomic transition of the futex value
1318 * and failed. The kernel side here does the whole locking operation:
1319 * if there are waiters then it will block, it does PI, etc. (Due to
1320 * races the kernel might see a 0 value of the futex too.)
1322 static int futex_lock_pi(u32 __user *uaddr, int fshared,
1323 int detect, ktime_t *time, int trylock)
1325 struct hrtimer_sleeper timeout, *to = NULL;
1326 struct task_struct *curr = current;
1327 struct futex_hash_bucket *hb;
1328 u32 uval, newval, curval;
1330 int ret, lock_taken, ownerdied = 0, attempt = 0;
1332 if (refill_pi_state_cache())
1337 hrtimer_init_on_stack(&to->timer, CLOCK_REALTIME,
1339 hrtimer_init_sleeper(to, current);
1340 to->timer.expires = *time;
1345 q.key = FUTEX_KEY_INIT;
1346 ret = get_futex_key(uaddr, fshared, &q.key);
1347 if (unlikely(ret != 0))
1348 goto out_release_sem;
1351 hb = queue_lock(&q);
1354 ret = lock_taken = 0;
1357 * To avoid races, we attempt to take the lock here again
1358 * (by doing a 0 -> TID atomic cmpxchg), while holding all
1359 * the locks. It will most likely not succeed.
1361 newval = task_pid_vnr(current);
1363 curval = cmpxchg_futex_value_locked(uaddr, 0, newval);
1365 if (unlikely(curval == -EFAULT))
1369 * Detect deadlocks. In case of REQUEUE_PI this is a valid
1370 * situation and we return success to user space.
1372 if (unlikely((curval & FUTEX_TID_MASK) == task_pid_vnr(current))) {
1374 goto out_unlock_release_sem;
1378 * Surprise - we got the lock. Just return to userspace:
1380 if (unlikely(!curval))
1381 goto out_unlock_release_sem;
1386 * Set the WAITERS flag, so the owner will know it has someone
1387 * to wake at next unlock
1389 newval = curval | FUTEX_WAITERS;
1392 * There are two cases, where a futex might have no owner (the
1393 * owner TID is 0): OWNER_DIED. We take over the futex in this
1394 * case. We also do an unconditional take over, when the owner
1395 * of the futex died.
1397 * This is safe as we are protected by the hash bucket lock !
1399 if (unlikely(ownerdied || !(curval & FUTEX_TID_MASK))) {
1400 /* Keep the OWNER_DIED bit */
1401 newval = (curval & ~FUTEX_TID_MASK) | task_pid_vnr(current);
1406 curval = cmpxchg_futex_value_locked(uaddr, uval, newval);
1408 if (unlikely(curval == -EFAULT))
1410 if (unlikely(curval != uval))
1414 * We took the lock due to owner died take over.
1416 if (unlikely(lock_taken))
1417 goto out_unlock_release_sem;
1420 * We dont have the lock. Look up the PI state (or create it if
1421 * we are the first waiter):
1423 ret = lookup_pi_state(uval, hb, &q.key, &q.pi_state);
1425 if (unlikely(ret)) {
1430 * Task is exiting and we just wait for the
1433 queue_unlock(&q, hb);
1439 * No owner found for this futex. Check if the
1440 * OWNER_DIED bit is set to figure out whether
1441 * this is a robust futex or not.
1443 if (get_futex_value_locked(&curval, uaddr))
1447 * We simply start over in case of a robust
1448 * futex. The code above will take the futex
1451 if (curval & FUTEX_OWNER_DIED) {
1456 goto out_unlock_release_sem;
1461 * Only actually queue now that the atomic ops are done:
1465 WARN_ON(!q.pi_state);
1467 * Block on the PI mutex:
1470 ret = rt_mutex_timed_lock(&q.pi_state->pi_mutex, to, 1);
1472 ret = rt_mutex_trylock(&q.pi_state->pi_mutex);
1473 /* Fixup the trylock return value: */
1474 ret = ret ? 0 : -EWOULDBLOCK;
1477 spin_lock(q.lock_ptr);
1481 * Got the lock. We might not be the anticipated owner
1482 * if we did a lock-steal - fix up the PI-state in
1485 if (q.pi_state->owner != curr)
1486 ret = fixup_pi_state_owner(uaddr, &q, curr, fshared);
1489 * Catch the rare case, where the lock was released
1490 * when we were on the way back before we locked the
1493 if (q.pi_state->owner == curr) {
1495 * Try to get the rt_mutex now. This might
1496 * fail as some other task acquired the
1497 * rt_mutex after we removed ourself from the
1498 * rt_mutex waiters list.
1500 if (rt_mutex_trylock(&q.pi_state->pi_mutex))
1504 * pi_state is incorrect, some other
1505 * task did a lock steal and we
1506 * returned due to timeout or signal
1507 * without taking the rt_mutex. Too
1508 * late. We can access the
1509 * rt_mutex_owner without locking, as
1510 * the other task is now blocked on
1511 * the hash bucket lock. Fix the state
1514 struct task_struct *owner;
1517 owner = rt_mutex_owner(&q.pi_state->pi_mutex);
1518 res = fixup_pi_state_owner(uaddr, &q, owner,
1521 /* propagate -EFAULT, if the fixup failed */
1527 * Paranoia check. If we did not take the lock
1528 * in the trylock above, then we should not be
1529 * the owner of the rtmutex, neither the real
1530 * nor the pending one:
1532 if (rt_mutex_owner(&q.pi_state->pi_mutex) == curr)
1533 printk(KERN_ERR "futex_lock_pi: ret = %d "
1534 "pi-mutex: %p pi-state %p\n", ret,
1535 q.pi_state->pi_mutex.owner,
1540 /* Unqueue and drop the lock */
1544 destroy_hrtimer_on_stack(&to->timer);
1545 return ret != -EINTR ? ret : -ERESTARTNOINTR;
1547 out_unlock_release_sem:
1548 queue_unlock(&q, hb);
1551 put_futex_key(fshared, &q.key);
1553 destroy_hrtimer_on_stack(&to->timer);
1558 * We have to r/w *(int __user *)uaddr, but we can't modify it
1559 * non-atomically. Therefore, if get_user below is not
1560 * enough, we need to handle the fault ourselves, while
1561 * still holding the mmap_sem.
1563 * ... and hb->lock. :-) --ANK
1565 queue_unlock(&q, hb);
1568 ret = futex_handle_fault((unsigned long)uaddr, attempt);
1570 goto out_release_sem;
1571 goto retry_unlocked;
1574 ret = get_user(uval, uaddr);
1575 if (!ret && (uval != -EFAULT))
1579 destroy_hrtimer_on_stack(&to->timer);
1584 * Userspace attempted a TID -> 0 atomic transition, and failed.
1585 * This is the in-kernel slowpath: we look up the PI state (if any),
1586 * and do the rt-mutex unlock.
1588 static int futex_unlock_pi(u32 __user *uaddr, int fshared)
1590 struct futex_hash_bucket *hb;
1591 struct futex_q *this, *next;
1593 struct plist_head *head;
1594 union futex_key key = FUTEX_KEY_INIT;
1595 int ret, attempt = 0;
1598 if (get_user(uval, uaddr))
1601 * We release only a lock we actually own:
1603 if ((uval & FUTEX_TID_MASK) != task_pid_vnr(current))
1606 ret = get_futex_key(uaddr, fshared, &key);
1607 if (unlikely(ret != 0))
1610 hb = hash_futex(&key);
1612 spin_lock(&hb->lock);
1615 * To avoid races, try to do the TID -> 0 atomic transition
1616 * again. If it succeeds then we can return without waking
1619 if (!(uval & FUTEX_OWNER_DIED))
1620 uval = cmpxchg_futex_value_locked(uaddr, task_pid_vnr(current), 0);
1623 if (unlikely(uval == -EFAULT))
1626 * Rare case: we managed to release the lock atomically,
1627 * no need to wake anyone else up:
1629 if (unlikely(uval == task_pid_vnr(current)))
1633 * Ok, other tasks may need to be woken up - check waiters
1634 * and do the wakeup if necessary:
1638 plist_for_each_entry_safe(this, next, head, list) {
1639 if (!match_futex (&this->key, &key))
1641 ret = wake_futex_pi(uaddr, uval, this);
1643 * The atomic access to the futex value
1644 * generated a pagefault, so retry the
1645 * user-access and the wakeup:
1652 * No waiters - kernel unlocks the futex:
1654 if (!(uval & FUTEX_OWNER_DIED)) {
1655 ret = unlock_futex_pi(uaddr, uval);
1661 spin_unlock(&hb->lock);
1663 put_futex_key(fshared, &key);
1669 * We have to r/w *(int __user *)uaddr, but we can't modify it
1670 * non-atomically. Therefore, if get_user below is not
1671 * enough, we need to handle the fault ourselves, while
1672 * still holding the mmap_sem.
1674 * ... and hb->lock. --ANK
1676 spin_unlock(&hb->lock);
1679 ret = futex_handle_fault((unsigned long)uaddr, attempt);
1683 goto retry_unlocked;
1686 ret = get_user(uval, uaddr);
1687 if (!ret && (uval != -EFAULT))
1694 * Support for robust futexes: the kernel cleans up held futexes at
1697 * Implementation: user-space maintains a per-thread list of locks it
1698 * is holding. Upon do_exit(), the kernel carefully walks this list,
1699 * and marks all locks that are owned by this thread with the
1700 * FUTEX_OWNER_DIED bit, and wakes up a waiter (if any). The list is
1701 * always manipulated with the lock held, so the list is private and
1702 * per-thread. Userspace also maintains a per-thread 'list_op_pending'
1703 * field, to allow the kernel to clean up if the thread dies after
1704 * acquiring the lock, but just before it could have added itself to
1705 * the list. There can only be one such pending lock.
1709 * sys_set_robust_list - set the robust-futex list head of a task
1710 * @head: pointer to the list-head
1711 * @len: length of the list-head, as userspace expects
1714 sys_set_robust_list(struct robust_list_head __user *head,
1717 if (!futex_cmpxchg_enabled)
1720 * The kernel knows only one size for now:
1722 if (unlikely(len != sizeof(*head)))
1725 current->robust_list = head;
1731 * sys_get_robust_list - get the robust-futex list head of a task
1732 * @pid: pid of the process [zero for current task]
1733 * @head_ptr: pointer to a list-head pointer, the kernel fills it in
1734 * @len_ptr: pointer to a length field, the kernel fills in the header size
1737 sys_get_robust_list(int pid, struct robust_list_head __user * __user *head_ptr,
1738 size_t __user *len_ptr)
1740 struct robust_list_head __user *head;
1743 if (!futex_cmpxchg_enabled)
1747 head = current->robust_list;
1749 struct task_struct *p;
1753 p = find_task_by_vpid(pid);
1757 if ((current->euid != p->euid) && (current->euid != p->uid) &&
1758 !capable(CAP_SYS_PTRACE))
1760 head = p->robust_list;
1764 if (put_user(sizeof(*head), len_ptr))
1766 return put_user(head, head_ptr);
1775 * Process a futex-list entry, check whether it's owned by the
1776 * dying task, and do notification if so:
1778 int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi)
1780 u32 uval, nval, mval;
1783 if (get_user(uval, uaddr))
1786 if ((uval & FUTEX_TID_MASK) == task_pid_vnr(curr)) {
1788 * Ok, this dying thread is truly holding a futex
1789 * of interest. Set the OWNER_DIED bit atomically
1790 * via cmpxchg, and if the value had FUTEX_WAITERS
1791 * set, wake up a waiter (if any). (We have to do a
1792 * futex_wake() even if OWNER_DIED is already set -
1793 * to handle the rare but possible case of recursive
1794 * thread-death.) The rest of the cleanup is done in
1797 mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
1798 nval = futex_atomic_cmpxchg_inatomic(uaddr, uval, mval);
1800 if (nval == -EFAULT)
1807 * Wake robust non-PI futexes here. The wakeup of
1808 * PI futexes happens in exit_pi_state():
1810 if (!pi && (uval & FUTEX_WAITERS))
1811 futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
1817 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
1819 static inline int fetch_robust_entry(struct robust_list __user **entry,
1820 struct robust_list __user * __user *head,
1823 unsigned long uentry;
1825 if (get_user(uentry, (unsigned long __user *)head))
1828 *entry = (void __user *)(uentry & ~1UL);
1835 * Walk curr->robust_list (very carefully, it's a userspace list!)
1836 * and mark any locks found there dead, and notify any waiters.
1838 * We silently return on any sign of list-walking problem.
1840 void exit_robust_list(struct task_struct *curr)
1842 struct robust_list_head __user *head = curr->robust_list;
1843 struct robust_list __user *entry, *next_entry, *pending;
1844 unsigned int limit = ROBUST_LIST_LIMIT, pi, next_pi, pip;
1845 unsigned long futex_offset;
1848 if (!futex_cmpxchg_enabled)
1852 * Fetch the list head (which was registered earlier, via
1853 * sys_set_robust_list()):
1855 if (fetch_robust_entry(&entry, &head->list.next, &pi))
1858 * Fetch the relative futex offset:
1860 if (get_user(futex_offset, &head->futex_offset))
1863 * Fetch any possibly pending lock-add first, and handle it
1866 if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
1869 next_entry = NULL; /* avoid warning with gcc */
1870 while (entry != &head->list) {
1872 * Fetch the next entry in the list before calling
1873 * handle_futex_death:
1875 rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
1877 * A pending lock might already be on the list, so
1878 * don't process it twice:
1880 if (entry != pending)
1881 if (handle_futex_death((void __user *)entry + futex_offset,
1889 * Avoid excessively long or circular lists:
1898 handle_futex_death((void __user *)pending + futex_offset,
1902 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
1903 u32 __user *uaddr2, u32 val2, u32 val3)
1906 int cmd = op & FUTEX_CMD_MASK;
1909 if (!(op & FUTEX_PRIVATE_FLAG))
1914 val3 = FUTEX_BITSET_MATCH_ANY;
1915 case FUTEX_WAIT_BITSET:
1916 ret = futex_wait(uaddr, fshared, val, timeout, val3);
1919 val3 = FUTEX_BITSET_MATCH_ANY;
1920 case FUTEX_WAKE_BITSET:
1921 ret = futex_wake(uaddr, fshared, val, val3);
1924 ret = futex_requeue(uaddr, fshared, uaddr2, val, val2, NULL);
1926 case FUTEX_CMP_REQUEUE:
1927 ret = futex_requeue(uaddr, fshared, uaddr2, val, val2, &val3);
1930 ret = futex_wake_op(uaddr, fshared, uaddr2, val, val2, val3);
1933 if (futex_cmpxchg_enabled)
1934 ret = futex_lock_pi(uaddr, fshared, val, timeout, 0);
1936 case FUTEX_UNLOCK_PI:
1937 if (futex_cmpxchg_enabled)
1938 ret = futex_unlock_pi(uaddr, fshared);
1940 case FUTEX_TRYLOCK_PI:
1941 if (futex_cmpxchg_enabled)
1942 ret = futex_lock_pi(uaddr, fshared, 0, timeout, 1);
1951 asmlinkage long sys_futex(u32 __user *uaddr, int op, u32 val,
1952 struct timespec __user *utime, u32 __user *uaddr2,
1956 ktime_t t, *tp = NULL;
1958 int cmd = op & FUTEX_CMD_MASK;
1960 if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
1961 cmd == FUTEX_WAIT_BITSET)) {
1962 if (copy_from_user(&ts, utime, sizeof(ts)) != 0)
1964 if (!timespec_valid(&ts))
1967 t = timespec_to_ktime(ts);
1968 if (cmd == FUTEX_WAIT)
1969 t = ktime_add_safe(ktime_get(), t);
1973 * requeue parameter in 'utime' if cmd == FUTEX_REQUEUE.
1974 * number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.
1976 if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
1977 cmd == FUTEX_WAKE_OP)
1978 val2 = (u32) (unsigned long) utime;
1980 return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
1983 static int __init futex_init(void)
1989 * This will fail and we want it. Some arch implementations do
1990 * runtime detection of the futex_atomic_cmpxchg_inatomic()
1991 * functionality. We want to know that before we call in any
1992 * of the complex code paths. Also we want to prevent
1993 * registration of robust lists in that case. NULL is
1994 * guaranteed to fault and we get -EFAULT on functional
1995 * implementation, the non functional ones will return
1998 curval = cmpxchg_futex_value_locked(NULL, 0, 0);
1999 if (curval == -EFAULT)
2000 futex_cmpxchg_enabled = 1;
2002 for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
2003 plist_head_init(&futex_queues[i].chain, &futex_queues[i].lock);
2004 spin_lock_init(&futex_queues[i].lock);
2009 __initcall(futex_init);