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 * Requeue-PI support by Darren Hart <dvhltc@us.ibm.com>
23 * Copyright (C) IBM Corporation, 2009
24 * Thanks to Thomas Gleixner for conceptual design and careful reviews.
26 * Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
27 * enough at me, Linus for the original (flawed) idea, Matthew
28 * Kirkwood for proof-of-concept implementation.
30 * "The futexes are also cursed."
31 * "But they come in a choice of three flavours!"
33 * This program is free software; you can redistribute it and/or modify
34 * it under the terms of the GNU General Public License as published by
35 * the Free Software Foundation; either version 2 of the License, or
36 * (at your option) any later version.
38 * This program is distributed in the hope that it will be useful,
39 * but WITHOUT ANY WARRANTY; without even the implied warranty of
40 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
41 * GNU General Public License for more details.
43 * You should have received a copy of the GNU General Public License
44 * along with this program; if not, write to the Free Software
45 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
47 #include <linux/slab.h>
48 #include <linux/poll.h>
50 #include <linux/file.h>
51 #include <linux/jhash.h>
52 #include <linux/init.h>
53 #include <linux/futex.h>
54 #include <linux/mount.h>
55 #include <linux/pagemap.h>
56 #include <linux/syscalls.h>
57 #include <linux/signal.h>
58 #include <linux/module.h>
59 #include <linux/magic.h>
60 #include <linux/pid.h>
61 #include <linux/nsproxy.h>
63 #include <asm/futex.h>
65 #include "rtmutex_common.h"
67 int __read_mostly futex_cmpxchg_enabled
;
69 #define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)
72 * Priority Inheritance state:
74 struct futex_pi_state
{
76 * list of 'owned' pi_state instances - these have to be
77 * cleaned up in do_exit() if the task exits prematurely:
79 struct list_head list
;
84 struct rt_mutex pi_mutex
;
86 struct task_struct
*owner
;
93 * We use this hashed waitqueue instead of a normal wait_queue_t, so
94 * we can wake only the relevant ones (hashed queues may be shared).
96 * A futex_q has a woken state, just like tasks have TASK_RUNNING.
97 * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
98 * The order of wakup is always to make the first condition true, then
99 * wake up q->waiter, then make the second condition true.
102 struct plist_node list
;
103 /* There can only be a single waiter */
104 wait_queue_head_t waiter
;
106 /* Which hash list lock to use: */
107 spinlock_t
*lock_ptr
;
109 /* Key which the futex is hashed on: */
112 /* Optional priority inheritance state: */
113 struct futex_pi_state
*pi_state
;
114 struct task_struct
*task
;
116 /* rt_waiter storage for requeue_pi: */
117 struct rt_mutex_waiter
*rt_waiter
;
119 /* Bitset for the optional bitmasked wakeup */
124 * Hash buckets are shared by all the futex_keys that hash to the same
125 * location. Each key may have multiple futex_q structures, one for each task
126 * waiting on a futex.
128 struct futex_hash_bucket
{
130 struct plist_head chain
;
133 static struct futex_hash_bucket futex_queues
[1<<FUTEX_HASHBITS
];
136 * We hash on the keys returned from get_futex_key (see below).
138 static struct futex_hash_bucket
*hash_futex(union futex_key
*key
)
140 u32 hash
= jhash2((u32
*)&key
->both
.word
,
141 (sizeof(key
->both
.word
)+sizeof(key
->both
.ptr
))/4,
143 return &futex_queues
[hash
& ((1 << FUTEX_HASHBITS
)-1)];
147 * Return 1 if two futex_keys are equal, 0 otherwise.
149 static inline int match_futex(union futex_key
*key1
, union futex_key
*key2
)
151 return (key1
->both
.word
== key2
->both
.word
152 && key1
->both
.ptr
== key2
->both
.ptr
153 && key1
->both
.offset
== key2
->both
.offset
);
157 * Take a reference to the resource addressed by a key.
158 * Can be called while holding spinlocks.
161 static void get_futex_key_refs(union futex_key
*key
)
166 switch (key
->both
.offset
& (FUT_OFF_INODE
|FUT_OFF_MMSHARED
)) {
168 atomic_inc(&key
->shared
.inode
->i_count
);
170 case FUT_OFF_MMSHARED
:
171 atomic_inc(&key
->private.mm
->mm_count
);
177 * Drop a reference to the resource addressed by a key.
178 * The hash bucket spinlock must not be held.
180 static void drop_futex_key_refs(union futex_key
*key
)
182 if (!key
->both
.ptr
) {
183 /* If we're here then we tried to put a key we failed to get */
188 switch (key
->both
.offset
& (FUT_OFF_INODE
|FUT_OFF_MMSHARED
)) {
190 iput(key
->shared
.inode
);
192 case FUT_OFF_MMSHARED
:
193 mmdrop(key
->private.mm
);
199 * get_futex_key - Get parameters which are the keys for a futex.
200 * @uaddr: virtual address of the futex
201 * @fshared: 0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED
202 * @key: address where result is stored.
204 * Returns a negative error code or 0
205 * The key words are stored in *key on success.
207 * For shared mappings, it's (page->index, vma->vm_file->f_path.dentry->d_inode,
208 * offset_within_page). For private mappings, it's (uaddr, current->mm).
209 * We can usually work out the index without swapping in the page.
211 * lock_page() might sleep, the caller should not hold a spinlock.
213 static int get_futex_key(u32 __user
*uaddr
, int fshared
, union futex_key
*key
)
215 unsigned long address
= (unsigned long)uaddr
;
216 struct mm_struct
*mm
= current
->mm
;
221 * The futex address must be "naturally" aligned.
223 key
->both
.offset
= address
% PAGE_SIZE
;
224 if (unlikely((address
% sizeof(u32
)) != 0))
226 address
-= key
->both
.offset
;
229 * PROCESS_PRIVATE futexes are fast.
230 * As the mm cannot disappear under us and the 'key' only needs
231 * virtual address, we dont even have to find the underlying vma.
232 * Note : We do have to check 'uaddr' is a valid user address,
233 * but access_ok() should be faster than find_vma()
236 if (unlikely(!access_ok(VERIFY_WRITE
, uaddr
, sizeof(u32
))))
238 key
->private.mm
= mm
;
239 key
->private.address
= address
;
240 get_futex_key_refs(key
);
245 err
= get_user_pages_fast(address
, 1, 0, &page
);
250 if (!page
->mapping
) {
257 * Private mappings are handled in a simple way.
259 * NOTE: When userspace waits on a MAP_SHARED mapping, even if
260 * it's a read-only handle, it's expected that futexes attach to
261 * the object not the particular process.
263 if (PageAnon(page
)) {
264 key
->both
.offset
|= FUT_OFF_MMSHARED
; /* ref taken on mm */
265 key
->private.mm
= mm
;
266 key
->private.address
= address
;
268 key
->both
.offset
|= FUT_OFF_INODE
; /* inode-based key */
269 key
->shared
.inode
= page
->mapping
->host
;
270 key
->shared
.pgoff
= page
->index
;
273 get_futex_key_refs(key
);
281 void put_futex_key(int fshared
, union futex_key
*key
)
283 drop_futex_key_refs(key
);
287 * futex_top_waiter() - Return the highest priority waiter on a futex
288 * @hb: the hash bucket the futex_q's reside in
289 * @key: the futex key (to distinguish it from other futex futex_q's)
291 * Must be called with the hb lock held.
293 static struct futex_q
*futex_top_waiter(struct futex_hash_bucket
*hb
,
294 union futex_key
*key
)
296 struct futex_q
*this;
298 plist_for_each_entry(this, &hb
->chain
, list
) {
299 if (match_futex(&this->key
, key
))
305 static u32
cmpxchg_futex_value_locked(u32 __user
*uaddr
, u32 uval
, u32 newval
)
310 curval
= futex_atomic_cmpxchg_inatomic(uaddr
, uval
, newval
);
316 static int get_futex_value_locked(u32
*dest
, u32 __user
*from
)
321 ret
= __copy_from_user_inatomic(dest
, from
, sizeof(u32
));
324 return ret
? -EFAULT
: 0;
331 static int refill_pi_state_cache(void)
333 struct futex_pi_state
*pi_state
;
335 if (likely(current
->pi_state_cache
))
338 pi_state
= kzalloc(sizeof(*pi_state
), GFP_KERNEL
);
343 INIT_LIST_HEAD(&pi_state
->list
);
344 /* pi_mutex gets initialized later */
345 pi_state
->owner
= NULL
;
346 atomic_set(&pi_state
->refcount
, 1);
347 pi_state
->key
= FUTEX_KEY_INIT
;
349 current
->pi_state_cache
= pi_state
;
354 static struct futex_pi_state
* alloc_pi_state(void)
356 struct futex_pi_state
*pi_state
= current
->pi_state_cache
;
359 current
->pi_state_cache
= NULL
;
364 static void free_pi_state(struct futex_pi_state
*pi_state
)
366 if (!atomic_dec_and_test(&pi_state
->refcount
))
370 * If pi_state->owner is NULL, the owner is most probably dying
371 * and has cleaned up the pi_state already
373 if (pi_state
->owner
) {
374 spin_lock_irq(&pi_state
->owner
->pi_lock
);
375 list_del_init(&pi_state
->list
);
376 spin_unlock_irq(&pi_state
->owner
->pi_lock
);
378 rt_mutex_proxy_unlock(&pi_state
->pi_mutex
, pi_state
->owner
);
381 if (current
->pi_state_cache
)
385 * pi_state->list is already empty.
386 * clear pi_state->owner.
387 * refcount is at 0 - put it back to 1.
389 pi_state
->owner
= NULL
;
390 atomic_set(&pi_state
->refcount
, 1);
391 current
->pi_state_cache
= pi_state
;
396 * Look up the task based on what TID userspace gave us.
399 static struct task_struct
* futex_find_get_task(pid_t pid
)
401 struct task_struct
*p
;
402 const struct cred
*cred
= current_cred(), *pcred
;
405 p
= find_task_by_vpid(pid
);
409 pcred
= __task_cred(p
);
410 if (cred
->euid
!= pcred
->euid
&&
411 cred
->euid
!= pcred
->uid
)
423 * This task is holding PI mutexes at exit time => bad.
424 * Kernel cleans up PI-state, but userspace is likely hosed.
425 * (Robust-futex cleanup is separate and might save the day for userspace.)
427 void exit_pi_state_list(struct task_struct
*curr
)
429 struct list_head
*next
, *head
= &curr
->pi_state_list
;
430 struct futex_pi_state
*pi_state
;
431 struct futex_hash_bucket
*hb
;
432 union futex_key key
= FUTEX_KEY_INIT
;
434 if (!futex_cmpxchg_enabled
)
437 * We are a ZOMBIE and nobody can enqueue itself on
438 * pi_state_list anymore, but we have to be careful
439 * versus waiters unqueueing themselves:
441 spin_lock_irq(&curr
->pi_lock
);
442 while (!list_empty(head
)) {
445 pi_state
= list_entry(next
, struct futex_pi_state
, list
);
447 hb
= hash_futex(&key
);
448 spin_unlock_irq(&curr
->pi_lock
);
450 spin_lock(&hb
->lock
);
452 spin_lock_irq(&curr
->pi_lock
);
454 * We dropped the pi-lock, so re-check whether this
455 * task still owns the PI-state:
457 if (head
->next
!= next
) {
458 spin_unlock(&hb
->lock
);
462 WARN_ON(pi_state
->owner
!= curr
);
463 WARN_ON(list_empty(&pi_state
->list
));
464 list_del_init(&pi_state
->list
);
465 pi_state
->owner
= NULL
;
466 spin_unlock_irq(&curr
->pi_lock
);
468 rt_mutex_unlock(&pi_state
->pi_mutex
);
470 spin_unlock(&hb
->lock
);
472 spin_lock_irq(&curr
->pi_lock
);
474 spin_unlock_irq(&curr
->pi_lock
);
478 lookup_pi_state(u32 uval
, struct futex_hash_bucket
*hb
,
479 union futex_key
*key
, struct futex_pi_state
**ps
)
481 struct futex_pi_state
*pi_state
= NULL
;
482 struct futex_q
*this, *next
;
483 struct plist_head
*head
;
484 struct task_struct
*p
;
485 pid_t pid
= uval
& FUTEX_TID_MASK
;
489 plist_for_each_entry_safe(this, next
, head
, list
) {
490 if (match_futex(&this->key
, key
)) {
492 * Another waiter already exists - bump up
493 * the refcount and return its pi_state:
495 pi_state
= this->pi_state
;
497 * Userspace might have messed up non PI and PI futexes
499 if (unlikely(!pi_state
))
502 WARN_ON(!atomic_read(&pi_state
->refcount
));
503 WARN_ON(pid
&& pi_state
->owner
&&
504 pi_state
->owner
->pid
!= pid
);
506 atomic_inc(&pi_state
->refcount
);
514 * We are the first waiter - try to look up the real owner and attach
515 * the new pi_state to it, but bail out when TID = 0
519 p
= futex_find_get_task(pid
);
524 * We need to look at the task state flags to figure out,
525 * whether the task is exiting. To protect against the do_exit
526 * change of the task flags, we do this protected by
529 spin_lock_irq(&p
->pi_lock
);
530 if (unlikely(p
->flags
& PF_EXITING
)) {
532 * The task is on the way out. When PF_EXITPIDONE is
533 * set, we know that the task has finished the
536 int ret
= (p
->flags
& PF_EXITPIDONE
) ? -ESRCH
: -EAGAIN
;
538 spin_unlock_irq(&p
->pi_lock
);
543 pi_state
= alloc_pi_state();
546 * Initialize the pi_mutex in locked state and make 'p'
549 rt_mutex_init_proxy_locked(&pi_state
->pi_mutex
, p
);
551 /* Store the key for possible exit cleanups: */
552 pi_state
->key
= *key
;
554 WARN_ON(!list_empty(&pi_state
->list
));
555 list_add(&pi_state
->list
, &p
->pi_state_list
);
557 spin_unlock_irq(&p
->pi_lock
);
567 * futex_lock_pi_atomic() - atomic work required to acquire a pi aware futex
568 * @uaddr: the pi futex user address
569 * @hb: the pi futex hash bucket
570 * @key: the futex key associated with uaddr and hb
571 * @ps: the pi_state pointer where we store the result of the lookup
572 * @task: the task to perform the atomic lock work for. This will be
573 * "current" except in the case of requeue pi.
577 * 1 - acquired the lock
580 * The hb->lock and futex_key refs shall be held by the caller.
582 static int futex_lock_pi_atomic(u32 __user
*uaddr
, struct futex_hash_bucket
*hb
,
583 union futex_key
*key
,
584 struct futex_pi_state
**ps
,
585 struct task_struct
*task
)
587 int lock_taken
, ret
, ownerdied
= 0;
588 u32 uval
, newval
, curval
;
591 ret
= lock_taken
= 0;
594 * To avoid races, we attempt to take the lock here again
595 * (by doing a 0 -> TID atomic cmpxchg), while holding all
596 * the locks. It will most likely not succeed.
598 newval
= task_pid_vnr(task
);
600 curval
= cmpxchg_futex_value_locked(uaddr
, 0, newval
);
602 if (unlikely(curval
== -EFAULT
))
608 if ((unlikely((curval
& FUTEX_TID_MASK
) == task_pid_vnr(task
))))
612 * Surprise - we got the lock. Just return to userspace:
614 if (unlikely(!curval
))
620 * Set the FUTEX_WAITERS flag, so the owner will know it has someone
621 * to wake at the next unlock.
623 newval
= curval
| FUTEX_WAITERS
;
626 * There are two cases, where a futex might have no owner (the
627 * owner TID is 0): OWNER_DIED. We take over the futex in this
628 * case. We also do an unconditional take over, when the owner
631 * This is safe as we are protected by the hash bucket lock !
633 if (unlikely(ownerdied
|| !(curval
& FUTEX_TID_MASK
))) {
634 /* Keep the OWNER_DIED bit */
635 newval
= (curval
& ~FUTEX_TID_MASK
) | task_pid_vnr(task
);
640 curval
= cmpxchg_futex_value_locked(uaddr
, uval
, newval
);
642 if (unlikely(curval
== -EFAULT
))
644 if (unlikely(curval
!= uval
))
648 * We took the lock due to owner died take over.
650 if (unlikely(lock_taken
))
654 * We dont have the lock. Look up the PI state (or create it if
655 * we are the first waiter):
657 ret
= lookup_pi_state(uval
, hb
, key
, ps
);
663 * No owner found for this futex. Check if the
664 * OWNER_DIED bit is set to figure out whether
665 * this is a robust futex or not.
667 if (get_futex_value_locked(&curval
, uaddr
))
671 * We simply start over in case of a robust
672 * futex. The code above will take the futex
675 if (curval
& FUTEX_OWNER_DIED
) {
688 * The hash bucket lock must be held when this is called.
689 * Afterwards, the futex_q must not be accessed.
691 static void wake_futex(struct futex_q
*q
)
693 plist_del(&q
->list
, &q
->list
.plist
);
695 * The lock in wake_up_all() is a crucial memory barrier after the
696 * plist_del() and also before assigning to q->lock_ptr.
700 * The waiting task can free the futex_q as soon as this is written,
701 * without taking any locks. This must come last.
703 * A memory barrier is required here to prevent the following store to
704 * lock_ptr from getting ahead of the wakeup. Clearing the lock at the
705 * end of wake_up() does not prevent this store from moving.
711 static int wake_futex_pi(u32 __user
*uaddr
, u32 uval
, struct futex_q
*this)
713 struct task_struct
*new_owner
;
714 struct futex_pi_state
*pi_state
= this->pi_state
;
720 spin_lock(&pi_state
->pi_mutex
.wait_lock
);
721 new_owner
= rt_mutex_next_owner(&pi_state
->pi_mutex
);
724 * This happens when we have stolen the lock and the original
725 * pending owner did not enqueue itself back on the rt_mutex.
726 * Thats not a tragedy. We know that way, that a lock waiter
727 * is on the fly. We make the futex_q waiter the pending owner.
730 new_owner
= this->task
;
733 * We pass it to the next owner. (The WAITERS bit is always
734 * kept enabled while there is PI state around. We must also
735 * preserve the owner died bit.)
737 if (!(uval
& FUTEX_OWNER_DIED
)) {
740 newval
= FUTEX_WAITERS
| task_pid_vnr(new_owner
);
742 curval
= cmpxchg_futex_value_locked(uaddr
, uval
, newval
);
744 if (curval
== -EFAULT
)
746 else if (curval
!= uval
)
749 spin_unlock(&pi_state
->pi_mutex
.wait_lock
);
754 spin_lock_irq(&pi_state
->owner
->pi_lock
);
755 WARN_ON(list_empty(&pi_state
->list
));
756 list_del_init(&pi_state
->list
);
757 spin_unlock_irq(&pi_state
->owner
->pi_lock
);
759 spin_lock_irq(&new_owner
->pi_lock
);
760 WARN_ON(!list_empty(&pi_state
->list
));
761 list_add(&pi_state
->list
, &new_owner
->pi_state_list
);
762 pi_state
->owner
= new_owner
;
763 spin_unlock_irq(&new_owner
->pi_lock
);
765 spin_unlock(&pi_state
->pi_mutex
.wait_lock
);
766 rt_mutex_unlock(&pi_state
->pi_mutex
);
771 static int unlock_futex_pi(u32 __user
*uaddr
, u32 uval
)
776 * There is no waiter, so we unlock the futex. The owner died
777 * bit has not to be preserved here. We are the owner:
779 oldval
= cmpxchg_futex_value_locked(uaddr
, uval
, 0);
781 if (oldval
== -EFAULT
)
790 * Express the locking dependencies for lockdep:
793 double_lock_hb(struct futex_hash_bucket
*hb1
, struct futex_hash_bucket
*hb2
)
796 spin_lock(&hb1
->lock
);
798 spin_lock_nested(&hb2
->lock
, SINGLE_DEPTH_NESTING
);
799 } else { /* hb1 > hb2 */
800 spin_lock(&hb2
->lock
);
801 spin_lock_nested(&hb1
->lock
, SINGLE_DEPTH_NESTING
);
806 double_unlock_hb(struct futex_hash_bucket
*hb1
, struct futex_hash_bucket
*hb2
)
808 spin_unlock(&hb1
->lock
);
810 spin_unlock(&hb2
->lock
);
814 * Wake up waiters matching bitset queued on this futex (uaddr).
816 static int futex_wake(u32 __user
*uaddr
, int fshared
, int nr_wake
, u32 bitset
)
818 struct futex_hash_bucket
*hb
;
819 struct futex_q
*this, *next
;
820 struct plist_head
*head
;
821 union futex_key key
= FUTEX_KEY_INIT
;
827 ret
= get_futex_key(uaddr
, fshared
, &key
);
828 if (unlikely(ret
!= 0))
831 hb
= hash_futex(&key
);
832 spin_lock(&hb
->lock
);
835 plist_for_each_entry_safe(this, next
, head
, list
) {
836 if (match_futex (&this->key
, &key
)) {
837 if (this->pi_state
|| this->rt_waiter
) {
842 /* Check if one of the bits is set in both bitsets */
843 if (!(this->bitset
& bitset
))
847 if (++ret
>= nr_wake
)
852 spin_unlock(&hb
->lock
);
853 put_futex_key(fshared
, &key
);
859 * Wake up all waiters hashed on the physical page that is mapped
860 * to this virtual address:
863 futex_wake_op(u32 __user
*uaddr1
, int fshared
, u32 __user
*uaddr2
,
864 int nr_wake
, int nr_wake2
, int op
)
866 union futex_key key1
= FUTEX_KEY_INIT
, key2
= FUTEX_KEY_INIT
;
867 struct futex_hash_bucket
*hb1
, *hb2
;
868 struct plist_head
*head
;
869 struct futex_q
*this, *next
;
873 ret
= get_futex_key(uaddr1
, fshared
, &key1
);
874 if (unlikely(ret
!= 0))
876 ret
= get_futex_key(uaddr2
, fshared
, &key2
);
877 if (unlikely(ret
!= 0))
880 hb1
= hash_futex(&key1
);
881 hb2
= hash_futex(&key2
);
883 double_lock_hb(hb1
, hb2
);
885 op_ret
= futex_atomic_op_inuser(op
, uaddr2
);
886 if (unlikely(op_ret
< 0)) {
889 double_unlock_hb(hb1
, hb2
);
893 * we don't get EFAULT from MMU faults if we don't have an MMU,
894 * but we might get them from range checking
900 if (unlikely(op_ret
!= -EFAULT
)) {
905 ret
= get_user(dummy
, uaddr2
);
912 put_futex_key(fshared
, &key2
);
913 put_futex_key(fshared
, &key1
);
919 plist_for_each_entry_safe(this, next
, head
, list
) {
920 if (match_futex (&this->key
, &key1
)) {
922 if (++ret
>= nr_wake
)
931 plist_for_each_entry_safe(this, next
, head
, list
) {
932 if (match_futex (&this->key
, &key2
)) {
934 if (++op_ret
>= nr_wake2
)
941 double_unlock_hb(hb1
, hb2
);
943 put_futex_key(fshared
, &key2
);
945 put_futex_key(fshared
, &key1
);
951 * requeue_futex() - Requeue a futex_q from one hb to another
952 * @q: the futex_q to requeue
953 * @hb1: the source hash_bucket
954 * @hb2: the target hash_bucket
955 * @key2: the new key for the requeued futex_q
958 void requeue_futex(struct futex_q
*q
, struct futex_hash_bucket
*hb1
,
959 struct futex_hash_bucket
*hb2
, union futex_key
*key2
)
963 * If key1 and key2 hash to the same bucket, no need to
966 if (likely(&hb1
->chain
!= &hb2
->chain
)) {
967 plist_del(&q
->list
, &hb1
->chain
);
968 plist_add(&q
->list
, &hb2
->chain
);
969 q
->lock_ptr
= &hb2
->lock
;
970 #ifdef CONFIG_DEBUG_PI_LIST
971 q
->list
.plist
.lock
= &hb2
->lock
;
974 get_futex_key_refs(key2
);
979 * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
981 * key: the key of the requeue target futex
983 * During futex_requeue, with requeue_pi=1, it is possible to acquire the
984 * target futex if it is uncontended or via a lock steal. Set the futex_q key
985 * to the requeue target futex so the waiter can detect the wakeup on the right
986 * futex, but remove it from the hb and NULL the rt_waiter so it can detect
987 * atomic lock acquisition. Must be called with the q->lock_ptr held.
990 void requeue_pi_wake_futex(struct futex_q
*q
, union futex_key
*key
)
992 drop_futex_key_refs(&q
->key
);
993 get_futex_key_refs(key
);
996 WARN_ON(plist_node_empty(&q
->list
));
997 plist_del(&q
->list
, &q
->list
.plist
);
999 WARN_ON(!q
->rt_waiter
);
1000 q
->rt_waiter
= NULL
;
1002 wake_up(&q
->waiter
);
1006 * futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
1007 * @pifutex: the user address of the to futex
1008 * @hb1: the from futex hash bucket, must be locked by the caller
1009 * @hb2: the to futex hash bucket, must be locked by the caller
1010 * @key1: the from futex key
1011 * @key2: the to futex key
1013 * Try and get the lock on behalf of the top waiter if we can do it atomically.
1014 * Wake the top waiter if we succeed. hb1 and hb2 must be held by the caller.
1017 * 0 - failed to acquire the lock atomicly
1018 * 1 - acquired the lock
1021 static int futex_proxy_trylock_atomic(u32 __user
*pifutex
,
1022 struct futex_hash_bucket
*hb1
,
1023 struct futex_hash_bucket
*hb2
,
1024 union futex_key
*key1
, union futex_key
*key2
,
1025 struct futex_pi_state
**ps
)
1027 struct futex_q
*top_waiter
;
1031 if (get_futex_value_locked(&curval
, pifutex
))
1034 top_waiter
= futex_top_waiter(hb1
, key1
);
1036 /* There are no waiters, nothing for us to do. */
1041 * Either take the lock for top_waiter or set the FUTEX_WAITERS bit.
1042 * The pi_state is returned in ps in contended cases.
1044 ret
= futex_lock_pi_atomic(pifutex
, hb2
, key2
, ps
, top_waiter
->task
);
1046 requeue_pi_wake_futex(top_waiter
, key2
);
1052 * futex_requeue() - Requeue waiters from uaddr1 to uaddr2
1053 * uaddr1: source futex user address
1054 * uaddr2: target futex user address
1055 * nr_wake: number of waiters to wake (must be 1 for requeue_pi)
1056 * nr_requeue: number of waiters to requeue (0-INT_MAX)
1057 * requeue_pi: if we are attempting to requeue from a non-pi futex to a
1058 * pi futex (pi to pi requeue is not supported)
1060 * Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
1061 * uaddr2 atomically on behalf of the top waiter.
1064 * >=0 - on success, the number of tasks requeued or woken
1067 static int futex_requeue(u32 __user
*uaddr1
, int fshared
, u32 __user
*uaddr2
,
1068 int nr_wake
, int nr_requeue
, u32
*cmpval
,
1071 union futex_key key1
= FUTEX_KEY_INIT
, key2
= FUTEX_KEY_INIT
;
1072 int drop_count
= 0, task_count
= 0, ret
;
1073 struct futex_pi_state
*pi_state
= NULL
;
1074 struct futex_hash_bucket
*hb1
, *hb2
;
1075 struct plist_head
*head1
;
1076 struct futex_q
*this, *next
;
1081 * requeue_pi requires a pi_state, try to allocate it now
1082 * without any locks in case it fails.
1084 if (refill_pi_state_cache())
1087 * requeue_pi must wake as many tasks as it can, up to nr_wake
1088 * + nr_requeue, since it acquires the rt_mutex prior to
1089 * returning to userspace, so as to not leave the rt_mutex with
1090 * waiters and no owner. However, second and third wake-ups
1091 * cannot be predicted as they involve race conditions with the
1092 * first wake and a fault while looking up the pi_state. Both
1093 * pthread_cond_signal() and pthread_cond_broadcast() should
1101 if (pi_state
!= NULL
) {
1103 * We will have to lookup the pi_state again, so free this one
1104 * to keep the accounting correct.
1106 free_pi_state(pi_state
);
1110 ret
= get_futex_key(uaddr1
, fshared
, &key1
);
1111 if (unlikely(ret
!= 0))
1113 ret
= get_futex_key(uaddr2
, fshared
, &key2
);
1114 if (unlikely(ret
!= 0))
1117 hb1
= hash_futex(&key1
);
1118 hb2
= hash_futex(&key2
);
1121 double_lock_hb(hb1
, hb2
);
1123 if (likely(cmpval
!= NULL
)) {
1126 ret
= get_futex_value_locked(&curval
, uaddr1
);
1128 if (unlikely(ret
)) {
1129 double_unlock_hb(hb1
, hb2
);
1131 ret
= get_user(curval
, uaddr1
);
1138 put_futex_key(fshared
, &key2
);
1139 put_futex_key(fshared
, &key1
);
1142 if (curval
!= *cmpval
) {
1148 if (requeue_pi
&& (task_count
- nr_wake
< nr_requeue
)) {
1149 /* Attempt to acquire uaddr2 and wake the top_waiter. */
1150 ret
= futex_proxy_trylock_atomic(uaddr2
, hb1
, hb2
, &key1
,
1154 * At this point the top_waiter has either taken uaddr2 or is
1155 * waiting on it. If the former, then the pi_state will not
1156 * exist yet, look it up one more time to ensure we have a
1162 ret
= get_futex_value_locked(&curval2
, uaddr2
);
1164 ret
= lookup_pi_state(curval2
, hb2
, &key2
,
1172 double_unlock_hb(hb1
, hb2
);
1173 put_futex_key(fshared
, &key2
);
1174 put_futex_key(fshared
, &key1
);
1175 ret
= get_user(curval2
, uaddr2
);
1180 /* The owner was exiting, try again. */
1181 double_unlock_hb(hb1
, hb2
);
1182 put_futex_key(fshared
, &key2
);
1183 put_futex_key(fshared
, &key1
);
1191 head1
= &hb1
->chain
;
1192 plist_for_each_entry_safe(this, next
, head1
, list
) {
1193 if (task_count
- nr_wake
>= nr_requeue
)
1196 if (!match_futex(&this->key
, &key1
))
1199 WARN_ON(!requeue_pi
&& this->rt_waiter
);
1200 WARN_ON(requeue_pi
&& !this->rt_waiter
);
1203 * Wake nr_wake waiters. For requeue_pi, if we acquired the
1204 * lock, we already woke the top_waiter. If not, it will be
1205 * woken by futex_unlock_pi().
1207 if (++task_count
<= nr_wake
&& !requeue_pi
) {
1213 * Requeue nr_requeue waiters and possibly one more in the case
1214 * of requeue_pi if we couldn't acquire the lock atomically.
1217 /* Prepare the waiter to take the rt_mutex. */
1218 atomic_inc(&pi_state
->refcount
);
1219 this->pi_state
= pi_state
;
1220 ret
= rt_mutex_start_proxy_lock(&pi_state
->pi_mutex
,
1224 /* We got the lock. */
1225 requeue_pi_wake_futex(this, &key2
);
1229 this->pi_state
= NULL
;
1230 free_pi_state(pi_state
);
1234 requeue_futex(this, hb1
, hb2
, &key2
);
1239 double_unlock_hb(hb1
, hb2
);
1241 /* drop_futex_key_refs() must be called outside the spinlocks. */
1242 while (--drop_count
>= 0)
1243 drop_futex_key_refs(&key1
);
1246 put_futex_key(fshared
, &key2
);
1248 put_futex_key(fshared
, &key1
);
1250 if (pi_state
!= NULL
)
1251 free_pi_state(pi_state
);
1252 return ret
? ret
: task_count
;
1255 /* The key must be already stored in q->key. */
1256 static inline struct futex_hash_bucket
*queue_lock(struct futex_q
*q
)
1258 struct futex_hash_bucket
*hb
;
1260 init_waitqueue_head(&q
->waiter
);
1262 get_futex_key_refs(&q
->key
);
1263 hb
= hash_futex(&q
->key
);
1264 q
->lock_ptr
= &hb
->lock
;
1266 spin_lock(&hb
->lock
);
1270 static inline void queue_me(struct futex_q
*q
, struct futex_hash_bucket
*hb
)
1275 * The priority used to register this element is
1276 * - either the real thread-priority for the real-time threads
1277 * (i.e. threads with a priority lower than MAX_RT_PRIO)
1278 * - or MAX_RT_PRIO for non-RT threads.
1279 * Thus, all RT-threads are woken first in priority order, and
1280 * the others are woken last, in FIFO order.
1282 prio
= min(current
->normal_prio
, MAX_RT_PRIO
);
1284 plist_node_init(&q
->list
, prio
);
1285 #ifdef CONFIG_DEBUG_PI_LIST
1286 q
->list
.plist
.lock
= &hb
->lock
;
1288 plist_add(&q
->list
, &hb
->chain
);
1290 spin_unlock(&hb
->lock
);
1294 queue_unlock(struct futex_q
*q
, struct futex_hash_bucket
*hb
)
1296 spin_unlock(&hb
->lock
);
1297 drop_futex_key_refs(&q
->key
);
1301 * queue_me and unqueue_me must be called as a pair, each
1302 * exactly once. They are called with the hashed spinlock held.
1305 /* Return 1 if we were still queued (ie. 0 means we were woken) */
1306 static int unqueue_me(struct futex_q
*q
)
1308 spinlock_t
*lock_ptr
;
1311 /* In the common case we don't take the spinlock, which is nice. */
1313 lock_ptr
= q
->lock_ptr
;
1315 if (lock_ptr
!= NULL
) {
1316 spin_lock(lock_ptr
);
1318 * q->lock_ptr can change between reading it and
1319 * spin_lock(), causing us to take the wrong lock. This
1320 * corrects the race condition.
1322 * Reasoning goes like this: if we have the wrong lock,
1323 * q->lock_ptr must have changed (maybe several times)
1324 * between reading it and the spin_lock(). It can
1325 * change again after the spin_lock() but only if it was
1326 * already changed before the spin_lock(). It cannot,
1327 * however, change back to the original value. Therefore
1328 * we can detect whether we acquired the correct lock.
1330 if (unlikely(lock_ptr
!= q
->lock_ptr
)) {
1331 spin_unlock(lock_ptr
);
1334 WARN_ON(plist_node_empty(&q
->list
));
1335 plist_del(&q
->list
, &q
->list
.plist
);
1337 BUG_ON(q
->pi_state
);
1339 spin_unlock(lock_ptr
);
1343 drop_futex_key_refs(&q
->key
);
1348 * PI futexes can not be requeued and must remove themself from the
1349 * hash bucket. The hash bucket lock (i.e. lock_ptr) is held on entry
1352 static void unqueue_me_pi(struct futex_q
*q
)
1354 WARN_ON(plist_node_empty(&q
->list
));
1355 plist_del(&q
->list
, &q
->list
.plist
);
1357 BUG_ON(!q
->pi_state
);
1358 free_pi_state(q
->pi_state
);
1361 spin_unlock(q
->lock_ptr
);
1363 drop_futex_key_refs(&q
->key
);
1367 * Fixup the pi_state owner with the new owner.
1369 * Must be called with hash bucket lock held and mm->sem held for non
1372 static int fixup_pi_state_owner(u32 __user
*uaddr
, struct futex_q
*q
,
1373 struct task_struct
*newowner
, int fshared
)
1375 u32 newtid
= task_pid_vnr(newowner
) | FUTEX_WAITERS
;
1376 struct futex_pi_state
*pi_state
= q
->pi_state
;
1377 struct task_struct
*oldowner
= pi_state
->owner
;
1378 u32 uval
, curval
, newval
;
1382 if (!pi_state
->owner
)
1383 newtid
|= FUTEX_OWNER_DIED
;
1386 * We are here either because we stole the rtmutex from the
1387 * pending owner or we are the pending owner which failed to
1388 * get the rtmutex. We have to replace the pending owner TID
1389 * in the user space variable. This must be atomic as we have
1390 * to preserve the owner died bit here.
1392 * Note: We write the user space value _before_ changing the pi_state
1393 * because we can fault here. Imagine swapped out pages or a fork
1394 * that marked all the anonymous memory readonly for cow.
1396 * Modifying pi_state _before_ the user space value would
1397 * leave the pi_state in an inconsistent state when we fault
1398 * here, because we need to drop the hash bucket lock to
1399 * handle the fault. This might be observed in the PID check
1400 * in lookup_pi_state.
1403 if (get_futex_value_locked(&uval
, uaddr
))
1407 newval
= (uval
& FUTEX_OWNER_DIED
) | newtid
;
1409 curval
= cmpxchg_futex_value_locked(uaddr
, uval
, newval
);
1411 if (curval
== -EFAULT
)
1419 * We fixed up user space. Now we need to fix the pi_state
1422 if (pi_state
->owner
!= NULL
) {
1423 spin_lock_irq(&pi_state
->owner
->pi_lock
);
1424 WARN_ON(list_empty(&pi_state
->list
));
1425 list_del_init(&pi_state
->list
);
1426 spin_unlock_irq(&pi_state
->owner
->pi_lock
);
1429 pi_state
->owner
= newowner
;
1431 spin_lock_irq(&newowner
->pi_lock
);
1432 WARN_ON(!list_empty(&pi_state
->list
));
1433 list_add(&pi_state
->list
, &newowner
->pi_state_list
);
1434 spin_unlock_irq(&newowner
->pi_lock
);
1438 * To handle the page fault we need to drop the hash bucket
1439 * lock here. That gives the other task (either the pending
1440 * owner itself or the task which stole the rtmutex) the
1441 * chance to try the fixup of the pi_state. So once we are
1442 * back from handling the fault we need to check the pi_state
1443 * after reacquiring the hash bucket lock and before trying to
1444 * do another fixup. When the fixup has been done already we
1448 spin_unlock(q
->lock_ptr
);
1450 ret
= get_user(uval
, uaddr
);
1452 spin_lock(q
->lock_ptr
);
1455 * Check if someone else fixed it for us:
1457 if (pi_state
->owner
!= oldowner
)
1467 * In case we must use restart_block to restart a futex_wait,
1468 * we encode in the 'flags' shared capability
1470 #define FLAGS_SHARED 0x01
1471 #define FLAGS_CLOCKRT 0x02
1472 #define FLAGS_HAS_TIMEOUT 0x04
1474 static long futex_wait_restart(struct restart_block
*restart
);
1475 static long futex_lock_pi_restart(struct restart_block
*restart
);
1478 * fixup_owner() - Post lock pi_state and corner case management
1479 * @uaddr: user address of the futex
1480 * @fshared: whether the futex is shared (1) or not (0)
1481 * @q: futex_q (contains pi_state and access to the rt_mutex)
1482 * @locked: if the attempt to take the rt_mutex succeeded (1) or not (0)
1484 * After attempting to lock an rt_mutex, this function is called to cleanup
1485 * the pi_state owner as well as handle race conditions that may allow us to
1486 * acquire the lock. Must be called with the hb lock held.
1489 * 1 - success, lock taken
1490 * 0 - success, lock not taken
1491 * <0 - on error (-EFAULT)
1493 static int fixup_owner(u32 __user
*uaddr
, int fshared
, struct futex_q
*q
,
1496 struct task_struct
*owner
;
1501 * Got the lock. We might not be the anticipated owner if we
1502 * did a lock-steal - fix up the PI-state in that case:
1504 if (q
->pi_state
->owner
!= current
)
1505 ret
= fixup_pi_state_owner(uaddr
, q
, current
, fshared
);
1510 * Catch the rare case, where the lock was released when we were on the
1511 * way back before we locked the hash bucket.
1513 if (q
->pi_state
->owner
== current
) {
1515 * Try to get the rt_mutex now. This might fail as some other
1516 * task acquired the rt_mutex after we removed ourself from the
1517 * rt_mutex waiters list.
1519 if (rt_mutex_trylock(&q
->pi_state
->pi_mutex
)) {
1525 * pi_state is incorrect, some other task did a lock steal and
1526 * we returned due to timeout or signal without taking the
1527 * rt_mutex. Too late. We can access the rt_mutex_owner without
1528 * locking, as the other task is now blocked on the hash bucket
1529 * lock. Fix the state up.
1531 owner
= rt_mutex_owner(&q
->pi_state
->pi_mutex
);
1532 ret
= fixup_pi_state_owner(uaddr
, q
, owner
, fshared
);
1537 * Paranoia check. If we did not take the lock, then we should not be
1538 * the owner, nor the pending owner, of the rt_mutex.
1540 if (rt_mutex_owner(&q
->pi_state
->pi_mutex
) == current
)
1541 printk(KERN_ERR
"fixup_owner: ret = %d pi-mutex: %p "
1542 "pi-state %p\n", ret
,
1543 q
->pi_state
->pi_mutex
.owner
,
1544 q
->pi_state
->owner
);
1547 return ret
? ret
: locked
;
1551 * futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal
1552 * @hb: the futex hash bucket, must be locked by the caller
1553 * @q: the futex_q to queue up on
1554 * @timeout: the prepared hrtimer_sleeper, or null for no timeout
1555 * @wait: the wait_queue to add to the futex_q after queueing in the hb
1557 static void futex_wait_queue_me(struct futex_hash_bucket
*hb
, struct futex_q
*q
,
1558 struct hrtimer_sleeper
*timeout
,
1564 * There might have been scheduling since the queue_me(), as we
1565 * cannot hold a spinlock across the get_user() in case it
1566 * faults, and we cannot just set TASK_INTERRUPTIBLE state when
1567 * queueing ourselves into the futex hash. This code thus has to
1568 * rely on the futex_wake() code removing us from hash when it
1572 /* add_wait_queue is the barrier after __set_current_state. */
1573 __set_current_state(TASK_INTERRUPTIBLE
);
1576 * Add current as the futex_q waiter. We don't remove ourselves from
1577 * the wait_queue because we are the only user of it.
1579 add_wait_queue(&q
->waiter
, wait
);
1583 hrtimer_start_expires(&timeout
->timer
, HRTIMER_MODE_ABS
);
1584 if (!hrtimer_active(&timeout
->timer
))
1585 timeout
->task
= NULL
;
1589 * !plist_node_empty() is safe here without any lock.
1590 * q.lock_ptr != 0 is not safe, because of ordering against wakeup.
1592 if (likely(!plist_node_empty(&q
->list
))) {
1594 * If the timer has already expired, current will already be
1595 * flagged for rescheduling. Only call schedule if there
1596 * is no timeout, or if it has yet to expire.
1598 if (!timeout
|| timeout
->task
)
1601 __set_current_state(TASK_RUNNING
);
1605 * futex_wait_setup() - Prepare to wait on a futex
1606 * @uaddr: the futex userspace address
1607 * @val: the expected value
1608 * @fshared: whether the futex is shared (1) or not (0)
1609 * @q: the associated futex_q
1610 * @hb: storage for hash_bucket pointer to be returned to caller
1612 * Setup the futex_q and locate the hash_bucket. Get the futex value and
1613 * compare it with the expected value. Handle atomic faults internally.
1614 * Return with the hb lock held and a q.key reference on success, and unlocked
1615 * with no q.key reference on failure.
1618 * 0 - uaddr contains val and hb has been locked
1619 * <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlcoked
1621 static int futex_wait_setup(u32 __user
*uaddr
, u32 val
, int fshared
,
1622 struct futex_q
*q
, struct futex_hash_bucket
**hb
)
1628 * Access the page AFTER the hash-bucket is locked.
1629 * Order is important:
1631 * Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
1632 * Userspace waker: if (cond(var)) { var = new; futex_wake(&var); }
1634 * The basic logical guarantee of a futex is that it blocks ONLY
1635 * if cond(var) is known to be true at the time of blocking, for
1636 * any cond. If we queued after testing *uaddr, that would open
1637 * a race condition where we could block indefinitely with
1638 * cond(var) false, which would violate the guarantee.
1640 * A consequence is that futex_wait() can return zero and absorb
1641 * a wakeup when *uaddr != val on entry to the syscall. This is
1645 q
->key
= FUTEX_KEY_INIT
;
1646 ret
= get_futex_key(uaddr
, fshared
, &q
->key
);
1647 if (unlikely(ret
!= 0))
1651 *hb
= queue_lock(q
);
1653 ret
= get_futex_value_locked(&uval
, uaddr
);
1656 queue_unlock(q
, *hb
);
1658 ret
= get_user(uval
, uaddr
);
1665 put_futex_key(fshared
, &q
->key
);
1670 queue_unlock(q
, *hb
);
1676 put_futex_key(fshared
, &q
->key
);
1680 static int futex_wait(u32 __user
*uaddr
, int fshared
,
1681 u32 val
, ktime_t
*abs_time
, u32 bitset
, int clockrt
)
1683 struct hrtimer_sleeper timeout
, *to
= NULL
;
1684 DECLARE_WAITQUEUE(wait
, current
);
1685 struct restart_block
*restart
;
1686 struct futex_hash_bucket
*hb
;
1700 hrtimer_init_on_stack(&to
->timer
, clockrt
? CLOCK_REALTIME
:
1701 CLOCK_MONOTONIC
, HRTIMER_MODE_ABS
);
1702 hrtimer_init_sleeper(to
, current
);
1703 hrtimer_set_expires_range_ns(&to
->timer
, *abs_time
,
1704 current
->timer_slack_ns
);
1707 /* Prepare to wait on uaddr. */
1708 ret
= futex_wait_setup(uaddr
, val
, fshared
, &q
, &hb
);
1712 /* queue_me and wait for wakeup, timeout, or a signal. */
1713 futex_wait_queue_me(hb
, &q
, to
, &wait
);
1715 /* If we were woken (and unqueued), we succeeded, whatever. */
1717 if (!unqueue_me(&q
))
1720 if (to
&& !to
->task
)
1724 * We expect signal_pending(current), but another thread may
1725 * have handled it for us already.
1731 restart
= ¤t_thread_info()->restart_block
;
1732 restart
->fn
= futex_wait_restart
;
1733 restart
->futex
.uaddr
= (u32
*)uaddr
;
1734 restart
->futex
.val
= val
;
1735 restart
->futex
.time
= abs_time
->tv64
;
1736 restart
->futex
.bitset
= bitset
;
1737 restart
->futex
.flags
= FLAGS_HAS_TIMEOUT
;
1740 restart
->futex
.flags
|= FLAGS_SHARED
;
1742 restart
->futex
.flags
|= FLAGS_CLOCKRT
;
1744 ret
= -ERESTART_RESTARTBLOCK
;
1747 put_futex_key(fshared
, &q
.key
);
1750 hrtimer_cancel(&to
->timer
);
1751 destroy_hrtimer_on_stack(&to
->timer
);
1757 static long futex_wait_restart(struct restart_block
*restart
)
1759 u32 __user
*uaddr
= (u32 __user
*)restart
->futex
.uaddr
;
1761 ktime_t t
, *tp
= NULL
;
1763 if (restart
->futex
.flags
& FLAGS_HAS_TIMEOUT
) {
1764 t
.tv64
= restart
->futex
.time
;
1767 restart
->fn
= do_no_restart_syscall
;
1768 if (restart
->futex
.flags
& FLAGS_SHARED
)
1770 return (long)futex_wait(uaddr
, fshared
, restart
->futex
.val
, tp
,
1771 restart
->futex
.bitset
,
1772 restart
->futex
.flags
& FLAGS_CLOCKRT
);
1777 * Userspace tried a 0 -> TID atomic transition of the futex value
1778 * and failed. The kernel side here does the whole locking operation:
1779 * if there are waiters then it will block, it does PI, etc. (Due to
1780 * races the kernel might see a 0 value of the futex too.)
1782 static int futex_lock_pi(u32 __user
*uaddr
, int fshared
,
1783 int detect
, ktime_t
*time
, int trylock
)
1785 struct hrtimer_sleeper timeout
, *to
= NULL
;
1786 struct futex_hash_bucket
*hb
;
1791 if (refill_pi_state_cache())
1796 hrtimer_init_on_stack(&to
->timer
, CLOCK_REALTIME
,
1798 hrtimer_init_sleeper(to
, current
);
1799 hrtimer_set_expires(&to
->timer
, *time
);
1805 q
.key
= FUTEX_KEY_INIT
;
1806 ret
= get_futex_key(uaddr
, fshared
, &q
.key
);
1807 if (unlikely(ret
!= 0))
1811 hb
= queue_lock(&q
);
1813 ret
= futex_lock_pi_atomic(uaddr
, hb
, &q
.key
, &q
.pi_state
, current
);
1814 if (unlikely(ret
)) {
1817 /* We got the lock. */
1819 goto out_unlock_put_key
;
1824 * Task is exiting and we just wait for the
1827 queue_unlock(&q
, hb
);
1828 put_futex_key(fshared
, &q
.key
);
1832 goto out_unlock_put_key
;
1837 * Only actually queue now that the atomic ops are done:
1841 WARN_ON(!q
.pi_state
);
1843 * Block on the PI mutex:
1846 ret
= rt_mutex_timed_lock(&q
.pi_state
->pi_mutex
, to
, 1);
1848 ret
= rt_mutex_trylock(&q
.pi_state
->pi_mutex
);
1849 /* Fixup the trylock return value: */
1850 ret
= ret
? 0 : -EWOULDBLOCK
;
1853 spin_lock(q
.lock_ptr
);
1855 * Fixup the pi_state owner and possibly acquire the lock if we
1858 res
= fixup_owner(uaddr
, fshared
, &q
, !ret
);
1860 * If fixup_owner() returned an error, proprogate that. If it acquired
1861 * the lock, clear our -ETIMEDOUT or -EINTR.
1864 ret
= (res
< 0) ? res
: 0;
1867 * If fixup_owner() faulted and was unable to handle the fault, unlock
1868 * it and return the fault to userspace.
1870 if (ret
&& (rt_mutex_owner(&q
.pi_state
->pi_mutex
) == current
))
1871 rt_mutex_unlock(&q
.pi_state
->pi_mutex
);
1873 /* Unqueue and drop the lock */
1879 queue_unlock(&q
, hb
);
1882 put_futex_key(fshared
, &q
.key
);
1885 destroy_hrtimer_on_stack(&to
->timer
);
1886 return ret
!= -EINTR
? ret
: -ERESTARTNOINTR
;
1890 * We have to r/w *(int __user *)uaddr, and we have to modify it
1891 * atomically. Therefore, if we continue to fault after get_user()
1892 * below, we need to handle the fault ourselves, while still holding
1893 * the mmap_sem. This can occur if the uaddr is under contention as
1894 * we have to drop the mmap_sem in order to call get_user().
1896 queue_unlock(&q
, hb
);
1898 ret
= get_user(uval
, uaddr
);
1905 put_futex_key(fshared
, &q
.key
);
1909 static long futex_lock_pi_restart(struct restart_block
*restart
)
1911 u32 __user
*uaddr
= (u32 __user
*)restart
->futex
.uaddr
;
1912 ktime_t t
, *tp
= NULL
;
1913 int fshared
= restart
->futex
.flags
& FLAGS_SHARED
;
1915 if (restart
->futex
.flags
& FLAGS_HAS_TIMEOUT
) {
1916 t
.tv64
= restart
->futex
.time
;
1919 restart
->fn
= do_no_restart_syscall
;
1921 return (long)futex_lock_pi(uaddr
, fshared
, restart
->futex
.val
, tp
, 0);
1925 * Userspace attempted a TID -> 0 atomic transition, and failed.
1926 * This is the in-kernel slowpath: we look up the PI state (if any),
1927 * and do the rt-mutex unlock.
1929 static int futex_unlock_pi(u32 __user
*uaddr
, int fshared
)
1931 struct futex_hash_bucket
*hb
;
1932 struct futex_q
*this, *next
;
1934 struct plist_head
*head
;
1935 union futex_key key
= FUTEX_KEY_INIT
;
1939 if (get_user(uval
, uaddr
))
1942 * We release only a lock we actually own:
1944 if ((uval
& FUTEX_TID_MASK
) != task_pid_vnr(current
))
1947 ret
= get_futex_key(uaddr
, fshared
, &key
);
1948 if (unlikely(ret
!= 0))
1951 hb
= hash_futex(&key
);
1952 spin_lock(&hb
->lock
);
1955 * To avoid races, try to do the TID -> 0 atomic transition
1956 * again. If it succeeds then we can return without waking
1959 if (!(uval
& FUTEX_OWNER_DIED
))
1960 uval
= cmpxchg_futex_value_locked(uaddr
, task_pid_vnr(current
), 0);
1963 if (unlikely(uval
== -EFAULT
))
1966 * Rare case: we managed to release the lock atomically,
1967 * no need to wake anyone else up:
1969 if (unlikely(uval
== task_pid_vnr(current
)))
1973 * Ok, other tasks may need to be woken up - check waiters
1974 * and do the wakeup if necessary:
1978 plist_for_each_entry_safe(this, next
, head
, list
) {
1979 if (!match_futex (&this->key
, &key
))
1981 ret
= wake_futex_pi(uaddr
, uval
, this);
1983 * The atomic access to the futex value
1984 * generated a pagefault, so retry the
1985 * user-access and the wakeup:
1992 * No waiters - kernel unlocks the futex:
1994 if (!(uval
& FUTEX_OWNER_DIED
)) {
1995 ret
= unlock_futex_pi(uaddr
, uval
);
2001 spin_unlock(&hb
->lock
);
2002 put_futex_key(fshared
, &key
);
2009 * We have to r/w *(int __user *)uaddr, and we have to modify it
2010 * atomically. Therefore, if we continue to fault after get_user()
2011 * below, we need to handle the fault ourselves, while still holding
2012 * the mmap_sem. This can occur if the uaddr is under contention as
2013 * we have to drop the mmap_sem in order to call get_user().
2015 spin_unlock(&hb
->lock
);
2016 put_futex_key(fshared
, &key
);
2018 ret
= get_user(uval
, uaddr
);
2026 * handle_early_requeue_pi_wakeup() - Detect early wakeup on the initial futex
2027 * @hb: the hash_bucket futex_q was original enqueued on
2028 * @q: the futex_q woken while waiting to be requeued
2029 * @key2: the futex_key of the requeue target futex
2030 * @timeout: the timeout associated with the wait (NULL if none)
2032 * Detect if the task was woken on the initial futex as opposed to the requeue
2033 * target futex. If so, determine if it was a timeout or a signal that caused
2034 * the wakeup and return the appropriate error code to the caller. Must be
2035 * called with the hb lock held.
2038 * 0 - no early wakeup detected
2039 * <0 - -ETIMEDOUT or -ERESTARTSYS (FIXME: or ERESTARTNOINTR?)
2042 int handle_early_requeue_pi_wakeup(struct futex_hash_bucket
*hb
,
2043 struct futex_q
*q
, union futex_key
*key2
,
2044 struct hrtimer_sleeper
*timeout
)
2049 * With the hb lock held, we avoid races while we process the wakeup.
2050 * We only need to hold hb (and not hb2) to ensure atomicity as the
2051 * wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
2052 * It can't be requeued from uaddr2 to something else since we don't
2053 * support a PI aware source futex for requeue.
2055 if (!match_futex(&q
->key
, key2
)) {
2056 WARN_ON(q
->lock_ptr
&& (&hb
->lock
!= q
->lock_ptr
));
2058 * We were woken prior to requeue by a timeout or a signal.
2059 * Unqueue the futex_q and determine which it was.
2061 plist_del(&q
->list
, &q
->list
.plist
);
2062 drop_futex_key_refs(&q
->key
);
2064 if (timeout
&& !timeout
->task
)
2068 * We expect signal_pending(current), but another
2069 * thread may have handled it for us already.
2071 /* FIXME: ERESTARTSYS or ERESTARTNOINTR? Do we care if
2072 * the user specified SA_RESTART or not? */
2080 * futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
2081 * @uaddr: the futex we initialyl wait on (non-pi)
2082 * @fshared: whether the futexes are shared (1) or not (0). They must be
2083 * the same type, no requeueing from private to shared, etc.
2084 * @val: the expected value of uaddr
2085 * @abs_time: absolute timeout
2086 * @bitset: 32 bit wakeup bitset set by userspace, defaults to all.
2087 * @clockrt: whether to use CLOCK_REALTIME (1) or CLOCK_MONOTONIC (0)
2088 * @uaddr2: the pi futex we will take prior to returning to user-space
2090 * The caller will wait on uaddr and will be requeued by futex_requeue() to
2091 * uaddr2 which must be PI aware. Normal wakeup will wake on uaddr2 and
2092 * complete the acquisition of the rt_mutex prior to returning to userspace.
2093 * This ensures the rt_mutex maintains an owner when it has waiters; without
2094 * one, the pi logic wouldn't know which task to boost/deboost, if there was a
2097 * We call schedule in futex_wait_queue_me() when we enqueue and return there
2098 * via the following:
2099 * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
2100 * 2) wakeup on uaddr2 after a requeue and subsequent unlock
2101 * 3) signal (before or after requeue)
2102 * 4) timeout (before or after requeue)
2104 * If 3, we setup a restart_block with futex_wait_requeue_pi() as the function.
2106 * If 2, we may then block on trying to take the rt_mutex and return via:
2107 * 5) successful lock
2110 * 8) other lock acquisition failure
2112 * If 6, we setup a restart_block with futex_lock_pi() as the function.
2114 * If 4 or 7, we cleanup and return with -ETIMEDOUT.
2120 static int futex_wait_requeue_pi(u32 __user
*uaddr
, int fshared
,
2121 u32 val
, ktime_t
*abs_time
, u32 bitset
,
2122 int clockrt
, u32 __user
*uaddr2
)
2124 struct hrtimer_sleeper timeout
, *to
= NULL
;
2125 struct rt_mutex_waiter rt_waiter
;
2126 struct rt_mutex
*pi_mutex
= NULL
;
2127 DECLARE_WAITQUEUE(wait
, current
);
2128 struct restart_block
*restart
;
2129 struct futex_hash_bucket
*hb
;
2130 union futex_key key2
;
2140 hrtimer_init_on_stack(&to
->timer
, clockrt
? CLOCK_REALTIME
:
2141 CLOCK_MONOTONIC
, HRTIMER_MODE_ABS
);
2142 hrtimer_init_sleeper(to
, current
);
2143 hrtimer_set_expires_range_ns(&to
->timer
, *abs_time
,
2144 current
->timer_slack_ns
);
2148 * The waiter is allocated on our stack, manipulated by the requeue
2149 * code while we sleep on uaddr.
2151 debug_rt_mutex_init_waiter(&rt_waiter
);
2152 rt_waiter
.task
= NULL
;
2156 q
.rt_waiter
= &rt_waiter
;
2158 key2
= FUTEX_KEY_INIT
;
2159 ret
= get_futex_key(uaddr2
, fshared
, &key2
);
2160 if (unlikely(ret
!= 0))
2163 /* Prepare to wait on uaddr. */
2164 ret
= futex_wait_setup(uaddr
, val
, fshared
, &q
, &hb
);
2166 put_futex_key(fshared
, &key2
);
2170 /* Queue the futex_q, drop the hb lock, wait for wakeup. */
2171 futex_wait_queue_me(hb
, &q
, to
, &wait
);
2173 spin_lock(&hb
->lock
);
2174 ret
= handle_early_requeue_pi_wakeup(hb
, &q
, &key2
, to
);
2175 spin_unlock(&hb
->lock
);
2180 * In order for us to be here, we know our q.key == key2, and since
2181 * we took the hb->lock above, we also know that futex_requeue() has
2182 * completed and we no longer have to concern ourselves with a wakeup
2183 * race with the atomic proxy lock acquition by the requeue code.
2186 /* Check if the requeue code acquired the second futex for us. */
2189 * Got the lock. We might not be the anticipated owner if we
2190 * did a lock-steal - fix up the PI-state in that case.
2192 if (q
.pi_state
&& (q
.pi_state
->owner
!= current
)) {
2193 spin_lock(q
.lock_ptr
);
2194 ret
= fixup_pi_state_owner(uaddr2
, &q
, current
,
2196 spin_unlock(q
.lock_ptr
);
2200 * We have been woken up by futex_unlock_pi(), a timeout, or a
2201 * signal. futex_unlock_pi() will not destroy the lock_ptr nor
2204 WARN_ON(!&q
.pi_state
);
2205 pi_mutex
= &q
.pi_state
->pi_mutex
;
2206 ret
= rt_mutex_finish_proxy_lock(pi_mutex
, to
, &rt_waiter
, 1);
2207 debug_rt_mutex_free_waiter(&rt_waiter
);
2209 spin_lock(q
.lock_ptr
);
2211 * Fixup the pi_state owner and possibly acquire the lock if we
2214 res
= fixup_owner(uaddr2
, fshared
, &q
, !ret
);
2216 * If fixup_owner() returned an error, proprogate that. If it
2217 * acquired the lock, clear our -ETIMEDOUT or -EINTR.
2220 ret
= (res
< 0) ? res
: 0;
2222 /* Unqueue and drop the lock. */
2227 * If fixup_pi_state_owner() faulted and was unable to handle the
2228 * fault, unlock the rt_mutex and return the fault to userspace.
2230 if (ret
== -EFAULT
) {
2231 if (rt_mutex_owner(pi_mutex
) == current
)
2232 rt_mutex_unlock(pi_mutex
);
2233 } else if (ret
== -EINTR
) {
2235 if (get_user(uval
, uaddr2
))
2239 * We've already been requeued, so restart by calling
2240 * futex_lock_pi() directly, rather then returning to this
2243 ret
= -ERESTART_RESTARTBLOCK
;
2244 restart
= ¤t_thread_info()->restart_block
;
2245 restart
->fn
= futex_lock_pi_restart
;
2246 restart
->futex
.uaddr
= (u32
*)uaddr2
;
2247 restart
->futex
.val
= uval
;
2248 restart
->futex
.flags
= 0;
2250 restart
->futex
.flags
|= FLAGS_HAS_TIMEOUT
;
2251 restart
->futex
.time
= abs_time
->tv64
;
2255 restart
->futex
.flags
|= FLAGS_SHARED
;
2257 restart
->futex
.flags
|= FLAGS_CLOCKRT
;
2261 put_futex_key(fshared
, &q
.key
);
2262 put_futex_key(fshared
, &key2
);
2266 hrtimer_cancel(&to
->timer
);
2267 destroy_hrtimer_on_stack(&to
->timer
);
2273 * Support for robust futexes: the kernel cleans up held futexes at
2276 * Implementation: user-space maintains a per-thread list of locks it
2277 * is holding. Upon do_exit(), the kernel carefully walks this list,
2278 * and marks all locks that are owned by this thread with the
2279 * FUTEX_OWNER_DIED bit, and wakes up a waiter (if any). The list is
2280 * always manipulated with the lock held, so the list is private and
2281 * per-thread. Userspace also maintains a per-thread 'list_op_pending'
2282 * field, to allow the kernel to clean up if the thread dies after
2283 * acquiring the lock, but just before it could have added itself to
2284 * the list. There can only be one such pending lock.
2288 * sys_set_robust_list - set the robust-futex list head of a task
2289 * @head: pointer to the list-head
2290 * @len: length of the list-head, as userspace expects
2292 SYSCALL_DEFINE2(set_robust_list
, struct robust_list_head __user
*, head
,
2295 if (!futex_cmpxchg_enabled
)
2298 * The kernel knows only one size for now:
2300 if (unlikely(len
!= sizeof(*head
)))
2303 current
->robust_list
= head
;
2309 * sys_get_robust_list - get the robust-futex list head of a task
2310 * @pid: pid of the process [zero for current task]
2311 * @head_ptr: pointer to a list-head pointer, the kernel fills it in
2312 * @len_ptr: pointer to a length field, the kernel fills in the header size
2314 SYSCALL_DEFINE3(get_robust_list
, int, pid
,
2315 struct robust_list_head __user
* __user
*, head_ptr
,
2316 size_t __user
*, len_ptr
)
2318 struct robust_list_head __user
*head
;
2320 const struct cred
*cred
= current_cred(), *pcred
;
2322 if (!futex_cmpxchg_enabled
)
2326 head
= current
->robust_list
;
2328 struct task_struct
*p
;
2332 p
= find_task_by_vpid(pid
);
2336 pcred
= __task_cred(p
);
2337 if (cred
->euid
!= pcred
->euid
&&
2338 cred
->euid
!= pcred
->uid
&&
2339 !capable(CAP_SYS_PTRACE
))
2341 head
= p
->robust_list
;
2345 if (put_user(sizeof(*head
), len_ptr
))
2347 return put_user(head
, head_ptr
);
2356 * Process a futex-list entry, check whether it's owned by the
2357 * dying task, and do notification if so:
2359 int handle_futex_death(u32 __user
*uaddr
, struct task_struct
*curr
, int pi
)
2361 u32 uval
, nval
, mval
;
2364 if (get_user(uval
, uaddr
))
2367 if ((uval
& FUTEX_TID_MASK
) == task_pid_vnr(curr
)) {
2369 * Ok, this dying thread is truly holding a futex
2370 * of interest. Set the OWNER_DIED bit atomically
2371 * via cmpxchg, and if the value had FUTEX_WAITERS
2372 * set, wake up a waiter (if any). (We have to do a
2373 * futex_wake() even if OWNER_DIED is already set -
2374 * to handle the rare but possible case of recursive
2375 * thread-death.) The rest of the cleanup is done in
2378 mval
= (uval
& FUTEX_WAITERS
) | FUTEX_OWNER_DIED
;
2379 nval
= futex_atomic_cmpxchg_inatomic(uaddr
, uval
, mval
);
2381 if (nval
== -EFAULT
)
2388 * Wake robust non-PI futexes here. The wakeup of
2389 * PI futexes happens in exit_pi_state():
2391 if (!pi
&& (uval
& FUTEX_WAITERS
))
2392 futex_wake(uaddr
, 1, 1, FUTEX_BITSET_MATCH_ANY
);
2398 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
2400 static inline int fetch_robust_entry(struct robust_list __user
**entry
,
2401 struct robust_list __user
* __user
*head
,
2404 unsigned long uentry
;
2406 if (get_user(uentry
, (unsigned long __user
*)head
))
2409 *entry
= (void __user
*)(uentry
& ~1UL);
2416 * Walk curr->robust_list (very carefully, it's a userspace list!)
2417 * and mark any locks found there dead, and notify any waiters.
2419 * We silently return on any sign of list-walking problem.
2421 void exit_robust_list(struct task_struct
*curr
)
2423 struct robust_list_head __user
*head
= curr
->robust_list
;
2424 struct robust_list __user
*entry
, *next_entry
, *pending
;
2425 unsigned int limit
= ROBUST_LIST_LIMIT
, pi
, next_pi
, pip
;
2426 unsigned long futex_offset
;
2429 if (!futex_cmpxchg_enabled
)
2433 * Fetch the list head (which was registered earlier, via
2434 * sys_set_robust_list()):
2436 if (fetch_robust_entry(&entry
, &head
->list
.next
, &pi
))
2439 * Fetch the relative futex offset:
2441 if (get_user(futex_offset
, &head
->futex_offset
))
2444 * Fetch any possibly pending lock-add first, and handle it
2447 if (fetch_robust_entry(&pending
, &head
->list_op_pending
, &pip
))
2450 next_entry
= NULL
; /* avoid warning with gcc */
2451 while (entry
!= &head
->list
) {
2453 * Fetch the next entry in the list before calling
2454 * handle_futex_death:
2456 rc
= fetch_robust_entry(&next_entry
, &entry
->next
, &next_pi
);
2458 * A pending lock might already be on the list, so
2459 * don't process it twice:
2461 if (entry
!= pending
)
2462 if (handle_futex_death((void __user
*)entry
+ futex_offset
,
2470 * Avoid excessively long or circular lists:
2479 handle_futex_death((void __user
*)pending
+ futex_offset
,
2483 long do_futex(u32 __user
*uaddr
, int op
, u32 val
, ktime_t
*timeout
,
2484 u32 __user
*uaddr2
, u32 val2
, u32 val3
)
2486 int clockrt
, ret
= -ENOSYS
;
2487 int cmd
= op
& FUTEX_CMD_MASK
;
2490 if (!(op
& FUTEX_PRIVATE_FLAG
))
2493 clockrt
= op
& FUTEX_CLOCK_REALTIME
;
2494 if (clockrt
&& cmd
!= FUTEX_WAIT_BITSET
&& cmd
!= FUTEX_WAIT_REQUEUE_PI
)
2499 val3
= FUTEX_BITSET_MATCH_ANY
;
2500 case FUTEX_WAIT_BITSET
:
2501 ret
= futex_wait(uaddr
, fshared
, val
, timeout
, val3
, clockrt
);
2504 val3
= FUTEX_BITSET_MATCH_ANY
;
2505 case FUTEX_WAKE_BITSET
:
2506 ret
= futex_wake(uaddr
, fshared
, val
, val3
);
2509 ret
= futex_requeue(uaddr
, fshared
, uaddr2
, val
, val2
, NULL
, 0);
2511 case FUTEX_CMP_REQUEUE
:
2512 ret
= futex_requeue(uaddr
, fshared
, uaddr2
, val
, val2
, &val3
,
2516 ret
= futex_wake_op(uaddr
, fshared
, uaddr2
, val
, val2
, val3
);
2519 if (futex_cmpxchg_enabled
)
2520 ret
= futex_lock_pi(uaddr
, fshared
, val
, timeout
, 0);
2522 case FUTEX_UNLOCK_PI
:
2523 if (futex_cmpxchg_enabled
)
2524 ret
= futex_unlock_pi(uaddr
, fshared
);
2526 case FUTEX_TRYLOCK_PI
:
2527 if (futex_cmpxchg_enabled
)
2528 ret
= futex_lock_pi(uaddr
, fshared
, 0, timeout
, 1);
2530 case FUTEX_WAIT_REQUEUE_PI
:
2531 val3
= FUTEX_BITSET_MATCH_ANY
;
2532 ret
= futex_wait_requeue_pi(uaddr
, fshared
, val
, timeout
, val3
,
2535 case FUTEX_REQUEUE_PI
:
2536 ret
= futex_requeue(uaddr
, fshared
, uaddr2
, val
, val2
, NULL
, 1);
2538 case FUTEX_CMP_REQUEUE_PI
:
2539 ret
= futex_requeue(uaddr
, fshared
, uaddr2
, val
, val2
, &val3
,
2549 SYSCALL_DEFINE6(futex
, u32 __user
*, uaddr
, int, op
, u32
, val
,
2550 struct timespec __user
*, utime
, u32 __user
*, uaddr2
,
2554 ktime_t t
, *tp
= NULL
;
2556 int cmd
= op
& FUTEX_CMD_MASK
;
2558 if (utime
&& (cmd
== FUTEX_WAIT
|| cmd
== FUTEX_LOCK_PI
||
2559 cmd
== FUTEX_WAIT_BITSET
||
2560 cmd
== FUTEX_WAIT_REQUEUE_PI
)) {
2561 if (copy_from_user(&ts
, utime
, sizeof(ts
)) != 0)
2563 if (!timespec_valid(&ts
))
2566 t
= timespec_to_ktime(ts
);
2567 if (cmd
== FUTEX_WAIT
)
2568 t
= ktime_add_safe(ktime_get(), t
);
2572 * requeue parameter in 'utime' if cmd == FUTEX_*_REQUEUE_*.
2573 * number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.
2575 if (cmd
== FUTEX_REQUEUE
|| cmd
== FUTEX_CMP_REQUEUE
||
2576 cmd
== FUTEX_REQUEUE_PI
|| cmd
== FUTEX_CMP_REQUEUE_PI
||
2577 cmd
== FUTEX_WAKE_OP
)
2578 val2
= (u32
) (unsigned long) utime
;
2580 return do_futex(uaddr
, op
, val
, tp
, uaddr2
, val2
, val3
);
2583 static int __init
futex_init(void)
2589 * This will fail and we want it. Some arch implementations do
2590 * runtime detection of the futex_atomic_cmpxchg_inatomic()
2591 * functionality. We want to know that before we call in any
2592 * of the complex code paths. Also we want to prevent
2593 * registration of robust lists in that case. NULL is
2594 * guaranteed to fault and we get -EFAULT on functional
2595 * implementation, the non functional ones will return
2598 curval
= cmpxchg_futex_value_locked(NULL
, 0, 0);
2599 if (curval
== -EFAULT
)
2600 futex_cmpxchg_enabled
= 1;
2602 for (i
= 0; i
< ARRAY_SIZE(futex_queues
); i
++) {
2603 plist_head_init(&futex_queues
[i
].chain
, &futex_queues
[i
].lock
);
2604 spin_lock_init(&futex_queues
[i
].lock
);
2609 __initcall(futex_init
);