1 // SPDX-License-Identifier: GPL-2.0-or-later
3 * Fast Userspace Mutexes (which I call "Futexes!").
4 * (C) Rusty Russell, IBM 2002
6 * Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
7 * (C) Copyright 2003 Red Hat Inc, All Rights Reserved
9 * Removed page pinning, fix privately mapped COW pages and other cleanups
10 * (C) Copyright 2003, 2004 Jamie Lokier
12 * Robust futex support started by Ingo Molnar
13 * (C) Copyright 2006 Red Hat Inc, All Rights Reserved
14 * Thanks to Thomas Gleixner for suggestions, analysis and fixes.
16 * PI-futex support started by Ingo Molnar and Thomas Gleixner
17 * Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
18 * Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
20 * PRIVATE futexes by Eric Dumazet
21 * Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.com>
23 * Requeue-PI support by Darren Hart <dvhltc@us.ibm.com>
24 * Copyright (C) IBM Corporation, 2009
25 * Thanks to Thomas Gleixner for conceptual design and careful reviews.
27 * Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
28 * enough at me, Linus for the original (flawed) idea, Matthew
29 * Kirkwood for proof-of-concept implementation.
31 * "The futexes are also cursed."
32 * "But they come in a choice of three flavours!"
34 #include <linux/compat.h>
35 #include <linux/jhash.h>
36 #include <linux/pagemap.h>
37 #include <linux/syscalls.h>
38 #include <linux/freezer.h>
39 #include <linux/memblock.h>
40 #include <linux/fault-inject.h>
41 #include <linux/time_namespace.h>
43 #include <asm/futex.h>
45 #include "locking/rtmutex_common.h"
48 * READ this before attempting to hack on futexes!
50 * Basic futex operation and ordering guarantees
51 * =============================================
53 * The waiter reads the futex value in user space and calls
54 * futex_wait(). This function computes the hash bucket and acquires
55 * the hash bucket lock. After that it reads the futex user space value
56 * again and verifies that the data has not changed. If it has not changed
57 * it enqueues itself into the hash bucket, releases the hash bucket lock
60 * The waker side modifies the user space value of the futex and calls
61 * futex_wake(). This function computes the hash bucket and acquires the
62 * hash bucket lock. Then it looks for waiters on that futex in the hash
63 * bucket and wakes them.
65 * In futex wake up scenarios where no tasks are blocked on a futex, taking
66 * the hb spinlock can be avoided and simply return. In order for this
67 * optimization to work, ordering guarantees must exist so that the waiter
68 * being added to the list is acknowledged when the list is concurrently being
69 * checked by the waker, avoiding scenarios like the following:
73 * sys_futex(WAIT, futex, val);
74 * futex_wait(futex, val);
77 * sys_futex(WAKE, futex);
82 * lock(hash_bucket(futex));
84 * unlock(hash_bucket(futex));
87 * This would cause the waiter on CPU 0 to wait forever because it
88 * missed the transition of the user space value from val to newval
89 * and the waker did not find the waiter in the hash bucket queue.
91 * The correct serialization ensures that a waiter either observes
92 * the changed user space value before blocking or is woken by a
97 * sys_futex(WAIT, futex, val);
98 * futex_wait(futex, val);
101 * smp_mb(); (A) <-- paired with -.
103 * lock(hash_bucket(futex)); |
107 * | sys_futex(WAKE, futex);
108 * | futex_wake(futex);
110 * `--------> smp_mb(); (B)
113 * unlock(hash_bucket(futex));
114 * schedule(); if (waiters)
115 * lock(hash_bucket(futex));
116 * else wake_waiters(futex);
117 * waiters--; (b) unlock(hash_bucket(futex));
119 * Where (A) orders the waiters increment and the futex value read through
120 * atomic operations (see hb_waiters_inc) and where (B) orders the write
121 * to futex and the waiters read (see hb_waiters_pending()).
123 * This yields the following case (where X:=waiters, Y:=futex):
131 * Which guarantees that x==0 && y==0 is impossible; which translates back into
132 * the guarantee that we cannot both miss the futex variable change and the
135 * Note that a new waiter is accounted for in (a) even when it is possible that
136 * the wait call can return error, in which case we backtrack from it in (b).
137 * Refer to the comment in queue_lock().
139 * Similarly, in order to account for waiters being requeued on another
140 * address we always increment the waiters for the destination bucket before
141 * acquiring the lock. It then decrements them again after releasing it -
142 * the code that actually moves the futex(es) between hash buckets (requeue_futex)
143 * will do the additional required waiter count housekeeping. This is done for
144 * double_lock_hb() and double_unlock_hb(), respectively.
147 #ifdef CONFIG_HAVE_FUTEX_CMPXCHG
148 #define futex_cmpxchg_enabled 1
150 static int __read_mostly futex_cmpxchg_enabled
;
154 * Futex flags used to encode options to functions and preserve them across
158 # define FLAGS_SHARED 0x01
161 * NOMMU does not have per process address space. Let the compiler optimize
164 # define FLAGS_SHARED 0x00
166 #define FLAGS_CLOCKRT 0x02
167 #define FLAGS_HAS_TIMEOUT 0x04
170 * Priority Inheritance state:
172 struct futex_pi_state
{
174 * list of 'owned' pi_state instances - these have to be
175 * cleaned up in do_exit() if the task exits prematurely:
177 struct list_head list
;
182 struct rt_mutex pi_mutex
;
184 struct task_struct
*owner
;
188 } __randomize_layout
;
191 * struct futex_q - The hashed futex queue entry, one per waiting task
192 * @list: priority-sorted list of tasks waiting on this futex
193 * @task: the task waiting on the futex
194 * @lock_ptr: the hash bucket lock
195 * @key: the key the futex is hashed on
196 * @pi_state: optional priority inheritance state
197 * @rt_waiter: rt_waiter storage for use with requeue_pi
198 * @requeue_pi_key: the requeue_pi target futex key
199 * @bitset: bitset for the optional bitmasked wakeup
201 * We use this hashed waitqueue, instead of a normal wait_queue_entry_t, so
202 * we can wake only the relevant ones (hashed queues may be shared).
204 * A futex_q has a woken state, just like tasks have TASK_RUNNING.
205 * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
206 * The order of wakeup is always to make the first condition true, then
209 * PI futexes are typically woken before they are removed from the hash list via
210 * the rt_mutex code. See unqueue_me_pi().
213 struct plist_node list
;
215 struct task_struct
*task
;
216 spinlock_t
*lock_ptr
;
218 struct futex_pi_state
*pi_state
;
219 struct rt_mutex_waiter
*rt_waiter
;
220 union futex_key
*requeue_pi_key
;
222 } __randomize_layout
;
224 static const struct futex_q futex_q_init
= {
225 /* list gets initialized in queue_me()*/
226 .key
= FUTEX_KEY_INIT
,
227 .bitset
= FUTEX_BITSET_MATCH_ANY
231 * Hash buckets are shared by all the futex_keys that hash to the same
232 * location. Each key may have multiple futex_q structures, one for each task
233 * waiting on a futex.
235 struct futex_hash_bucket
{
238 struct plist_head chain
;
239 } ____cacheline_aligned_in_smp
;
242 * The base of the bucket array and its size are always used together
243 * (after initialization only in hash_futex()), so ensure that they
244 * reside in the same cacheline.
247 struct futex_hash_bucket
*queues
;
248 unsigned long hashsize
;
249 } __futex_data __read_mostly
__aligned(2*sizeof(long));
250 #define futex_queues (__futex_data.queues)
251 #define futex_hashsize (__futex_data.hashsize)
255 * Fault injections for futexes.
257 #ifdef CONFIG_FAIL_FUTEX
260 struct fault_attr attr
;
264 .attr
= FAULT_ATTR_INITIALIZER
,
265 .ignore_private
= false,
268 static int __init
setup_fail_futex(char *str
)
270 return setup_fault_attr(&fail_futex
.attr
, str
);
272 __setup("fail_futex=", setup_fail_futex
);
274 static bool should_fail_futex(bool fshared
)
276 if (fail_futex
.ignore_private
&& !fshared
)
279 return should_fail(&fail_futex
.attr
, 1);
282 #ifdef CONFIG_FAULT_INJECTION_DEBUG_FS
284 static int __init
fail_futex_debugfs(void)
286 umode_t mode
= S_IFREG
| S_IRUSR
| S_IWUSR
;
289 dir
= fault_create_debugfs_attr("fail_futex", NULL
,
294 debugfs_create_bool("ignore-private", mode
, dir
,
295 &fail_futex
.ignore_private
);
299 late_initcall(fail_futex_debugfs
);
301 #endif /* CONFIG_FAULT_INJECTION_DEBUG_FS */
304 static inline bool should_fail_futex(bool fshared
)
308 #endif /* CONFIG_FAIL_FUTEX */
311 static void compat_exit_robust_list(struct task_struct
*curr
);
315 * Reflects a new waiter being added to the waitqueue.
317 static inline void hb_waiters_inc(struct futex_hash_bucket
*hb
)
320 atomic_inc(&hb
->waiters
);
322 * Full barrier (A), see the ordering comment above.
324 smp_mb__after_atomic();
329 * Reflects a waiter being removed from the waitqueue by wakeup
332 static inline void hb_waiters_dec(struct futex_hash_bucket
*hb
)
335 atomic_dec(&hb
->waiters
);
339 static inline int hb_waiters_pending(struct futex_hash_bucket
*hb
)
343 * Full barrier (B), see the ordering comment above.
346 return atomic_read(&hb
->waiters
);
353 * hash_futex - Return the hash bucket in the global hash
354 * @key: Pointer to the futex key for which the hash is calculated
356 * We hash on the keys returned from get_futex_key (see below) and return the
357 * corresponding hash bucket in the global hash.
359 static struct futex_hash_bucket
*hash_futex(union futex_key
*key
)
361 u32 hash
= jhash2((u32
*)key
, offsetof(typeof(*key
), both
.offset
) / 4,
364 return &futex_queues
[hash
& (futex_hashsize
- 1)];
369 * match_futex - Check whether two futex keys are equal
370 * @key1: Pointer to key1
371 * @key2: Pointer to key2
373 * Return 1 if two futex_keys are equal, 0 otherwise.
375 static inline int match_futex(union futex_key
*key1
, union futex_key
*key2
)
378 && key1
->both
.word
== key2
->both
.word
379 && key1
->both
.ptr
== key2
->both
.ptr
380 && key1
->both
.offset
== key2
->both
.offset
);
389 * futex_setup_timer - set up the sleeping hrtimer.
390 * @time: ptr to the given timeout value
391 * @timeout: the hrtimer_sleeper structure to be set up
392 * @flags: futex flags
393 * @range_ns: optional range in ns
395 * Return: Initialized hrtimer_sleeper structure or NULL if no timeout
398 static inline struct hrtimer_sleeper
*
399 futex_setup_timer(ktime_t
*time
, struct hrtimer_sleeper
*timeout
,
400 int flags
, u64 range_ns
)
405 hrtimer_init_sleeper_on_stack(timeout
, (flags
& FLAGS_CLOCKRT
) ?
406 CLOCK_REALTIME
: CLOCK_MONOTONIC
,
409 * If range_ns is 0, calling hrtimer_set_expires_range_ns() is
410 * effectively the same as calling hrtimer_set_expires().
412 hrtimer_set_expires_range_ns(&timeout
->timer
, *time
, range_ns
);
418 * Generate a machine wide unique identifier for this inode.
420 * This relies on u64 not wrapping in the life-time of the machine; which with
421 * 1ns resolution means almost 585 years.
423 * This further relies on the fact that a well formed program will not unmap
424 * the file while it has a (shared) futex waiting on it. This mapping will have
425 * a file reference which pins the mount and inode.
427 * If for some reason an inode gets evicted and read back in again, it will get
428 * a new sequence number and will _NOT_ match, even though it is the exact same
431 * It is important that match_futex() will never have a false-positive, esp.
432 * for PI futexes that can mess up the state. The above argues that false-negatives
433 * are only possible for malformed programs.
435 static u64
get_inode_sequence_number(struct inode
*inode
)
437 static atomic64_t i_seq
;
440 /* Does the inode already have a sequence number? */
441 old
= atomic64_read(&inode
->i_sequence
);
446 u64
new = atomic64_add_return(1, &i_seq
);
447 if (WARN_ON_ONCE(!new))
450 old
= atomic64_cmpxchg_relaxed(&inode
->i_sequence
, 0, new);
458 * get_futex_key() - Get parameters which are the keys for a futex
459 * @uaddr: virtual address of the futex
460 * @fshared: false for a PROCESS_PRIVATE futex, true for PROCESS_SHARED
461 * @key: address where result is stored.
462 * @rw: mapping needs to be read/write (values: FUTEX_READ,
465 * Return: a negative error code or 0
467 * The key words are stored in @key on success.
469 * For shared mappings (when @fshared), the key is:
471 * ( inode->i_sequence, page->index, offset_within_page )
473 * [ also see get_inode_sequence_number() ]
475 * For private mappings (or when !@fshared), the key is:
477 * ( current->mm, address, 0 )
479 * This allows (cross process, where applicable) identification of the futex
480 * without keeping the page pinned for the duration of the FUTEX_WAIT.
482 * lock_page() might sleep, the caller should not hold a spinlock.
484 static int get_futex_key(u32 __user
*uaddr
, bool fshared
, union futex_key
*key
,
485 enum futex_access rw
)
487 unsigned long address
= (unsigned long)uaddr
;
488 struct mm_struct
*mm
= current
->mm
;
489 struct page
*page
, *tail
;
490 struct address_space
*mapping
;
494 * The futex address must be "naturally" aligned.
496 key
->both
.offset
= address
% PAGE_SIZE
;
497 if (unlikely((address
% sizeof(u32
)) != 0))
499 address
-= key
->both
.offset
;
501 if (unlikely(!access_ok(uaddr
, sizeof(u32
))))
504 if (unlikely(should_fail_futex(fshared
)))
508 * PROCESS_PRIVATE futexes are fast.
509 * As the mm cannot disappear under us and the 'key' only needs
510 * virtual address, we dont even have to find the underlying vma.
511 * Note : We do have to check 'uaddr' is a valid user address,
512 * but access_ok() should be faster than find_vma()
515 key
->private.mm
= mm
;
516 key
->private.address
= address
;
521 /* Ignore any VERIFY_READ mapping (futex common case) */
522 if (unlikely(should_fail_futex(true)))
525 err
= get_user_pages_fast(address
, 1, FOLL_WRITE
, &page
);
527 * If write access is not required (eg. FUTEX_WAIT), try
528 * and get read-only access.
530 if (err
== -EFAULT
&& rw
== FUTEX_READ
) {
531 err
= get_user_pages_fast(address
, 1, 0, &page
);
540 * The treatment of mapping from this point on is critical. The page
541 * lock protects many things but in this context the page lock
542 * stabilizes mapping, prevents inode freeing in the shared
543 * file-backed region case and guards against movement to swap cache.
545 * Strictly speaking the page lock is not needed in all cases being
546 * considered here and page lock forces unnecessarily serialization
547 * From this point on, mapping will be re-verified if necessary and
548 * page lock will be acquired only if it is unavoidable
550 * Mapping checks require the head page for any compound page so the
551 * head page and mapping is looked up now. For anonymous pages, it
552 * does not matter if the page splits in the future as the key is
553 * based on the address. For filesystem-backed pages, the tail is
554 * required as the index of the page determines the key. For
555 * base pages, there is no tail page and tail == page.
558 page
= compound_head(page
);
559 mapping
= READ_ONCE(page
->mapping
);
562 * If page->mapping is NULL, then it cannot be a PageAnon
563 * page; but it might be the ZERO_PAGE or in the gate area or
564 * in a special mapping (all cases which we are happy to fail);
565 * or it may have been a good file page when get_user_pages_fast
566 * found it, but truncated or holepunched or subjected to
567 * invalidate_complete_page2 before we got the page lock (also
568 * cases which we are happy to fail). And we hold a reference,
569 * so refcount care in invalidate_complete_page's remove_mapping
570 * prevents drop_caches from setting mapping to NULL beneath us.
572 * The case we do have to guard against is when memory pressure made
573 * shmem_writepage move it from filecache to swapcache beneath us:
574 * an unlikely race, but we do need to retry for page->mapping.
576 if (unlikely(!mapping
)) {
580 * Page lock is required to identify which special case above
581 * applies. If this is really a shmem page then the page lock
582 * will prevent unexpected transitions.
585 shmem_swizzled
= PageSwapCache(page
) || page
->mapping
;
596 * Private mappings are handled in a simple way.
598 * If the futex key is stored on an anonymous page, then the associated
599 * object is the mm which is implicitly pinned by the calling process.
601 * NOTE: When userspace waits on a MAP_SHARED mapping, even if
602 * it's a read-only handle, it's expected that futexes attach to
603 * the object not the particular process.
605 if (PageAnon(page
)) {
607 * A RO anonymous page will never change and thus doesn't make
608 * sense for futex operations.
610 if (unlikely(should_fail_futex(true)) || ro
) {
615 key
->both
.offset
|= FUT_OFF_MMSHARED
; /* ref taken on mm */
616 key
->private.mm
= mm
;
617 key
->private.address
= address
;
623 * The associated futex object in this case is the inode and
624 * the page->mapping must be traversed. Ordinarily this should
625 * be stabilised under page lock but it's not strictly
626 * necessary in this case as we just want to pin the inode, not
627 * update the radix tree or anything like that.
629 * The RCU read lock is taken as the inode is finally freed
630 * under RCU. If the mapping still matches expectations then the
631 * mapping->host can be safely accessed as being a valid inode.
635 if (READ_ONCE(page
->mapping
) != mapping
) {
642 inode
= READ_ONCE(mapping
->host
);
650 key
->both
.offset
|= FUT_OFF_INODE
; /* inode-based key */
651 key
->shared
.i_seq
= get_inode_sequence_number(inode
);
652 key
->shared
.pgoff
= page_to_pgoff(tail
);
662 * fault_in_user_writeable() - Fault in user address and verify RW access
663 * @uaddr: pointer to faulting user space address
665 * Slow path to fixup the fault we just took in the atomic write
668 * We have no generic implementation of a non-destructive write to the
669 * user address. We know that we faulted in the atomic pagefault
670 * disabled section so we can as well avoid the #PF overhead by
671 * calling get_user_pages() right away.
673 static int fault_in_user_writeable(u32 __user
*uaddr
)
675 struct mm_struct
*mm
= current
->mm
;
679 ret
= fixup_user_fault(mm
, (unsigned long)uaddr
,
680 FAULT_FLAG_WRITE
, NULL
);
681 mmap_read_unlock(mm
);
683 return ret
< 0 ? ret
: 0;
687 * futex_top_waiter() - Return the highest priority waiter on a futex
688 * @hb: the hash bucket the futex_q's reside in
689 * @key: the futex key (to distinguish it from other futex futex_q's)
691 * Must be called with the hb lock held.
693 static struct futex_q
*futex_top_waiter(struct futex_hash_bucket
*hb
,
694 union futex_key
*key
)
696 struct futex_q
*this;
698 plist_for_each_entry(this, &hb
->chain
, list
) {
699 if (match_futex(&this->key
, key
))
705 static int cmpxchg_futex_value_locked(u32
*curval
, u32 __user
*uaddr
,
706 u32 uval
, u32 newval
)
711 ret
= futex_atomic_cmpxchg_inatomic(curval
, uaddr
, uval
, newval
);
717 static int get_futex_value_locked(u32
*dest
, u32 __user
*from
)
722 ret
= __get_user(*dest
, from
);
725 return ret
? -EFAULT
: 0;
732 static int refill_pi_state_cache(void)
734 struct futex_pi_state
*pi_state
;
736 if (likely(current
->pi_state_cache
))
739 pi_state
= kzalloc(sizeof(*pi_state
), GFP_KERNEL
);
744 INIT_LIST_HEAD(&pi_state
->list
);
745 /* pi_mutex gets initialized later */
746 pi_state
->owner
= NULL
;
747 refcount_set(&pi_state
->refcount
, 1);
748 pi_state
->key
= FUTEX_KEY_INIT
;
750 current
->pi_state_cache
= pi_state
;
755 static struct futex_pi_state
*alloc_pi_state(void)
757 struct futex_pi_state
*pi_state
= current
->pi_state_cache
;
760 current
->pi_state_cache
= NULL
;
765 static void pi_state_update_owner(struct futex_pi_state
*pi_state
,
766 struct task_struct
*new_owner
)
768 struct task_struct
*old_owner
= pi_state
->owner
;
770 lockdep_assert_held(&pi_state
->pi_mutex
.wait_lock
);
773 raw_spin_lock(&old_owner
->pi_lock
);
774 WARN_ON(list_empty(&pi_state
->list
));
775 list_del_init(&pi_state
->list
);
776 raw_spin_unlock(&old_owner
->pi_lock
);
780 raw_spin_lock(&new_owner
->pi_lock
);
781 WARN_ON(!list_empty(&pi_state
->list
));
782 list_add(&pi_state
->list
, &new_owner
->pi_state_list
);
783 pi_state
->owner
= new_owner
;
784 raw_spin_unlock(&new_owner
->pi_lock
);
788 static void get_pi_state(struct futex_pi_state
*pi_state
)
790 WARN_ON_ONCE(!refcount_inc_not_zero(&pi_state
->refcount
));
794 * Drops a reference to the pi_state object and frees or caches it
795 * when the last reference is gone.
797 static void put_pi_state(struct futex_pi_state
*pi_state
)
802 if (!refcount_dec_and_test(&pi_state
->refcount
))
806 * If pi_state->owner is NULL, the owner is most probably dying
807 * and has cleaned up the pi_state already
809 if (pi_state
->owner
) {
812 raw_spin_lock_irqsave(&pi_state
->pi_mutex
.wait_lock
, flags
);
813 pi_state_update_owner(pi_state
, NULL
);
814 rt_mutex_proxy_unlock(&pi_state
->pi_mutex
);
815 raw_spin_unlock_irqrestore(&pi_state
->pi_mutex
.wait_lock
, flags
);
818 if (current
->pi_state_cache
) {
822 * pi_state->list is already empty.
823 * clear pi_state->owner.
824 * refcount is at 0 - put it back to 1.
826 pi_state
->owner
= NULL
;
827 refcount_set(&pi_state
->refcount
, 1);
828 current
->pi_state_cache
= pi_state
;
832 #ifdef CONFIG_FUTEX_PI
835 * This task is holding PI mutexes at exit time => bad.
836 * Kernel cleans up PI-state, but userspace is likely hosed.
837 * (Robust-futex cleanup is separate and might save the day for userspace.)
839 static void exit_pi_state_list(struct task_struct
*curr
)
841 struct list_head
*next
, *head
= &curr
->pi_state_list
;
842 struct futex_pi_state
*pi_state
;
843 struct futex_hash_bucket
*hb
;
844 union futex_key key
= FUTEX_KEY_INIT
;
846 if (!futex_cmpxchg_enabled
)
849 * We are a ZOMBIE and nobody can enqueue itself on
850 * pi_state_list anymore, but we have to be careful
851 * versus waiters unqueueing themselves:
853 raw_spin_lock_irq(&curr
->pi_lock
);
854 while (!list_empty(head
)) {
856 pi_state
= list_entry(next
, struct futex_pi_state
, list
);
858 hb
= hash_futex(&key
);
861 * We can race against put_pi_state() removing itself from the
862 * list (a waiter going away). put_pi_state() will first
863 * decrement the reference count and then modify the list, so
864 * its possible to see the list entry but fail this reference
867 * In that case; drop the locks to let put_pi_state() make
868 * progress and retry the loop.
870 if (!refcount_inc_not_zero(&pi_state
->refcount
)) {
871 raw_spin_unlock_irq(&curr
->pi_lock
);
873 raw_spin_lock_irq(&curr
->pi_lock
);
876 raw_spin_unlock_irq(&curr
->pi_lock
);
878 spin_lock(&hb
->lock
);
879 raw_spin_lock_irq(&pi_state
->pi_mutex
.wait_lock
);
880 raw_spin_lock(&curr
->pi_lock
);
882 * We dropped the pi-lock, so re-check whether this
883 * task still owns the PI-state:
885 if (head
->next
!= next
) {
886 /* retain curr->pi_lock for the loop invariant */
887 raw_spin_unlock(&pi_state
->pi_mutex
.wait_lock
);
888 spin_unlock(&hb
->lock
);
889 put_pi_state(pi_state
);
893 WARN_ON(pi_state
->owner
!= curr
);
894 WARN_ON(list_empty(&pi_state
->list
));
895 list_del_init(&pi_state
->list
);
896 pi_state
->owner
= NULL
;
898 raw_spin_unlock(&curr
->pi_lock
);
899 raw_spin_unlock_irq(&pi_state
->pi_mutex
.wait_lock
);
900 spin_unlock(&hb
->lock
);
902 rt_mutex_futex_unlock(&pi_state
->pi_mutex
);
903 put_pi_state(pi_state
);
905 raw_spin_lock_irq(&curr
->pi_lock
);
907 raw_spin_unlock_irq(&curr
->pi_lock
);
910 static inline void exit_pi_state_list(struct task_struct
*curr
) { }
914 * We need to check the following states:
916 * Waiter | pi_state | pi->owner | uTID | uODIED | ?
918 * [1] NULL | --- | --- | 0 | 0/1 | Valid
919 * [2] NULL | --- | --- | >0 | 0/1 | Valid
921 * [3] Found | NULL | -- | Any | 0/1 | Invalid
923 * [4] Found | Found | NULL | 0 | 1 | Valid
924 * [5] Found | Found | NULL | >0 | 1 | Invalid
926 * [6] Found | Found | task | 0 | 1 | Valid
928 * [7] Found | Found | NULL | Any | 0 | Invalid
930 * [8] Found | Found | task | ==taskTID | 0/1 | Valid
931 * [9] Found | Found | task | 0 | 0 | Invalid
932 * [10] Found | Found | task | !=taskTID | 0/1 | Invalid
934 * [1] Indicates that the kernel can acquire the futex atomically. We
935 * came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.
937 * [2] Valid, if TID does not belong to a kernel thread. If no matching
938 * thread is found then it indicates that the owner TID has died.
940 * [3] Invalid. The waiter is queued on a non PI futex
942 * [4] Valid state after exit_robust_list(), which sets the user space
943 * value to FUTEX_WAITERS | FUTEX_OWNER_DIED.
945 * [5] The user space value got manipulated between exit_robust_list()
946 * and exit_pi_state_list()
948 * [6] Valid state after exit_pi_state_list() which sets the new owner in
949 * the pi_state but cannot access the user space value.
951 * [7] pi_state->owner can only be NULL when the OWNER_DIED bit is set.
953 * [8] Owner and user space value match
955 * [9] There is no transient state which sets the user space TID to 0
956 * except exit_robust_list(), but this is indicated by the
957 * FUTEX_OWNER_DIED bit. See [4]
959 * [10] There is no transient state which leaves owner and user space
960 * TID out of sync. Except one error case where the kernel is denied
961 * write access to the user address, see fixup_pi_state_owner().
964 * Serialization and lifetime rules:
968 * hb -> futex_q, relation
969 * futex_q -> pi_state, relation
971 * (cannot be raw because hb can contain arbitrary amount
974 * pi_mutex->wait_lock:
978 * (and pi_mutex 'obviously')
982 * p->pi_state_list -> pi_state->list, relation
984 * pi_state->refcount:
992 * pi_mutex->wait_lock
998 * Validate that the existing waiter has a pi_state and sanity check
999 * the pi_state against the user space value. If correct, attach to
1002 static int attach_to_pi_state(u32 __user
*uaddr
, u32 uval
,
1003 struct futex_pi_state
*pi_state
,
1004 struct futex_pi_state
**ps
)
1006 pid_t pid
= uval
& FUTEX_TID_MASK
;
1011 * Userspace might have messed up non-PI and PI futexes [3]
1013 if (unlikely(!pi_state
))
1017 * We get here with hb->lock held, and having found a
1018 * futex_top_waiter(). This means that futex_lock_pi() of said futex_q
1019 * has dropped the hb->lock in between queue_me() and unqueue_me_pi(),
1020 * which in turn means that futex_lock_pi() still has a reference on
1023 * The waiter holding a reference on @pi_state also protects against
1024 * the unlocked put_pi_state() in futex_unlock_pi(), futex_lock_pi()
1025 * and futex_wait_requeue_pi() as it cannot go to 0 and consequently
1026 * free pi_state before we can take a reference ourselves.
1028 WARN_ON(!refcount_read(&pi_state
->refcount
));
1031 * Now that we have a pi_state, we can acquire wait_lock
1032 * and do the state validation.
1034 raw_spin_lock_irq(&pi_state
->pi_mutex
.wait_lock
);
1037 * Since {uval, pi_state} is serialized by wait_lock, and our current
1038 * uval was read without holding it, it can have changed. Verify it
1039 * still is what we expect it to be, otherwise retry the entire
1042 if (get_futex_value_locked(&uval2
, uaddr
))
1049 * Handle the owner died case:
1051 if (uval
& FUTEX_OWNER_DIED
) {
1053 * exit_pi_state_list sets owner to NULL and wakes the
1054 * topmost waiter. The task which acquires the
1055 * pi_state->rt_mutex will fixup owner.
1057 if (!pi_state
->owner
) {
1059 * No pi state owner, but the user space TID
1060 * is not 0. Inconsistent state. [5]
1065 * Take a ref on the state and return success. [4]
1071 * If TID is 0, then either the dying owner has not
1072 * yet executed exit_pi_state_list() or some waiter
1073 * acquired the rtmutex in the pi state, but did not
1074 * yet fixup the TID in user space.
1076 * Take a ref on the state and return success. [6]
1082 * If the owner died bit is not set, then the pi_state
1083 * must have an owner. [7]
1085 if (!pi_state
->owner
)
1090 * Bail out if user space manipulated the futex value. If pi
1091 * state exists then the owner TID must be the same as the
1092 * user space TID. [9/10]
1094 if (pid
!= task_pid_vnr(pi_state
->owner
))
1098 get_pi_state(pi_state
);
1099 raw_spin_unlock_irq(&pi_state
->pi_mutex
.wait_lock
);
1116 raw_spin_unlock_irq(&pi_state
->pi_mutex
.wait_lock
);
1121 * wait_for_owner_exiting - Block until the owner has exited
1122 * @ret: owner's current futex lock status
1123 * @exiting: Pointer to the exiting task
1125 * Caller must hold a refcount on @exiting.
1127 static void wait_for_owner_exiting(int ret
, struct task_struct
*exiting
)
1129 if (ret
!= -EBUSY
) {
1130 WARN_ON_ONCE(exiting
);
1134 if (WARN_ON_ONCE(ret
== -EBUSY
&& !exiting
))
1137 mutex_lock(&exiting
->futex_exit_mutex
);
1139 * No point in doing state checking here. If the waiter got here
1140 * while the task was in exec()->exec_futex_release() then it can
1141 * have any FUTEX_STATE_* value when the waiter has acquired the
1142 * mutex. OK, if running, EXITING or DEAD if it reached exit()
1143 * already. Highly unlikely and not a problem. Just one more round
1144 * through the futex maze.
1146 mutex_unlock(&exiting
->futex_exit_mutex
);
1148 put_task_struct(exiting
);
1151 static int handle_exit_race(u32 __user
*uaddr
, u32 uval
,
1152 struct task_struct
*tsk
)
1157 * If the futex exit state is not yet FUTEX_STATE_DEAD, tell the
1158 * caller that the alleged owner is busy.
1160 if (tsk
&& tsk
->futex_state
!= FUTEX_STATE_DEAD
)
1164 * Reread the user space value to handle the following situation:
1168 * sys_exit() sys_futex()
1169 * do_exit() futex_lock_pi()
1170 * futex_lock_pi_atomic()
1171 * exit_signals(tsk) No waiters:
1172 * tsk->flags |= PF_EXITING; *uaddr == 0x00000PID
1173 * mm_release(tsk) Set waiter bit
1174 * exit_robust_list(tsk) { *uaddr = 0x80000PID;
1175 * Set owner died attach_to_pi_owner() {
1176 * *uaddr = 0xC0000000; tsk = get_task(PID);
1177 * } if (!tsk->flags & PF_EXITING) {
1179 * tsk->futex_state = } else {
1180 * FUTEX_STATE_DEAD; if (tsk->futex_state !=
1183 * return -ESRCH; <--- FAIL
1186 * Returning ESRCH unconditionally is wrong here because the
1187 * user space value has been changed by the exiting task.
1189 * The same logic applies to the case where the exiting task is
1192 if (get_futex_value_locked(&uval2
, uaddr
))
1195 /* If the user space value has changed, try again. */
1200 * The exiting task did not have a robust list, the robust list was
1201 * corrupted or the user space value in *uaddr is simply bogus.
1202 * Give up and tell user space.
1208 * Lookup the task for the TID provided from user space and attach to
1209 * it after doing proper sanity checks.
1211 static int attach_to_pi_owner(u32 __user
*uaddr
, u32 uval
, union futex_key
*key
,
1212 struct futex_pi_state
**ps
,
1213 struct task_struct
**exiting
)
1215 pid_t pid
= uval
& FUTEX_TID_MASK
;
1216 struct futex_pi_state
*pi_state
;
1217 struct task_struct
*p
;
1220 * We are the first waiter - try to look up the real owner and attach
1221 * the new pi_state to it, but bail out when TID = 0 [1]
1223 * The !pid check is paranoid. None of the call sites should end up
1224 * with pid == 0, but better safe than sorry. Let the caller retry
1228 p
= find_get_task_by_vpid(pid
);
1230 return handle_exit_race(uaddr
, uval
, NULL
);
1232 if (unlikely(p
->flags
& PF_KTHREAD
)) {
1238 * We need to look at the task state to figure out, whether the
1239 * task is exiting. To protect against the change of the task state
1240 * in futex_exit_release(), we do this protected by p->pi_lock:
1242 raw_spin_lock_irq(&p
->pi_lock
);
1243 if (unlikely(p
->futex_state
!= FUTEX_STATE_OK
)) {
1245 * The task is on the way out. When the futex state is
1246 * FUTEX_STATE_DEAD, we know that the task has finished
1249 int ret
= handle_exit_race(uaddr
, uval
, p
);
1251 raw_spin_unlock_irq(&p
->pi_lock
);
1253 * If the owner task is between FUTEX_STATE_EXITING and
1254 * FUTEX_STATE_DEAD then store the task pointer and keep
1255 * the reference on the task struct. The calling code will
1256 * drop all locks, wait for the task to reach
1257 * FUTEX_STATE_DEAD and then drop the refcount. This is
1258 * required to prevent a live lock when the current task
1259 * preempted the exiting task between the two states.
1269 * No existing pi state. First waiter. [2]
1271 * This creates pi_state, we have hb->lock held, this means nothing can
1272 * observe this state, wait_lock is irrelevant.
1274 pi_state
= alloc_pi_state();
1277 * Initialize the pi_mutex in locked state and make @p
1280 rt_mutex_init_proxy_locked(&pi_state
->pi_mutex
, p
);
1282 /* Store the key for possible exit cleanups: */
1283 pi_state
->key
= *key
;
1285 WARN_ON(!list_empty(&pi_state
->list
));
1286 list_add(&pi_state
->list
, &p
->pi_state_list
);
1288 * Assignment without holding pi_state->pi_mutex.wait_lock is safe
1289 * because there is no concurrency as the object is not published yet.
1291 pi_state
->owner
= p
;
1292 raw_spin_unlock_irq(&p
->pi_lock
);
1301 static int lookup_pi_state(u32 __user
*uaddr
, u32 uval
,
1302 struct futex_hash_bucket
*hb
,
1303 union futex_key
*key
, struct futex_pi_state
**ps
,
1304 struct task_struct
**exiting
)
1306 struct futex_q
*top_waiter
= futex_top_waiter(hb
, key
);
1309 * If there is a waiter on that futex, validate it and
1310 * attach to the pi_state when the validation succeeds.
1313 return attach_to_pi_state(uaddr
, uval
, top_waiter
->pi_state
, ps
);
1316 * We are the first waiter - try to look up the owner based on
1317 * @uval and attach to it.
1319 return attach_to_pi_owner(uaddr
, uval
, key
, ps
, exiting
);
1322 static int lock_pi_update_atomic(u32 __user
*uaddr
, u32 uval
, u32 newval
)
1327 if (unlikely(should_fail_futex(true)))
1330 err
= cmpxchg_futex_value_locked(&curval
, uaddr
, uval
, newval
);
1334 /* If user space value changed, let the caller retry */
1335 return curval
!= uval
? -EAGAIN
: 0;
1339 * futex_lock_pi_atomic() - Atomic work required to acquire a pi aware futex
1340 * @uaddr: the pi futex user address
1341 * @hb: the pi futex hash bucket
1342 * @key: the futex key associated with uaddr and hb
1343 * @ps: the pi_state pointer where we store the result of the
1345 * @task: the task to perform the atomic lock work for. This will
1346 * be "current" except in the case of requeue pi.
1347 * @exiting: Pointer to store the task pointer of the owner task
1348 * which is in the middle of exiting
1349 * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
1352 * - 0 - ready to wait;
1353 * - 1 - acquired the lock;
1356 * The hb->lock and futex_key refs shall be held by the caller.
1358 * @exiting is only set when the return value is -EBUSY. If so, this holds
1359 * a refcount on the exiting task on return and the caller needs to drop it
1360 * after waiting for the exit to complete.
1362 static int futex_lock_pi_atomic(u32 __user
*uaddr
, struct futex_hash_bucket
*hb
,
1363 union futex_key
*key
,
1364 struct futex_pi_state
**ps
,
1365 struct task_struct
*task
,
1366 struct task_struct
**exiting
,
1369 u32 uval
, newval
, vpid
= task_pid_vnr(task
);
1370 struct futex_q
*top_waiter
;
1374 * Read the user space value first so we can validate a few
1375 * things before proceeding further.
1377 if (get_futex_value_locked(&uval
, uaddr
))
1380 if (unlikely(should_fail_futex(true)))
1386 if ((unlikely((uval
& FUTEX_TID_MASK
) == vpid
)))
1389 if ((unlikely(should_fail_futex(true))))
1393 * Lookup existing state first. If it exists, try to attach to
1396 top_waiter
= futex_top_waiter(hb
, key
);
1398 return attach_to_pi_state(uaddr
, uval
, top_waiter
->pi_state
, ps
);
1401 * No waiter and user TID is 0. We are here because the
1402 * waiters or the owner died bit is set or called from
1403 * requeue_cmp_pi or for whatever reason something took the
1406 if (!(uval
& FUTEX_TID_MASK
)) {
1408 * We take over the futex. No other waiters and the user space
1409 * TID is 0. We preserve the owner died bit.
1411 newval
= uval
& FUTEX_OWNER_DIED
;
1414 /* The futex requeue_pi code can enforce the waiters bit */
1416 newval
|= FUTEX_WAITERS
;
1418 ret
= lock_pi_update_atomic(uaddr
, uval
, newval
);
1419 /* If the take over worked, return 1 */
1420 return ret
< 0 ? ret
: 1;
1424 * First waiter. Set the waiters bit before attaching ourself to
1425 * the owner. If owner tries to unlock, it will be forced into
1426 * the kernel and blocked on hb->lock.
1428 newval
= uval
| FUTEX_WAITERS
;
1429 ret
= lock_pi_update_atomic(uaddr
, uval
, newval
);
1433 * If the update of the user space value succeeded, we try to
1434 * attach to the owner. If that fails, no harm done, we only
1435 * set the FUTEX_WAITERS bit in the user space variable.
1437 return attach_to_pi_owner(uaddr
, newval
, key
, ps
, exiting
);
1441 * __unqueue_futex() - Remove the futex_q from its futex_hash_bucket
1442 * @q: The futex_q to unqueue
1444 * The q->lock_ptr must not be NULL and must be held by the caller.
1446 static void __unqueue_futex(struct futex_q
*q
)
1448 struct futex_hash_bucket
*hb
;
1450 if (WARN_ON_SMP(!q
->lock_ptr
) || WARN_ON(plist_node_empty(&q
->list
)))
1452 lockdep_assert_held(q
->lock_ptr
);
1454 hb
= container_of(q
->lock_ptr
, struct futex_hash_bucket
, lock
);
1455 plist_del(&q
->list
, &hb
->chain
);
1460 * The hash bucket lock must be held when this is called.
1461 * Afterwards, the futex_q must not be accessed. Callers
1462 * must ensure to later call wake_up_q() for the actual
1465 static void mark_wake_futex(struct wake_q_head
*wake_q
, struct futex_q
*q
)
1467 struct task_struct
*p
= q
->task
;
1469 if (WARN(q
->pi_state
|| q
->rt_waiter
, "refusing to wake PI futex\n"))
1475 * The waiting task can free the futex_q as soon as q->lock_ptr = NULL
1476 * is written, without taking any locks. This is possible in the event
1477 * of a spurious wakeup, for example. A memory barrier is required here
1478 * to prevent the following store to lock_ptr from getting ahead of the
1479 * plist_del in __unqueue_futex().
1481 smp_store_release(&q
->lock_ptr
, NULL
);
1484 * Queue the task for later wakeup for after we've released
1487 wake_q_add_safe(wake_q
, p
);
1491 * Caller must hold a reference on @pi_state.
1493 static int wake_futex_pi(u32 __user
*uaddr
, u32 uval
, struct futex_pi_state
*pi_state
)
1496 struct task_struct
*new_owner
;
1497 bool postunlock
= false;
1498 DEFINE_WAKE_Q(wake_q
);
1501 new_owner
= rt_mutex_next_owner(&pi_state
->pi_mutex
);
1502 if (WARN_ON_ONCE(!new_owner
)) {
1504 * As per the comment in futex_unlock_pi() this should not happen.
1506 * When this happens, give up our locks and try again, giving
1507 * the futex_lock_pi() instance time to complete, either by
1508 * waiting on the rtmutex or removing itself from the futex
1516 * We pass it to the next owner. The WAITERS bit is always kept
1517 * enabled while there is PI state around. We cleanup the owner
1518 * died bit, because we are the owner.
1520 newval
= FUTEX_WAITERS
| task_pid_vnr(new_owner
);
1522 if (unlikely(should_fail_futex(true))) {
1527 ret
= cmpxchg_futex_value_locked(&curval
, uaddr
, uval
, newval
);
1528 if (!ret
&& (curval
!= uval
)) {
1530 * If a unconditional UNLOCK_PI operation (user space did not
1531 * try the TID->0 transition) raced with a waiter setting the
1532 * FUTEX_WAITERS flag between get_user() and locking the hash
1533 * bucket lock, retry the operation.
1535 if ((FUTEX_TID_MASK
& curval
) == uval
)
1543 * This is a point of no return; once we modified the uval
1544 * there is no going back and subsequent operations must
1547 pi_state_update_owner(pi_state
, new_owner
);
1548 postunlock
= __rt_mutex_futex_unlock(&pi_state
->pi_mutex
, &wake_q
);
1552 raw_spin_unlock_irq(&pi_state
->pi_mutex
.wait_lock
);
1555 rt_mutex_postunlock(&wake_q
);
1561 * Express the locking dependencies for lockdep:
1564 double_lock_hb(struct futex_hash_bucket
*hb1
, struct futex_hash_bucket
*hb2
)
1567 spin_lock(&hb1
->lock
);
1569 spin_lock_nested(&hb2
->lock
, SINGLE_DEPTH_NESTING
);
1570 } else { /* hb1 > hb2 */
1571 spin_lock(&hb2
->lock
);
1572 spin_lock_nested(&hb1
->lock
, SINGLE_DEPTH_NESTING
);
1577 double_unlock_hb(struct futex_hash_bucket
*hb1
, struct futex_hash_bucket
*hb2
)
1579 spin_unlock(&hb1
->lock
);
1581 spin_unlock(&hb2
->lock
);
1585 * Wake up waiters matching bitset queued on this futex (uaddr).
1588 futex_wake(u32 __user
*uaddr
, unsigned int flags
, int nr_wake
, u32 bitset
)
1590 struct futex_hash_bucket
*hb
;
1591 struct futex_q
*this, *next
;
1592 union futex_key key
= FUTEX_KEY_INIT
;
1594 DEFINE_WAKE_Q(wake_q
);
1599 ret
= get_futex_key(uaddr
, flags
& FLAGS_SHARED
, &key
, FUTEX_READ
);
1600 if (unlikely(ret
!= 0))
1603 hb
= hash_futex(&key
);
1605 /* Make sure we really have tasks to wakeup */
1606 if (!hb_waiters_pending(hb
))
1609 spin_lock(&hb
->lock
);
1611 plist_for_each_entry_safe(this, next
, &hb
->chain
, list
) {
1612 if (match_futex (&this->key
, &key
)) {
1613 if (this->pi_state
|| this->rt_waiter
) {
1618 /* Check if one of the bits is set in both bitsets */
1619 if (!(this->bitset
& bitset
))
1622 mark_wake_futex(&wake_q
, this);
1623 if (++ret
>= nr_wake
)
1628 spin_unlock(&hb
->lock
);
1633 static int futex_atomic_op_inuser(unsigned int encoded_op
, u32 __user
*uaddr
)
1635 unsigned int op
= (encoded_op
& 0x70000000) >> 28;
1636 unsigned int cmp
= (encoded_op
& 0x0f000000) >> 24;
1637 int oparg
= sign_extend32((encoded_op
& 0x00fff000) >> 12, 11);
1638 int cmparg
= sign_extend32(encoded_op
& 0x00000fff, 11);
1641 if (encoded_op
& (FUTEX_OP_OPARG_SHIFT
<< 28)) {
1642 if (oparg
< 0 || oparg
> 31) {
1643 char comm
[sizeof(current
->comm
)];
1645 * kill this print and return -EINVAL when userspace
1648 pr_info_ratelimited("futex_wake_op: %s tries to shift op by %d; fix this program\n",
1649 get_task_comm(comm
, current
), oparg
);
1655 pagefault_disable();
1656 ret
= arch_futex_atomic_op_inuser(op
, oparg
, &oldval
, uaddr
);
1662 case FUTEX_OP_CMP_EQ
:
1663 return oldval
== cmparg
;
1664 case FUTEX_OP_CMP_NE
:
1665 return oldval
!= cmparg
;
1666 case FUTEX_OP_CMP_LT
:
1667 return oldval
< cmparg
;
1668 case FUTEX_OP_CMP_GE
:
1669 return oldval
>= cmparg
;
1670 case FUTEX_OP_CMP_LE
:
1671 return oldval
<= cmparg
;
1672 case FUTEX_OP_CMP_GT
:
1673 return oldval
> cmparg
;
1680 * Wake up all waiters hashed on the physical page that is mapped
1681 * to this virtual address:
1684 futex_wake_op(u32 __user
*uaddr1
, unsigned int flags
, u32 __user
*uaddr2
,
1685 int nr_wake
, int nr_wake2
, int op
)
1687 union futex_key key1
= FUTEX_KEY_INIT
, key2
= FUTEX_KEY_INIT
;
1688 struct futex_hash_bucket
*hb1
, *hb2
;
1689 struct futex_q
*this, *next
;
1691 DEFINE_WAKE_Q(wake_q
);
1694 ret
= get_futex_key(uaddr1
, flags
& FLAGS_SHARED
, &key1
, FUTEX_READ
);
1695 if (unlikely(ret
!= 0))
1697 ret
= get_futex_key(uaddr2
, flags
& FLAGS_SHARED
, &key2
, FUTEX_WRITE
);
1698 if (unlikely(ret
!= 0))
1701 hb1
= hash_futex(&key1
);
1702 hb2
= hash_futex(&key2
);
1705 double_lock_hb(hb1
, hb2
);
1706 op_ret
= futex_atomic_op_inuser(op
, uaddr2
);
1707 if (unlikely(op_ret
< 0)) {
1708 double_unlock_hb(hb1
, hb2
);
1710 if (!IS_ENABLED(CONFIG_MMU
) ||
1711 unlikely(op_ret
!= -EFAULT
&& op_ret
!= -EAGAIN
)) {
1713 * we don't get EFAULT from MMU faults if we don't have
1714 * an MMU, but we might get them from range checking
1720 if (op_ret
== -EFAULT
) {
1721 ret
= fault_in_user_writeable(uaddr2
);
1726 if (!(flags
& FLAGS_SHARED
)) {
1735 plist_for_each_entry_safe(this, next
, &hb1
->chain
, list
) {
1736 if (match_futex (&this->key
, &key1
)) {
1737 if (this->pi_state
|| this->rt_waiter
) {
1741 mark_wake_futex(&wake_q
, this);
1742 if (++ret
>= nr_wake
)
1749 plist_for_each_entry_safe(this, next
, &hb2
->chain
, list
) {
1750 if (match_futex (&this->key
, &key2
)) {
1751 if (this->pi_state
|| this->rt_waiter
) {
1755 mark_wake_futex(&wake_q
, this);
1756 if (++op_ret
>= nr_wake2
)
1764 double_unlock_hb(hb1
, hb2
);
1770 * requeue_futex() - Requeue a futex_q from one hb to another
1771 * @q: the futex_q to requeue
1772 * @hb1: the source hash_bucket
1773 * @hb2: the target hash_bucket
1774 * @key2: the new key for the requeued futex_q
1777 void requeue_futex(struct futex_q
*q
, struct futex_hash_bucket
*hb1
,
1778 struct futex_hash_bucket
*hb2
, union futex_key
*key2
)
1782 * If key1 and key2 hash to the same bucket, no need to
1785 if (likely(&hb1
->chain
!= &hb2
->chain
)) {
1786 plist_del(&q
->list
, &hb1
->chain
);
1787 hb_waiters_dec(hb1
);
1788 hb_waiters_inc(hb2
);
1789 plist_add(&q
->list
, &hb2
->chain
);
1790 q
->lock_ptr
= &hb2
->lock
;
1796 * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
1798 * @key: the key of the requeue target futex
1799 * @hb: the hash_bucket of the requeue target futex
1801 * During futex_requeue, with requeue_pi=1, it is possible to acquire the
1802 * target futex if it is uncontended or via a lock steal. Set the futex_q key
1803 * to the requeue target futex so the waiter can detect the wakeup on the right
1804 * futex, but remove it from the hb and NULL the rt_waiter so it can detect
1805 * atomic lock acquisition. Set the q->lock_ptr to the requeue target hb->lock
1806 * to protect access to the pi_state to fixup the owner later. Must be called
1807 * with both q->lock_ptr and hb->lock held.
1810 void requeue_pi_wake_futex(struct futex_q
*q
, union futex_key
*key
,
1811 struct futex_hash_bucket
*hb
)
1817 WARN_ON(!q
->rt_waiter
);
1818 q
->rt_waiter
= NULL
;
1820 q
->lock_ptr
= &hb
->lock
;
1822 wake_up_state(q
->task
, TASK_NORMAL
);
1826 * futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
1827 * @pifutex: the user address of the to futex
1828 * @hb1: the from futex hash bucket, must be locked by the caller
1829 * @hb2: the to futex hash bucket, must be locked by the caller
1830 * @key1: the from futex key
1831 * @key2: the to futex key
1832 * @ps: address to store the pi_state pointer
1833 * @exiting: Pointer to store the task pointer of the owner task
1834 * which is in the middle of exiting
1835 * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
1837 * Try and get the lock on behalf of the top waiter if we can do it atomically.
1838 * Wake the top waiter if we succeed. If the caller specified set_waiters,
1839 * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
1840 * hb1 and hb2 must be held by the caller.
1842 * @exiting is only set when the return value is -EBUSY. If so, this holds
1843 * a refcount on the exiting task on return and the caller needs to drop it
1844 * after waiting for the exit to complete.
1847 * - 0 - failed to acquire the lock atomically;
1848 * - >0 - acquired the lock, return value is vpid of the top_waiter
1852 futex_proxy_trylock_atomic(u32 __user
*pifutex
, struct futex_hash_bucket
*hb1
,
1853 struct futex_hash_bucket
*hb2
, union futex_key
*key1
,
1854 union futex_key
*key2
, struct futex_pi_state
**ps
,
1855 struct task_struct
**exiting
, int set_waiters
)
1857 struct futex_q
*top_waiter
= NULL
;
1861 if (get_futex_value_locked(&curval
, pifutex
))
1864 if (unlikely(should_fail_futex(true)))
1868 * Find the top_waiter and determine if there are additional waiters.
1869 * If the caller intends to requeue more than 1 waiter to pifutex,
1870 * force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
1871 * as we have means to handle the possible fault. If not, don't set
1872 * the bit unecessarily as it will force the subsequent unlock to enter
1875 top_waiter
= futex_top_waiter(hb1
, key1
);
1877 /* There are no waiters, nothing for us to do. */
1881 /* Ensure we requeue to the expected futex. */
1882 if (!match_futex(top_waiter
->requeue_pi_key
, key2
))
1886 * Try to take the lock for top_waiter. Set the FUTEX_WAITERS bit in
1887 * the contended case or if set_waiters is 1. The pi_state is returned
1888 * in ps in contended cases.
1890 vpid
= task_pid_vnr(top_waiter
->task
);
1891 ret
= futex_lock_pi_atomic(pifutex
, hb2
, key2
, ps
, top_waiter
->task
,
1892 exiting
, set_waiters
);
1894 requeue_pi_wake_futex(top_waiter
, key2
, hb2
);
1901 * futex_requeue() - Requeue waiters from uaddr1 to uaddr2
1902 * @uaddr1: source futex user address
1903 * @flags: futex flags (FLAGS_SHARED, etc.)
1904 * @uaddr2: target futex user address
1905 * @nr_wake: number of waiters to wake (must be 1 for requeue_pi)
1906 * @nr_requeue: number of waiters to requeue (0-INT_MAX)
1907 * @cmpval: @uaddr1 expected value (or %NULL)
1908 * @requeue_pi: if we are attempting to requeue from a non-pi futex to a
1909 * pi futex (pi to pi requeue is not supported)
1911 * Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
1912 * uaddr2 atomically on behalf of the top waiter.
1915 * - >=0 - on success, the number of tasks requeued or woken;
1918 static int futex_requeue(u32 __user
*uaddr1
, unsigned int flags
,
1919 u32 __user
*uaddr2
, int nr_wake
, int nr_requeue
,
1920 u32
*cmpval
, int requeue_pi
)
1922 union futex_key key1
= FUTEX_KEY_INIT
, key2
= FUTEX_KEY_INIT
;
1923 int task_count
= 0, ret
;
1924 struct futex_pi_state
*pi_state
= NULL
;
1925 struct futex_hash_bucket
*hb1
, *hb2
;
1926 struct futex_q
*this, *next
;
1927 DEFINE_WAKE_Q(wake_q
);
1929 if (nr_wake
< 0 || nr_requeue
< 0)
1933 * When PI not supported: return -ENOSYS if requeue_pi is true,
1934 * consequently the compiler knows requeue_pi is always false past
1935 * this point which will optimize away all the conditional code
1938 if (!IS_ENABLED(CONFIG_FUTEX_PI
) && requeue_pi
)
1943 * Requeue PI only works on two distinct uaddrs. This
1944 * check is only valid for private futexes. See below.
1946 if (uaddr1
== uaddr2
)
1950 * requeue_pi requires a pi_state, try to allocate it now
1951 * without any locks in case it fails.
1953 if (refill_pi_state_cache())
1956 * requeue_pi must wake as many tasks as it can, up to nr_wake
1957 * + nr_requeue, since it acquires the rt_mutex prior to
1958 * returning to userspace, so as to not leave the rt_mutex with
1959 * waiters and no owner. However, second and third wake-ups
1960 * cannot be predicted as they involve race conditions with the
1961 * first wake and a fault while looking up the pi_state. Both
1962 * pthread_cond_signal() and pthread_cond_broadcast() should
1970 ret
= get_futex_key(uaddr1
, flags
& FLAGS_SHARED
, &key1
, FUTEX_READ
);
1971 if (unlikely(ret
!= 0))
1973 ret
= get_futex_key(uaddr2
, flags
& FLAGS_SHARED
, &key2
,
1974 requeue_pi
? FUTEX_WRITE
: FUTEX_READ
);
1975 if (unlikely(ret
!= 0))
1979 * The check above which compares uaddrs is not sufficient for
1980 * shared futexes. We need to compare the keys:
1982 if (requeue_pi
&& match_futex(&key1
, &key2
))
1985 hb1
= hash_futex(&key1
);
1986 hb2
= hash_futex(&key2
);
1989 hb_waiters_inc(hb2
);
1990 double_lock_hb(hb1
, hb2
);
1992 if (likely(cmpval
!= NULL
)) {
1995 ret
= get_futex_value_locked(&curval
, uaddr1
);
1997 if (unlikely(ret
)) {
1998 double_unlock_hb(hb1
, hb2
);
1999 hb_waiters_dec(hb2
);
2001 ret
= get_user(curval
, uaddr1
);
2005 if (!(flags
& FLAGS_SHARED
))
2010 if (curval
!= *cmpval
) {
2016 if (requeue_pi
&& (task_count
- nr_wake
< nr_requeue
)) {
2017 struct task_struct
*exiting
= NULL
;
2020 * Attempt to acquire uaddr2 and wake the top waiter. If we
2021 * intend to requeue waiters, force setting the FUTEX_WAITERS
2022 * bit. We force this here where we are able to easily handle
2023 * faults rather in the requeue loop below.
2025 ret
= futex_proxy_trylock_atomic(uaddr2
, hb1
, hb2
, &key1
,
2027 &exiting
, nr_requeue
);
2030 * At this point the top_waiter has either taken uaddr2 or is
2031 * waiting on it. If the former, then the pi_state will not
2032 * exist yet, look it up one more time to ensure we have a
2033 * reference to it. If the lock was taken, ret contains the
2034 * vpid of the top waiter task.
2035 * If the lock was not taken, we have pi_state and an initial
2036 * refcount on it. In case of an error we have nothing.
2042 * If we acquired the lock, then the user space value
2043 * of uaddr2 should be vpid. It cannot be changed by
2044 * the top waiter as it is blocked on hb2 lock if it
2045 * tries to do so. If something fiddled with it behind
2046 * our back the pi state lookup might unearth it. So
2047 * we rather use the known value than rereading and
2048 * handing potential crap to lookup_pi_state.
2050 * If that call succeeds then we have pi_state and an
2051 * initial refcount on it.
2053 ret
= lookup_pi_state(uaddr2
, ret
, hb2
, &key2
,
2054 &pi_state
, &exiting
);
2059 /* We hold a reference on the pi state. */
2062 /* If the above failed, then pi_state is NULL */
2064 double_unlock_hb(hb1
, hb2
);
2065 hb_waiters_dec(hb2
);
2066 ret
= fault_in_user_writeable(uaddr2
);
2073 * Two reasons for this:
2074 * - EBUSY: Owner is exiting and we just wait for the
2076 * - EAGAIN: The user space value changed.
2078 double_unlock_hb(hb1
, hb2
);
2079 hb_waiters_dec(hb2
);
2081 * Handle the case where the owner is in the middle of
2082 * exiting. Wait for the exit to complete otherwise
2083 * this task might loop forever, aka. live lock.
2085 wait_for_owner_exiting(ret
, exiting
);
2093 plist_for_each_entry_safe(this, next
, &hb1
->chain
, list
) {
2094 if (task_count
- nr_wake
>= nr_requeue
)
2097 if (!match_futex(&this->key
, &key1
))
2101 * FUTEX_WAIT_REQEUE_PI and FUTEX_CMP_REQUEUE_PI should always
2102 * be paired with each other and no other futex ops.
2104 * We should never be requeueing a futex_q with a pi_state,
2105 * which is awaiting a futex_unlock_pi().
2107 if ((requeue_pi
&& !this->rt_waiter
) ||
2108 (!requeue_pi
&& this->rt_waiter
) ||
2115 * Wake nr_wake waiters. For requeue_pi, if we acquired the
2116 * lock, we already woke the top_waiter. If not, it will be
2117 * woken by futex_unlock_pi().
2119 if (++task_count
<= nr_wake
&& !requeue_pi
) {
2120 mark_wake_futex(&wake_q
, this);
2124 /* Ensure we requeue to the expected futex for requeue_pi. */
2125 if (requeue_pi
&& !match_futex(this->requeue_pi_key
, &key2
)) {
2131 * Requeue nr_requeue waiters and possibly one more in the case
2132 * of requeue_pi if we couldn't acquire the lock atomically.
2136 * Prepare the waiter to take the rt_mutex. Take a
2137 * refcount on the pi_state and store the pointer in
2138 * the futex_q object of the waiter.
2140 get_pi_state(pi_state
);
2141 this->pi_state
= pi_state
;
2142 ret
= rt_mutex_start_proxy_lock(&pi_state
->pi_mutex
,
2147 * We got the lock. We do neither drop the
2148 * refcount on pi_state nor clear
2149 * this->pi_state because the waiter needs the
2150 * pi_state for cleaning up the user space
2151 * value. It will drop the refcount after
2154 requeue_pi_wake_futex(this, &key2
, hb2
);
2158 * rt_mutex_start_proxy_lock() detected a
2159 * potential deadlock when we tried to queue
2160 * that waiter. Drop the pi_state reference
2161 * which we took above and remove the pointer
2162 * to the state from the waiters futex_q
2165 this->pi_state
= NULL
;
2166 put_pi_state(pi_state
);
2168 * We stop queueing more waiters and let user
2169 * space deal with the mess.
2174 requeue_futex(this, hb1
, hb2
, &key2
);
2178 * We took an extra initial reference to the pi_state either
2179 * in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
2180 * need to drop it here again.
2182 put_pi_state(pi_state
);
2185 double_unlock_hb(hb1
, hb2
);
2187 hb_waiters_dec(hb2
);
2188 return ret
? ret
: task_count
;
2191 /* The key must be already stored in q->key. */
2192 static inline struct futex_hash_bucket
*queue_lock(struct futex_q
*q
)
2193 __acquires(&hb
->lock
)
2195 struct futex_hash_bucket
*hb
;
2197 hb
= hash_futex(&q
->key
);
2200 * Increment the counter before taking the lock so that
2201 * a potential waker won't miss a to-be-slept task that is
2202 * waiting for the spinlock. This is safe as all queue_lock()
2203 * users end up calling queue_me(). Similarly, for housekeeping,
2204 * decrement the counter at queue_unlock() when some error has
2205 * occurred and we don't end up adding the task to the list.
2207 hb_waiters_inc(hb
); /* implies smp_mb(); (A) */
2209 q
->lock_ptr
= &hb
->lock
;
2211 spin_lock(&hb
->lock
);
2216 queue_unlock(struct futex_hash_bucket
*hb
)
2217 __releases(&hb
->lock
)
2219 spin_unlock(&hb
->lock
);
2223 static inline void __queue_me(struct futex_q
*q
, struct futex_hash_bucket
*hb
)
2228 * The priority used to register this element is
2229 * - either the real thread-priority for the real-time threads
2230 * (i.e. threads with a priority lower than MAX_RT_PRIO)
2231 * - or MAX_RT_PRIO for non-RT threads.
2232 * Thus, all RT-threads are woken first in priority order, and
2233 * the others are woken last, in FIFO order.
2235 prio
= min(current
->normal_prio
, MAX_RT_PRIO
);
2237 plist_node_init(&q
->list
, prio
);
2238 plist_add(&q
->list
, &hb
->chain
);
2243 * queue_me() - Enqueue the futex_q on the futex_hash_bucket
2244 * @q: The futex_q to enqueue
2245 * @hb: The destination hash bucket
2247 * The hb->lock must be held by the caller, and is released here. A call to
2248 * queue_me() is typically paired with exactly one call to unqueue_me(). The
2249 * exceptions involve the PI related operations, which may use unqueue_me_pi()
2250 * or nothing if the unqueue is done as part of the wake process and the unqueue
2251 * state is implicit in the state of woken task (see futex_wait_requeue_pi() for
2254 static inline void queue_me(struct futex_q
*q
, struct futex_hash_bucket
*hb
)
2255 __releases(&hb
->lock
)
2258 spin_unlock(&hb
->lock
);
2262 * unqueue_me() - Remove the futex_q from its futex_hash_bucket
2263 * @q: The futex_q to unqueue
2265 * The q->lock_ptr must not be held by the caller. A call to unqueue_me() must
2266 * be paired with exactly one earlier call to queue_me().
2269 * - 1 - if the futex_q was still queued (and we removed unqueued it);
2270 * - 0 - if the futex_q was already removed by the waking thread
2272 static int unqueue_me(struct futex_q
*q
)
2274 spinlock_t
*lock_ptr
;
2277 /* In the common case we don't take the spinlock, which is nice. */
2280 * q->lock_ptr can change between this read and the following spin_lock.
2281 * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
2282 * optimizing lock_ptr out of the logic below.
2284 lock_ptr
= READ_ONCE(q
->lock_ptr
);
2285 if (lock_ptr
!= NULL
) {
2286 spin_lock(lock_ptr
);
2288 * q->lock_ptr can change between reading it and
2289 * spin_lock(), causing us to take the wrong lock. This
2290 * corrects the race condition.
2292 * Reasoning goes like this: if we have the wrong lock,
2293 * q->lock_ptr must have changed (maybe several times)
2294 * between reading it and the spin_lock(). It can
2295 * change again after the spin_lock() but only if it was
2296 * already changed before the spin_lock(). It cannot,
2297 * however, change back to the original value. Therefore
2298 * we can detect whether we acquired the correct lock.
2300 if (unlikely(lock_ptr
!= q
->lock_ptr
)) {
2301 spin_unlock(lock_ptr
);
2306 BUG_ON(q
->pi_state
);
2308 spin_unlock(lock_ptr
);
2316 * PI futexes can not be requeued and must remove themself from the
2317 * hash bucket. The hash bucket lock (i.e. lock_ptr) is held on entry
2320 static void unqueue_me_pi(struct futex_q
*q
)
2321 __releases(q
->lock_ptr
)
2325 BUG_ON(!q
->pi_state
);
2326 put_pi_state(q
->pi_state
);
2329 spin_unlock(q
->lock_ptr
);
2332 static int __fixup_pi_state_owner(u32 __user
*uaddr
, struct futex_q
*q
,
2333 struct task_struct
*argowner
)
2335 struct futex_pi_state
*pi_state
= q
->pi_state
;
2336 struct task_struct
*oldowner
, *newowner
;
2337 u32 uval
, curval
, newval
, newtid
;
2340 oldowner
= pi_state
->owner
;
2343 * We are here because either:
2345 * - we stole the lock and pi_state->owner needs updating to reflect
2346 * that (@argowner == current),
2350 * - someone stole our lock and we need to fix things to point to the
2351 * new owner (@argowner == NULL).
2353 * Either way, we have to replace the TID in the user space variable.
2354 * This must be atomic as we have to preserve the owner died bit here.
2356 * Note: We write the user space value _before_ changing the pi_state
2357 * because we can fault here. Imagine swapped out pages or a fork
2358 * that marked all the anonymous memory readonly for cow.
2360 * Modifying pi_state _before_ the user space value would leave the
2361 * pi_state in an inconsistent state when we fault here, because we
2362 * need to drop the locks to handle the fault. This might be observed
2363 * in the PID check in lookup_pi_state.
2367 if (oldowner
!= current
) {
2369 * We raced against a concurrent self; things are
2370 * already fixed up. Nothing to do.
2375 if (__rt_mutex_futex_trylock(&pi_state
->pi_mutex
)) {
2376 /* We got the lock. pi_state is correct. Tell caller. */
2381 * The trylock just failed, so either there is an owner or
2382 * there is a higher priority waiter than this one.
2384 newowner
= rt_mutex_owner(&pi_state
->pi_mutex
);
2386 * If the higher priority waiter has not yet taken over the
2387 * rtmutex then newowner is NULL. We can't return here with
2388 * that state because it's inconsistent vs. the user space
2389 * state. So drop the locks and try again. It's a valid
2390 * situation and not any different from the other retry
2393 if (unlikely(!newowner
)) {
2398 WARN_ON_ONCE(argowner
!= current
);
2399 if (oldowner
== current
) {
2401 * We raced against a concurrent self; things are
2402 * already fixed up. Nothing to do.
2406 newowner
= argowner
;
2409 newtid
= task_pid_vnr(newowner
) | FUTEX_WAITERS
;
2411 if (!pi_state
->owner
)
2412 newtid
|= FUTEX_OWNER_DIED
;
2414 err
= get_futex_value_locked(&uval
, uaddr
);
2419 newval
= (uval
& FUTEX_OWNER_DIED
) | newtid
;
2421 err
= cmpxchg_futex_value_locked(&curval
, uaddr
, uval
, newval
);
2431 * We fixed up user space. Now we need to fix the pi_state
2434 pi_state_update_owner(pi_state
, newowner
);
2436 return argowner
== current
;
2439 * In order to reschedule or handle a page fault, we need to drop the
2440 * locks here. In the case of a fault, this gives the other task
2441 * (either the highest priority waiter itself or the task which stole
2442 * the rtmutex) the chance to try the fixup of the pi_state. So once we
2443 * are back from handling the fault we need to check the pi_state after
2444 * reacquiring the locks and before trying to do another fixup. When
2445 * the fixup has been done already we simply return.
2447 * Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely
2448 * drop hb->lock since the caller owns the hb -> futex_q relation.
2449 * Dropping the pi_mutex->wait_lock requires the state revalidate.
2452 raw_spin_unlock_irq(&pi_state
->pi_mutex
.wait_lock
);
2453 spin_unlock(q
->lock_ptr
);
2457 err
= fault_in_user_writeable(uaddr
);
2470 spin_lock(q
->lock_ptr
);
2471 raw_spin_lock_irq(&pi_state
->pi_mutex
.wait_lock
);
2474 * Check if someone else fixed it for us:
2476 if (pi_state
->owner
!= oldowner
)
2477 return argowner
== current
;
2479 /* Retry if err was -EAGAIN or the fault in succeeded */
2484 * fault_in_user_writeable() failed so user state is immutable. At
2485 * best we can make the kernel state consistent but user state will
2486 * be most likely hosed and any subsequent unlock operation will be
2487 * rejected due to PI futex rule [10].
2489 * Ensure that the rtmutex owner is also the pi_state owner despite
2490 * the user space value claiming something different. There is no
2491 * point in unlocking the rtmutex if current is the owner as it
2492 * would need to wait until the next waiter has taken the rtmutex
2493 * to guarantee consistent state. Keep it simple. Userspace asked
2494 * for this wreckaged state.
2496 * The rtmutex has an owner - either current or some other
2497 * task. See the EAGAIN loop above.
2499 pi_state_update_owner(pi_state
, rt_mutex_owner(&pi_state
->pi_mutex
));
2504 static int fixup_pi_state_owner(u32 __user
*uaddr
, struct futex_q
*q
,
2505 struct task_struct
*argowner
)
2507 struct futex_pi_state
*pi_state
= q
->pi_state
;
2510 lockdep_assert_held(q
->lock_ptr
);
2512 raw_spin_lock_irq(&pi_state
->pi_mutex
.wait_lock
);
2513 ret
= __fixup_pi_state_owner(uaddr
, q
, argowner
);
2514 raw_spin_unlock_irq(&pi_state
->pi_mutex
.wait_lock
);
2518 static long futex_wait_restart(struct restart_block
*restart
);
2521 * fixup_owner() - Post lock pi_state and corner case management
2522 * @uaddr: user address of the futex
2523 * @q: futex_q (contains pi_state and access to the rt_mutex)
2524 * @locked: if the attempt to take the rt_mutex succeeded (1) or not (0)
2526 * After attempting to lock an rt_mutex, this function is called to cleanup
2527 * the pi_state owner as well as handle race conditions that may allow us to
2528 * acquire the lock. Must be called with the hb lock held.
2531 * - 1 - success, lock taken;
2532 * - 0 - success, lock not taken;
2533 * - <0 - on error (-EFAULT)
2535 static int fixup_owner(u32 __user
*uaddr
, struct futex_q
*q
, int locked
)
2539 * Got the lock. We might not be the anticipated owner if we
2540 * did a lock-steal - fix up the PI-state in that case:
2542 * Speculative pi_state->owner read (we don't hold wait_lock);
2543 * since we own the lock pi_state->owner == current is the
2544 * stable state, anything else needs more attention.
2546 if (q
->pi_state
->owner
!= current
)
2547 return fixup_pi_state_owner(uaddr
, q
, current
);
2552 * If we didn't get the lock; check if anybody stole it from us. In
2553 * that case, we need to fix up the uval to point to them instead of
2554 * us, otherwise bad things happen. [10]
2556 * Another speculative read; pi_state->owner == current is unstable
2557 * but needs our attention.
2559 if (q
->pi_state
->owner
== current
)
2560 return fixup_pi_state_owner(uaddr
, q
, NULL
);
2563 * Paranoia check. If we did not take the lock, then we should not be
2564 * the owner of the rt_mutex. Warn and establish consistent state.
2566 if (WARN_ON_ONCE(rt_mutex_owner(&q
->pi_state
->pi_mutex
) == current
))
2567 return fixup_pi_state_owner(uaddr
, q
, current
);
2573 * futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal
2574 * @hb: the futex hash bucket, must be locked by the caller
2575 * @q: the futex_q to queue up on
2576 * @timeout: the prepared hrtimer_sleeper, or null for no timeout
2578 static void futex_wait_queue_me(struct futex_hash_bucket
*hb
, struct futex_q
*q
,
2579 struct hrtimer_sleeper
*timeout
)
2582 * The task state is guaranteed to be set before another task can
2583 * wake it. set_current_state() is implemented using smp_store_mb() and
2584 * queue_me() calls spin_unlock() upon completion, both serializing
2585 * access to the hash list and forcing another memory barrier.
2587 set_current_state(TASK_INTERRUPTIBLE
);
2592 hrtimer_sleeper_start_expires(timeout
, HRTIMER_MODE_ABS
);
2595 * If we have been removed from the hash list, then another task
2596 * has tried to wake us, and we can skip the call to schedule().
2598 if (likely(!plist_node_empty(&q
->list
))) {
2600 * If the timer has already expired, current will already be
2601 * flagged for rescheduling. Only call schedule if there
2602 * is no timeout, or if it has yet to expire.
2604 if (!timeout
|| timeout
->task
)
2605 freezable_schedule();
2607 __set_current_state(TASK_RUNNING
);
2611 * futex_wait_setup() - Prepare to wait on a futex
2612 * @uaddr: the futex userspace address
2613 * @val: the expected value
2614 * @flags: futex flags (FLAGS_SHARED, etc.)
2615 * @q: the associated futex_q
2616 * @hb: storage for hash_bucket pointer to be returned to caller
2618 * Setup the futex_q and locate the hash_bucket. Get the futex value and
2619 * compare it with the expected value. Handle atomic faults internally.
2620 * Return with the hb lock held and a q.key reference on success, and unlocked
2621 * with no q.key reference on failure.
2624 * - 0 - uaddr contains val and hb has been locked;
2625 * - <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked
2627 static int futex_wait_setup(u32 __user
*uaddr
, u32 val
, unsigned int flags
,
2628 struct futex_q
*q
, struct futex_hash_bucket
**hb
)
2634 * Access the page AFTER the hash-bucket is locked.
2635 * Order is important:
2637 * Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
2638 * Userspace waker: if (cond(var)) { var = new; futex_wake(&var); }
2640 * The basic logical guarantee of a futex is that it blocks ONLY
2641 * if cond(var) is known to be true at the time of blocking, for
2642 * any cond. If we locked the hash-bucket after testing *uaddr, that
2643 * would open a race condition where we could block indefinitely with
2644 * cond(var) false, which would violate the guarantee.
2646 * On the other hand, we insert q and release the hash-bucket only
2647 * after testing *uaddr. This guarantees that futex_wait() will NOT
2648 * absorb a wakeup if *uaddr does not match the desired values
2649 * while the syscall executes.
2652 ret
= get_futex_key(uaddr
, flags
& FLAGS_SHARED
, &q
->key
, FUTEX_READ
);
2653 if (unlikely(ret
!= 0))
2657 *hb
= queue_lock(q
);
2659 ret
= get_futex_value_locked(&uval
, uaddr
);
2664 ret
= get_user(uval
, uaddr
);
2668 if (!(flags
& FLAGS_SHARED
))
2682 static int futex_wait(u32 __user
*uaddr
, unsigned int flags
, u32 val
,
2683 ktime_t
*abs_time
, u32 bitset
)
2685 struct hrtimer_sleeper timeout
, *to
;
2686 struct restart_block
*restart
;
2687 struct futex_hash_bucket
*hb
;
2688 struct futex_q q
= futex_q_init
;
2695 to
= futex_setup_timer(abs_time
, &timeout
, flags
,
2696 current
->timer_slack_ns
);
2699 * Prepare to wait on uaddr. On success, holds hb lock and increments
2702 ret
= futex_wait_setup(uaddr
, val
, flags
, &q
, &hb
);
2706 /* queue_me and wait for wakeup, timeout, or a signal. */
2707 futex_wait_queue_me(hb
, &q
, to
);
2709 /* If we were woken (and unqueued), we succeeded, whatever. */
2711 /* unqueue_me() drops q.key ref */
2712 if (!unqueue_me(&q
))
2715 if (to
&& !to
->task
)
2719 * We expect signal_pending(current), but we might be the
2720 * victim of a spurious wakeup as well.
2722 if (!signal_pending(current
))
2729 restart
= ¤t
->restart_block
;
2730 restart
->futex
.uaddr
= uaddr
;
2731 restart
->futex
.val
= val
;
2732 restart
->futex
.time
= *abs_time
;
2733 restart
->futex
.bitset
= bitset
;
2734 restart
->futex
.flags
= flags
| FLAGS_HAS_TIMEOUT
;
2736 ret
= set_restart_fn(restart
, futex_wait_restart
);
2740 hrtimer_cancel(&to
->timer
);
2741 destroy_hrtimer_on_stack(&to
->timer
);
2747 static long futex_wait_restart(struct restart_block
*restart
)
2749 u32 __user
*uaddr
= restart
->futex
.uaddr
;
2750 ktime_t t
, *tp
= NULL
;
2752 if (restart
->futex
.flags
& FLAGS_HAS_TIMEOUT
) {
2753 t
= restart
->futex
.time
;
2756 restart
->fn
= do_no_restart_syscall
;
2758 return (long)futex_wait(uaddr
, restart
->futex
.flags
,
2759 restart
->futex
.val
, tp
, restart
->futex
.bitset
);
2764 * Userspace tried a 0 -> TID atomic transition of the futex value
2765 * and failed. The kernel side here does the whole locking operation:
2766 * if there are waiters then it will block as a consequence of relying
2767 * on rt-mutexes, it does PI, etc. (Due to races the kernel might see
2768 * a 0 value of the futex too.).
2770 * Also serves as futex trylock_pi()'ing, and due semantics.
2772 static int futex_lock_pi(u32 __user
*uaddr
, unsigned int flags
,
2773 ktime_t
*time
, int trylock
)
2775 struct hrtimer_sleeper timeout
, *to
;
2776 struct task_struct
*exiting
= NULL
;
2777 struct rt_mutex_waiter rt_waiter
;
2778 struct futex_hash_bucket
*hb
;
2779 struct futex_q q
= futex_q_init
;
2782 if (!IS_ENABLED(CONFIG_FUTEX_PI
))
2785 if (refill_pi_state_cache())
2788 to
= futex_setup_timer(time
, &timeout
, FLAGS_CLOCKRT
, 0);
2791 ret
= get_futex_key(uaddr
, flags
& FLAGS_SHARED
, &q
.key
, FUTEX_WRITE
);
2792 if (unlikely(ret
!= 0))
2796 hb
= queue_lock(&q
);
2798 ret
= futex_lock_pi_atomic(uaddr
, hb
, &q
.key
, &q
.pi_state
, current
,
2800 if (unlikely(ret
)) {
2802 * Atomic work succeeded and we got the lock,
2803 * or failed. Either way, we do _not_ block.
2807 /* We got the lock. */
2809 goto out_unlock_put_key
;
2815 * Two reasons for this:
2816 * - EBUSY: Task is exiting and we just wait for the
2818 * - EAGAIN: The user space value changed.
2822 * Handle the case where the owner is in the middle of
2823 * exiting. Wait for the exit to complete otherwise
2824 * this task might loop forever, aka. live lock.
2826 wait_for_owner_exiting(ret
, exiting
);
2830 goto out_unlock_put_key
;
2834 WARN_ON(!q
.pi_state
);
2837 * Only actually queue now that the atomic ops are done:
2842 ret
= rt_mutex_futex_trylock(&q
.pi_state
->pi_mutex
);
2843 /* Fixup the trylock return value: */
2844 ret
= ret
? 0 : -EWOULDBLOCK
;
2848 rt_mutex_init_waiter(&rt_waiter
);
2851 * On PREEMPT_RT_FULL, when hb->lock becomes an rt_mutex, we must not
2852 * hold it while doing rt_mutex_start_proxy(), because then it will
2853 * include hb->lock in the blocking chain, even through we'll not in
2854 * fact hold it while blocking. This will lead it to report -EDEADLK
2855 * and BUG when futex_unlock_pi() interleaves with this.
2857 * Therefore acquire wait_lock while holding hb->lock, but drop the
2858 * latter before calling __rt_mutex_start_proxy_lock(). This
2859 * interleaves with futex_unlock_pi() -- which does a similar lock
2860 * handoff -- such that the latter can observe the futex_q::pi_state
2861 * before __rt_mutex_start_proxy_lock() is done.
2863 raw_spin_lock_irq(&q
.pi_state
->pi_mutex
.wait_lock
);
2864 spin_unlock(q
.lock_ptr
);
2866 * __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
2867 * such that futex_unlock_pi() is guaranteed to observe the waiter when
2868 * it sees the futex_q::pi_state.
2870 ret
= __rt_mutex_start_proxy_lock(&q
.pi_state
->pi_mutex
, &rt_waiter
, current
);
2871 raw_spin_unlock_irq(&q
.pi_state
->pi_mutex
.wait_lock
);
2880 hrtimer_sleeper_start_expires(to
, HRTIMER_MODE_ABS
);
2882 ret
= rt_mutex_wait_proxy_lock(&q
.pi_state
->pi_mutex
, to
, &rt_waiter
);
2885 spin_lock(q
.lock_ptr
);
2887 * If we failed to acquire the lock (deadlock/signal/timeout), we must
2888 * first acquire the hb->lock before removing the lock from the
2889 * rt_mutex waitqueue, such that we can keep the hb and rt_mutex wait
2892 * In particular; it is important that futex_unlock_pi() can not
2893 * observe this inconsistency.
2895 if (ret
&& !rt_mutex_cleanup_proxy_lock(&q
.pi_state
->pi_mutex
, &rt_waiter
))
2900 * Fixup the pi_state owner and possibly acquire the lock if we
2903 res
= fixup_owner(uaddr
, &q
, !ret
);
2905 * If fixup_owner() returned an error, proprogate that. If it acquired
2906 * the lock, clear our -ETIMEDOUT or -EINTR.
2909 ret
= (res
< 0) ? res
: 0;
2911 /* Unqueue and drop the lock */
2920 hrtimer_cancel(&to
->timer
);
2921 destroy_hrtimer_on_stack(&to
->timer
);
2923 return ret
!= -EINTR
? ret
: -ERESTARTNOINTR
;
2928 ret
= fault_in_user_writeable(uaddr
);
2932 if (!(flags
& FLAGS_SHARED
))
2939 * Userspace attempted a TID -> 0 atomic transition, and failed.
2940 * This is the in-kernel slowpath: we look up the PI state (if any),
2941 * and do the rt-mutex unlock.
2943 static int futex_unlock_pi(u32 __user
*uaddr
, unsigned int flags
)
2945 u32 curval
, uval
, vpid
= task_pid_vnr(current
);
2946 union futex_key key
= FUTEX_KEY_INIT
;
2947 struct futex_hash_bucket
*hb
;
2948 struct futex_q
*top_waiter
;
2951 if (!IS_ENABLED(CONFIG_FUTEX_PI
))
2955 if (get_user(uval
, uaddr
))
2958 * We release only a lock we actually own:
2960 if ((uval
& FUTEX_TID_MASK
) != vpid
)
2963 ret
= get_futex_key(uaddr
, flags
& FLAGS_SHARED
, &key
, FUTEX_WRITE
);
2967 hb
= hash_futex(&key
);
2968 spin_lock(&hb
->lock
);
2971 * Check waiters first. We do not trust user space values at
2972 * all and we at least want to know if user space fiddled
2973 * with the futex value instead of blindly unlocking.
2975 top_waiter
= futex_top_waiter(hb
, &key
);
2977 struct futex_pi_state
*pi_state
= top_waiter
->pi_state
;
2984 * If current does not own the pi_state then the futex is
2985 * inconsistent and user space fiddled with the futex value.
2987 if (pi_state
->owner
!= current
)
2990 get_pi_state(pi_state
);
2992 * By taking wait_lock while still holding hb->lock, we ensure
2993 * there is no point where we hold neither; and therefore
2994 * wake_futex_pi() must observe a state consistent with what we
2997 * In particular; this forces __rt_mutex_start_proxy() to
2998 * complete such that we're guaranteed to observe the
2999 * rt_waiter. Also see the WARN in wake_futex_pi().
3001 raw_spin_lock_irq(&pi_state
->pi_mutex
.wait_lock
);
3002 spin_unlock(&hb
->lock
);
3004 /* drops pi_state->pi_mutex.wait_lock */
3005 ret
= wake_futex_pi(uaddr
, uval
, pi_state
);
3007 put_pi_state(pi_state
);
3010 * Success, we're done! No tricky corner cases.
3015 * The atomic access to the futex value generated a
3016 * pagefault, so retry the user-access and the wakeup:
3021 * A unconditional UNLOCK_PI op raced against a waiter
3022 * setting the FUTEX_WAITERS bit. Try again.
3027 * wake_futex_pi has detected invalid state. Tell user
3034 * We have no kernel internal state, i.e. no waiters in the
3035 * kernel. Waiters which are about to queue themselves are stuck
3036 * on hb->lock. So we can safely ignore them. We do neither
3037 * preserve the WAITERS bit not the OWNER_DIED one. We are the
3040 if ((ret
= cmpxchg_futex_value_locked(&curval
, uaddr
, uval
, 0))) {
3041 spin_unlock(&hb
->lock
);
3056 * If uval has changed, let user space handle it.
3058 ret
= (curval
== uval
) ? 0 : -EAGAIN
;
3061 spin_unlock(&hb
->lock
);
3071 ret
= fault_in_user_writeable(uaddr
);
3079 * handle_early_requeue_pi_wakeup() - Detect early wakeup on the initial futex
3080 * @hb: the hash_bucket futex_q was original enqueued on
3081 * @q: the futex_q woken while waiting to be requeued
3082 * @key2: the futex_key of the requeue target futex
3083 * @timeout: the timeout associated with the wait (NULL if none)
3085 * Detect if the task was woken on the initial futex as opposed to the requeue
3086 * target futex. If so, determine if it was a timeout or a signal that caused
3087 * the wakeup and return the appropriate error code to the caller. Must be
3088 * called with the hb lock held.
3091 * - 0 = no early wakeup detected;
3092 * - <0 = -ETIMEDOUT or -ERESTARTNOINTR
3095 int handle_early_requeue_pi_wakeup(struct futex_hash_bucket
*hb
,
3096 struct futex_q
*q
, union futex_key
*key2
,
3097 struct hrtimer_sleeper
*timeout
)
3102 * With the hb lock held, we avoid races while we process the wakeup.
3103 * We only need to hold hb (and not hb2) to ensure atomicity as the
3104 * wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
3105 * It can't be requeued from uaddr2 to something else since we don't
3106 * support a PI aware source futex for requeue.
3108 if (!match_futex(&q
->key
, key2
)) {
3109 WARN_ON(q
->lock_ptr
&& (&hb
->lock
!= q
->lock_ptr
));
3111 * We were woken prior to requeue by a timeout or a signal.
3112 * Unqueue the futex_q and determine which it was.
3114 plist_del(&q
->list
, &hb
->chain
);
3117 /* Handle spurious wakeups gracefully */
3119 if (timeout
&& !timeout
->task
)
3121 else if (signal_pending(current
))
3122 ret
= -ERESTARTNOINTR
;
3128 * futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
3129 * @uaddr: the futex we initially wait on (non-pi)
3130 * @flags: futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be
3131 * the same type, no requeueing from private to shared, etc.
3132 * @val: the expected value of uaddr
3133 * @abs_time: absolute timeout
3134 * @bitset: 32 bit wakeup bitset set by userspace, defaults to all
3135 * @uaddr2: the pi futex we will take prior to returning to user-space
3137 * The caller will wait on uaddr and will be requeued by futex_requeue() to
3138 * uaddr2 which must be PI aware and unique from uaddr. Normal wakeup will wake
3139 * on uaddr2 and complete the acquisition of the rt_mutex prior to returning to
3140 * userspace. This ensures the rt_mutex maintains an owner when it has waiters;
3141 * without one, the pi logic would not know which task to boost/deboost, if
3142 * there was a need to.
3144 * We call schedule in futex_wait_queue_me() when we enqueue and return there
3145 * via the following--
3146 * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
3147 * 2) wakeup on uaddr2 after a requeue
3151 * If 3, cleanup and return -ERESTARTNOINTR.
3153 * If 2, we may then block on trying to take the rt_mutex and return via:
3154 * 5) successful lock
3157 * 8) other lock acquisition failure
3159 * If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
3161 * If 4 or 7, we cleanup and return with -ETIMEDOUT.
3167 static int futex_wait_requeue_pi(u32 __user
*uaddr
, unsigned int flags
,
3168 u32 val
, ktime_t
*abs_time
, u32 bitset
,
3171 struct hrtimer_sleeper timeout
, *to
;
3172 struct rt_mutex_waiter rt_waiter
;
3173 struct futex_hash_bucket
*hb
;
3174 union futex_key key2
= FUTEX_KEY_INIT
;
3175 struct futex_q q
= futex_q_init
;
3178 if (!IS_ENABLED(CONFIG_FUTEX_PI
))
3181 if (uaddr
== uaddr2
)
3187 to
= futex_setup_timer(abs_time
, &timeout
, flags
,
3188 current
->timer_slack_ns
);
3191 * The waiter is allocated on our stack, manipulated by the requeue
3192 * code while we sleep on uaddr.
3194 rt_mutex_init_waiter(&rt_waiter
);
3196 ret
= get_futex_key(uaddr2
, flags
& FLAGS_SHARED
, &key2
, FUTEX_WRITE
);
3197 if (unlikely(ret
!= 0))
3201 q
.rt_waiter
= &rt_waiter
;
3202 q
.requeue_pi_key
= &key2
;
3205 * Prepare to wait on uaddr. On success, increments q.key (key1) ref
3208 ret
= futex_wait_setup(uaddr
, val
, flags
, &q
, &hb
);
3213 * The check above which compares uaddrs is not sufficient for
3214 * shared futexes. We need to compare the keys:
3216 if (match_futex(&q
.key
, &key2
)) {
3222 /* Queue the futex_q, drop the hb lock, wait for wakeup. */
3223 futex_wait_queue_me(hb
, &q
, to
);
3225 spin_lock(&hb
->lock
);
3226 ret
= handle_early_requeue_pi_wakeup(hb
, &q
, &key2
, to
);
3227 spin_unlock(&hb
->lock
);
3232 * In order for us to be here, we know our q.key == key2, and since
3233 * we took the hb->lock above, we also know that futex_requeue() has
3234 * completed and we no longer have to concern ourselves with a wakeup
3235 * race with the atomic proxy lock acquisition by the requeue code. The
3236 * futex_requeue dropped our key1 reference and incremented our key2
3240 /* Check if the requeue code acquired the second futex for us. */
3243 * Got the lock. We might not be the anticipated owner if we
3244 * did a lock-steal - fix up the PI-state in that case.
3246 if (q
.pi_state
&& (q
.pi_state
->owner
!= current
)) {
3247 spin_lock(q
.lock_ptr
);
3248 ret
= fixup_pi_state_owner(uaddr2
, &q
, current
);
3250 * Drop the reference to the pi state which
3251 * the requeue_pi() code acquired for us.
3253 put_pi_state(q
.pi_state
);
3254 spin_unlock(q
.lock_ptr
);
3256 * Adjust the return value. It's either -EFAULT or
3257 * success (1) but the caller expects 0 for success.
3259 ret
= ret
< 0 ? ret
: 0;
3262 struct rt_mutex
*pi_mutex
;
3265 * We have been woken up by futex_unlock_pi(), a timeout, or a
3266 * signal. futex_unlock_pi() will not destroy the lock_ptr nor
3269 WARN_ON(!q
.pi_state
);
3270 pi_mutex
= &q
.pi_state
->pi_mutex
;
3271 ret
= rt_mutex_wait_proxy_lock(pi_mutex
, to
, &rt_waiter
);
3273 spin_lock(q
.lock_ptr
);
3274 if (ret
&& !rt_mutex_cleanup_proxy_lock(pi_mutex
, &rt_waiter
))
3277 debug_rt_mutex_free_waiter(&rt_waiter
);
3279 * Fixup the pi_state owner and possibly acquire the lock if we
3282 res
= fixup_owner(uaddr2
, &q
, !ret
);
3284 * If fixup_owner() returned an error, proprogate that. If it
3285 * acquired the lock, clear -ETIMEDOUT or -EINTR.
3288 ret
= (res
< 0) ? res
: 0;
3290 /* Unqueue and drop the lock. */
3294 if (ret
== -EINTR
) {
3296 * We've already been requeued, but cannot restart by calling
3297 * futex_lock_pi() directly. We could restart this syscall, but
3298 * it would detect that the user space "val" changed and return
3299 * -EWOULDBLOCK. Save the overhead of the restart and return
3300 * -EWOULDBLOCK directly.
3307 hrtimer_cancel(&to
->timer
);
3308 destroy_hrtimer_on_stack(&to
->timer
);
3314 * Support for robust futexes: the kernel cleans up held futexes at
3317 * Implementation: user-space maintains a per-thread list of locks it
3318 * is holding. Upon do_exit(), the kernel carefully walks this list,
3319 * and marks all locks that are owned by this thread with the
3320 * FUTEX_OWNER_DIED bit, and wakes up a waiter (if any). The list is
3321 * always manipulated with the lock held, so the list is private and
3322 * per-thread. Userspace also maintains a per-thread 'list_op_pending'
3323 * field, to allow the kernel to clean up if the thread dies after
3324 * acquiring the lock, but just before it could have added itself to
3325 * the list. There can only be one such pending lock.
3329 * sys_set_robust_list() - Set the robust-futex list head of a task
3330 * @head: pointer to the list-head
3331 * @len: length of the list-head, as userspace expects
3333 SYSCALL_DEFINE2(set_robust_list
, struct robust_list_head __user
*, head
,
3336 if (!futex_cmpxchg_enabled
)
3339 * The kernel knows only one size for now:
3341 if (unlikely(len
!= sizeof(*head
)))
3344 current
->robust_list
= head
;
3350 * sys_get_robust_list() - Get the robust-futex list head of a task
3351 * @pid: pid of the process [zero for current task]
3352 * @head_ptr: pointer to a list-head pointer, the kernel fills it in
3353 * @len_ptr: pointer to a length field, the kernel fills in the header size
3355 SYSCALL_DEFINE3(get_robust_list
, int, pid
,
3356 struct robust_list_head __user
* __user
*, head_ptr
,
3357 size_t __user
*, len_ptr
)
3359 struct robust_list_head __user
*head
;
3361 struct task_struct
*p
;
3363 if (!futex_cmpxchg_enabled
)
3372 p
= find_task_by_vpid(pid
);
3378 if (!ptrace_may_access(p
, PTRACE_MODE_READ_REALCREDS
))
3381 head
= p
->robust_list
;
3384 if (put_user(sizeof(*head
), len_ptr
))
3386 return put_user(head
, head_ptr
);
3394 /* Constants for the pending_op argument of handle_futex_death */
3395 #define HANDLE_DEATH_PENDING true
3396 #define HANDLE_DEATH_LIST false
3399 * Process a futex-list entry, check whether it's owned by the
3400 * dying task, and do notification if so:
3402 static int handle_futex_death(u32 __user
*uaddr
, struct task_struct
*curr
,
3403 bool pi
, bool pending_op
)
3405 u32 uval
, nval
, mval
;
3408 /* Futex address must be 32bit aligned */
3409 if ((((unsigned long)uaddr
) % sizeof(*uaddr
)) != 0)
3413 if (get_user(uval
, uaddr
))
3417 * Special case for regular (non PI) futexes. The unlock path in
3418 * user space has two race scenarios:
3420 * 1. The unlock path releases the user space futex value and
3421 * before it can execute the futex() syscall to wake up
3422 * waiters it is killed.
3424 * 2. A woken up waiter is killed before it can acquire the
3425 * futex in user space.
3427 * In both cases the TID validation below prevents a wakeup of
3428 * potential waiters which can cause these waiters to block
3431 * In both cases the following conditions are met:
3433 * 1) task->robust_list->list_op_pending != NULL
3434 * @pending_op == true
3435 * 2) User space futex value == 0
3436 * 3) Regular futex: @pi == false
3438 * If these conditions are met, it is safe to attempt waking up a
3439 * potential waiter without touching the user space futex value and
3440 * trying to set the OWNER_DIED bit. The user space futex value is
3441 * uncontended and the rest of the user space mutex state is
3442 * consistent, so a woken waiter will just take over the
3443 * uncontended futex. Setting the OWNER_DIED bit would create
3444 * inconsistent state and malfunction of the user space owner died
3447 if (pending_op
&& !pi
&& !uval
) {
3448 futex_wake(uaddr
, 1, 1, FUTEX_BITSET_MATCH_ANY
);
3452 if ((uval
& FUTEX_TID_MASK
) != task_pid_vnr(curr
))
3456 * Ok, this dying thread is truly holding a futex
3457 * of interest. Set the OWNER_DIED bit atomically
3458 * via cmpxchg, and if the value had FUTEX_WAITERS
3459 * set, wake up a waiter (if any). (We have to do a
3460 * futex_wake() even if OWNER_DIED is already set -
3461 * to handle the rare but possible case of recursive
3462 * thread-death.) The rest of the cleanup is done in
3465 mval
= (uval
& FUTEX_WAITERS
) | FUTEX_OWNER_DIED
;
3468 * We are not holding a lock here, but we want to have
3469 * the pagefault_disable/enable() protection because
3470 * we want to handle the fault gracefully. If the
3471 * access fails we try to fault in the futex with R/W
3472 * verification via get_user_pages. get_user() above
3473 * does not guarantee R/W access. If that fails we
3474 * give up and leave the futex locked.
3476 if ((err
= cmpxchg_futex_value_locked(&nval
, uaddr
, uval
, mval
))) {
3479 if (fault_in_user_writeable(uaddr
))
3497 * Wake robust non-PI futexes here. The wakeup of
3498 * PI futexes happens in exit_pi_state():
3500 if (!pi
&& (uval
& FUTEX_WAITERS
))
3501 futex_wake(uaddr
, 1, 1, FUTEX_BITSET_MATCH_ANY
);
3507 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
3509 static inline int fetch_robust_entry(struct robust_list __user
**entry
,
3510 struct robust_list __user
* __user
*head
,
3513 unsigned long uentry
;
3515 if (get_user(uentry
, (unsigned long __user
*)head
))
3518 *entry
= (void __user
*)(uentry
& ~1UL);
3525 * Walk curr->robust_list (very carefully, it's a userspace list!)
3526 * and mark any locks found there dead, and notify any waiters.
3528 * We silently return on any sign of list-walking problem.
3530 static void exit_robust_list(struct task_struct
*curr
)
3532 struct robust_list_head __user
*head
= curr
->robust_list
;
3533 struct robust_list __user
*entry
, *next_entry
, *pending
;
3534 unsigned int limit
= ROBUST_LIST_LIMIT
, pi
, pip
;
3535 unsigned int next_pi
;
3536 unsigned long futex_offset
;
3539 if (!futex_cmpxchg_enabled
)
3543 * Fetch the list head (which was registered earlier, via
3544 * sys_set_robust_list()):
3546 if (fetch_robust_entry(&entry
, &head
->list
.next
, &pi
))
3549 * Fetch the relative futex offset:
3551 if (get_user(futex_offset
, &head
->futex_offset
))
3554 * Fetch any possibly pending lock-add first, and handle it
3557 if (fetch_robust_entry(&pending
, &head
->list_op_pending
, &pip
))
3560 next_entry
= NULL
; /* avoid warning with gcc */
3561 while (entry
!= &head
->list
) {
3563 * Fetch the next entry in the list before calling
3564 * handle_futex_death:
3566 rc
= fetch_robust_entry(&next_entry
, &entry
->next
, &next_pi
);
3568 * A pending lock might already be on the list, so
3569 * don't process it twice:
3571 if (entry
!= pending
) {
3572 if (handle_futex_death((void __user
*)entry
+ futex_offset
,
3573 curr
, pi
, HANDLE_DEATH_LIST
))
3581 * Avoid excessively long or circular lists:
3590 handle_futex_death((void __user
*)pending
+ futex_offset
,
3591 curr
, pip
, HANDLE_DEATH_PENDING
);
3595 static void futex_cleanup(struct task_struct
*tsk
)
3597 if (unlikely(tsk
->robust_list
)) {
3598 exit_robust_list(tsk
);
3599 tsk
->robust_list
= NULL
;
3602 #ifdef CONFIG_COMPAT
3603 if (unlikely(tsk
->compat_robust_list
)) {
3604 compat_exit_robust_list(tsk
);
3605 tsk
->compat_robust_list
= NULL
;
3609 if (unlikely(!list_empty(&tsk
->pi_state_list
)))
3610 exit_pi_state_list(tsk
);
3614 * futex_exit_recursive - Set the tasks futex state to FUTEX_STATE_DEAD
3615 * @tsk: task to set the state on
3617 * Set the futex exit state of the task lockless. The futex waiter code
3618 * observes that state when a task is exiting and loops until the task has
3619 * actually finished the futex cleanup. The worst case for this is that the
3620 * waiter runs through the wait loop until the state becomes visible.
3622 * This is called from the recursive fault handling path in do_exit().
3624 * This is best effort. Either the futex exit code has run already or
3625 * not. If the OWNER_DIED bit has been set on the futex then the waiter can
3626 * take it over. If not, the problem is pushed back to user space. If the
3627 * futex exit code did not run yet, then an already queued waiter might
3628 * block forever, but there is nothing which can be done about that.
3630 void futex_exit_recursive(struct task_struct
*tsk
)
3632 /* If the state is FUTEX_STATE_EXITING then futex_exit_mutex is held */
3633 if (tsk
->futex_state
== FUTEX_STATE_EXITING
)
3634 mutex_unlock(&tsk
->futex_exit_mutex
);
3635 tsk
->futex_state
= FUTEX_STATE_DEAD
;
3638 static void futex_cleanup_begin(struct task_struct
*tsk
)
3641 * Prevent various race issues against a concurrent incoming waiter
3642 * including live locks by forcing the waiter to block on
3643 * tsk->futex_exit_mutex when it observes FUTEX_STATE_EXITING in
3644 * attach_to_pi_owner().
3646 mutex_lock(&tsk
->futex_exit_mutex
);
3649 * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
3651 * This ensures that all subsequent checks of tsk->futex_state in
3652 * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
3653 * tsk->pi_lock held.
3655 * It guarantees also that a pi_state which was queued right before
3656 * the state change under tsk->pi_lock by a concurrent waiter must
3657 * be observed in exit_pi_state_list().
3659 raw_spin_lock_irq(&tsk
->pi_lock
);
3660 tsk
->futex_state
= FUTEX_STATE_EXITING
;
3661 raw_spin_unlock_irq(&tsk
->pi_lock
);
3664 static void futex_cleanup_end(struct task_struct
*tsk
, int state
)
3667 * Lockless store. The only side effect is that an observer might
3668 * take another loop until it becomes visible.
3670 tsk
->futex_state
= state
;
3672 * Drop the exit protection. This unblocks waiters which observed
3673 * FUTEX_STATE_EXITING to reevaluate the state.
3675 mutex_unlock(&tsk
->futex_exit_mutex
);
3678 void futex_exec_release(struct task_struct
*tsk
)
3681 * The state handling is done for consistency, but in the case of
3682 * exec() there is no way to prevent futher damage as the PID stays
3683 * the same. But for the unlikely and arguably buggy case that a
3684 * futex is held on exec(), this provides at least as much state
3685 * consistency protection which is possible.
3687 futex_cleanup_begin(tsk
);
3690 * Reset the state to FUTEX_STATE_OK. The task is alive and about
3691 * exec a new binary.
3693 futex_cleanup_end(tsk
, FUTEX_STATE_OK
);
3696 void futex_exit_release(struct task_struct
*tsk
)
3698 futex_cleanup_begin(tsk
);
3700 futex_cleanup_end(tsk
, FUTEX_STATE_DEAD
);
3703 long do_futex(u32 __user
*uaddr
, int op
, u32 val
, ktime_t
*timeout
,
3704 u32 __user
*uaddr2
, u32 val2
, u32 val3
)
3706 int cmd
= op
& FUTEX_CMD_MASK
;
3707 unsigned int flags
= 0;
3709 if (!(op
& FUTEX_PRIVATE_FLAG
))
3710 flags
|= FLAGS_SHARED
;
3712 if (op
& FUTEX_CLOCK_REALTIME
) {
3713 flags
|= FLAGS_CLOCKRT
;
3714 if (cmd
!= FUTEX_WAIT_BITSET
&& cmd
!= FUTEX_WAIT_REQUEUE_PI
)
3720 case FUTEX_UNLOCK_PI
:
3721 case FUTEX_TRYLOCK_PI
:
3722 case FUTEX_WAIT_REQUEUE_PI
:
3723 case FUTEX_CMP_REQUEUE_PI
:
3724 if (!futex_cmpxchg_enabled
)
3730 val3
= FUTEX_BITSET_MATCH_ANY
;
3732 case FUTEX_WAIT_BITSET
:
3733 return futex_wait(uaddr
, flags
, val
, timeout
, val3
);
3735 val3
= FUTEX_BITSET_MATCH_ANY
;
3737 case FUTEX_WAKE_BITSET
:
3738 return futex_wake(uaddr
, flags
, val
, val3
);
3740 return futex_requeue(uaddr
, flags
, uaddr2
, val
, val2
, NULL
, 0);
3741 case FUTEX_CMP_REQUEUE
:
3742 return futex_requeue(uaddr
, flags
, uaddr2
, val
, val2
, &val3
, 0);
3744 return futex_wake_op(uaddr
, flags
, uaddr2
, val
, val2
, val3
);
3746 return futex_lock_pi(uaddr
, flags
, timeout
, 0);
3747 case FUTEX_UNLOCK_PI
:
3748 return futex_unlock_pi(uaddr
, flags
);
3749 case FUTEX_TRYLOCK_PI
:
3750 return futex_lock_pi(uaddr
, flags
, NULL
, 1);
3751 case FUTEX_WAIT_REQUEUE_PI
:
3752 val3
= FUTEX_BITSET_MATCH_ANY
;
3753 return futex_wait_requeue_pi(uaddr
, flags
, val
, timeout
, val3
,
3755 case FUTEX_CMP_REQUEUE_PI
:
3756 return futex_requeue(uaddr
, flags
, uaddr2
, val
, val2
, &val3
, 1);
3762 SYSCALL_DEFINE6(futex
, u32 __user
*, uaddr
, int, op
, u32
, val
,
3763 struct __kernel_timespec __user
*, utime
, u32 __user
*, uaddr2
,
3766 struct timespec64 ts
;
3767 ktime_t t
, *tp
= NULL
;
3769 int cmd
= op
& FUTEX_CMD_MASK
;
3771 if (utime
&& (cmd
== FUTEX_WAIT
|| cmd
== FUTEX_LOCK_PI
||
3772 cmd
== FUTEX_WAIT_BITSET
||
3773 cmd
== FUTEX_WAIT_REQUEUE_PI
)) {
3774 if (unlikely(should_fail_futex(!(op
& FUTEX_PRIVATE_FLAG
))))
3776 if (get_timespec64(&ts
, utime
))
3778 if (!timespec64_valid(&ts
))
3781 t
= timespec64_to_ktime(ts
);
3782 if (cmd
== FUTEX_WAIT
)
3783 t
= ktime_add_safe(ktime_get(), t
);
3784 else if (cmd
!= FUTEX_LOCK_PI
&& !(op
& FUTEX_CLOCK_REALTIME
))
3785 t
= timens_ktime_to_host(CLOCK_MONOTONIC
, t
);
3789 * requeue parameter in 'utime' if cmd == FUTEX_*_REQUEUE_*.
3790 * number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.
3792 if (cmd
== FUTEX_REQUEUE
|| cmd
== FUTEX_CMP_REQUEUE
||
3793 cmd
== FUTEX_CMP_REQUEUE_PI
|| cmd
== FUTEX_WAKE_OP
)
3794 val2
= (u32
) (unsigned long) utime
;
3796 return do_futex(uaddr
, op
, val
, tp
, uaddr2
, val2
, val3
);
3799 #ifdef CONFIG_COMPAT
3801 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
3804 compat_fetch_robust_entry(compat_uptr_t
*uentry
, struct robust_list __user
**entry
,
3805 compat_uptr_t __user
*head
, unsigned int *pi
)
3807 if (get_user(*uentry
, head
))
3810 *entry
= compat_ptr((*uentry
) & ~1);
3811 *pi
= (unsigned int)(*uentry
) & 1;
3816 static void __user
*futex_uaddr(struct robust_list __user
*entry
,
3817 compat_long_t futex_offset
)
3819 compat_uptr_t base
= ptr_to_compat(entry
);
3820 void __user
*uaddr
= compat_ptr(base
+ futex_offset
);
3826 * Walk curr->robust_list (very carefully, it's a userspace list!)
3827 * and mark any locks found there dead, and notify any waiters.
3829 * We silently return on any sign of list-walking problem.
3831 static void compat_exit_robust_list(struct task_struct
*curr
)
3833 struct compat_robust_list_head __user
*head
= curr
->compat_robust_list
;
3834 struct robust_list __user
*entry
, *next_entry
, *pending
;
3835 unsigned int limit
= ROBUST_LIST_LIMIT
, pi
, pip
;
3836 unsigned int next_pi
;
3837 compat_uptr_t uentry
, next_uentry
, upending
;
3838 compat_long_t futex_offset
;
3841 if (!futex_cmpxchg_enabled
)
3845 * Fetch the list head (which was registered earlier, via
3846 * sys_set_robust_list()):
3848 if (compat_fetch_robust_entry(&uentry
, &entry
, &head
->list
.next
, &pi
))
3851 * Fetch the relative futex offset:
3853 if (get_user(futex_offset
, &head
->futex_offset
))
3856 * Fetch any possibly pending lock-add first, and handle it
3859 if (compat_fetch_robust_entry(&upending
, &pending
,
3860 &head
->list_op_pending
, &pip
))
3863 next_entry
= NULL
; /* avoid warning with gcc */
3864 while (entry
!= (struct robust_list __user
*) &head
->list
) {
3866 * Fetch the next entry in the list before calling
3867 * handle_futex_death:
3869 rc
= compat_fetch_robust_entry(&next_uentry
, &next_entry
,
3870 (compat_uptr_t __user
*)&entry
->next
, &next_pi
);
3872 * A pending lock might already be on the list, so
3873 * dont process it twice:
3875 if (entry
!= pending
) {
3876 void __user
*uaddr
= futex_uaddr(entry
, futex_offset
);
3878 if (handle_futex_death(uaddr
, curr
, pi
,
3884 uentry
= next_uentry
;
3888 * Avoid excessively long or circular lists:
3896 void __user
*uaddr
= futex_uaddr(pending
, futex_offset
);
3898 handle_futex_death(uaddr
, curr
, pip
, HANDLE_DEATH_PENDING
);
3902 COMPAT_SYSCALL_DEFINE2(set_robust_list
,
3903 struct compat_robust_list_head __user
*, head
,
3906 if (!futex_cmpxchg_enabled
)
3909 if (unlikely(len
!= sizeof(*head
)))
3912 current
->compat_robust_list
= head
;
3917 COMPAT_SYSCALL_DEFINE3(get_robust_list
, int, pid
,
3918 compat_uptr_t __user
*, head_ptr
,
3919 compat_size_t __user
*, len_ptr
)
3921 struct compat_robust_list_head __user
*head
;
3923 struct task_struct
*p
;
3925 if (!futex_cmpxchg_enabled
)
3934 p
= find_task_by_vpid(pid
);
3940 if (!ptrace_may_access(p
, PTRACE_MODE_READ_REALCREDS
))
3943 head
= p
->compat_robust_list
;
3946 if (put_user(sizeof(*head
), len_ptr
))
3948 return put_user(ptr_to_compat(head
), head_ptr
);
3955 #endif /* CONFIG_COMPAT */
3957 #ifdef CONFIG_COMPAT_32BIT_TIME
3958 SYSCALL_DEFINE6(futex_time32
, u32 __user
*, uaddr
, int, op
, u32
, val
,
3959 struct old_timespec32 __user
*, utime
, u32 __user
*, uaddr2
,
3962 struct timespec64 ts
;
3963 ktime_t t
, *tp
= NULL
;
3965 int cmd
= op
& FUTEX_CMD_MASK
;
3967 if (utime
&& (cmd
== FUTEX_WAIT
|| cmd
== FUTEX_LOCK_PI
||
3968 cmd
== FUTEX_WAIT_BITSET
||
3969 cmd
== FUTEX_WAIT_REQUEUE_PI
)) {
3970 if (get_old_timespec32(&ts
, utime
))
3972 if (!timespec64_valid(&ts
))
3975 t
= timespec64_to_ktime(ts
);
3976 if (cmd
== FUTEX_WAIT
)
3977 t
= ktime_add_safe(ktime_get(), t
);
3978 else if (cmd
!= FUTEX_LOCK_PI
&& !(op
& FUTEX_CLOCK_REALTIME
))
3979 t
= timens_ktime_to_host(CLOCK_MONOTONIC
, t
);
3982 if (cmd
== FUTEX_REQUEUE
|| cmd
== FUTEX_CMP_REQUEUE
||
3983 cmd
== FUTEX_CMP_REQUEUE_PI
|| cmd
== FUTEX_WAKE_OP
)
3984 val2
= (int) (unsigned long) utime
;
3986 return do_futex(uaddr
, op
, val
, tp
, uaddr2
, val2
, val3
);
3988 #endif /* CONFIG_COMPAT_32BIT_TIME */
3990 static void __init
futex_detect_cmpxchg(void)
3992 #ifndef CONFIG_HAVE_FUTEX_CMPXCHG
3996 * This will fail and we want it. Some arch implementations do
3997 * runtime detection of the futex_atomic_cmpxchg_inatomic()
3998 * functionality. We want to know that before we call in any
3999 * of the complex code paths. Also we want to prevent
4000 * registration of robust lists in that case. NULL is
4001 * guaranteed to fault and we get -EFAULT on functional
4002 * implementation, the non-functional ones will return
4005 if (cmpxchg_futex_value_locked(&curval
, NULL
, 0, 0) == -EFAULT
)
4006 futex_cmpxchg_enabled
= 1;
4010 static int __init
futex_init(void)
4012 unsigned int futex_shift
;
4015 #if CONFIG_BASE_SMALL
4016 futex_hashsize
= 16;
4018 futex_hashsize
= roundup_pow_of_two(256 * num_possible_cpus());
4021 futex_queues
= alloc_large_system_hash("futex", sizeof(*futex_queues
),
4023 futex_hashsize
< 256 ? HASH_SMALL
: 0,
4025 futex_hashsize
, futex_hashsize
);
4026 futex_hashsize
= 1UL << futex_shift
;
4028 futex_detect_cmpxchg();
4030 for (i
= 0; i
< futex_hashsize
; i
++) {
4031 atomic_set(&futex_queues
[i
].waiters
, 0);
4032 plist_head_init(&futex_queues
[i
].chain
);
4033 spin_lock_init(&futex_queues
[i
].lock
);
4038 core_initcall(futex_init
);