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/memblock.h>
38 #include <linux/fault-inject.h>
39 #include <linux/slab.h>
42 #include "../locking/rtmutex_common.h"
45 * The base of the bucket array and its size are always used together
46 * (after initialization only in futex_hash()), so ensure that they
47 * reside in the same cacheline.
50 struct futex_hash_bucket *queues;
51 unsigned long hashsize;
52 } __futex_data __read_mostly __aligned(2*sizeof(long));
53 #define futex_queues (__futex_data.queues)
54 #define futex_hashsize (__futex_data.hashsize)
58 * Fault injections for futexes.
60 #ifdef CONFIG_FAIL_FUTEX
63 struct fault_attr attr;
67 .attr = FAULT_ATTR_INITIALIZER,
68 .ignore_private = false,
71 static int __init setup_fail_futex(char *str)
73 return setup_fault_attr(&fail_futex.attr, str);
75 __setup("fail_futex=", setup_fail_futex);
77 bool should_fail_futex(bool fshared)
79 if (fail_futex.ignore_private && !fshared)
82 return should_fail(&fail_futex.attr, 1);
85 #ifdef CONFIG_FAULT_INJECTION_DEBUG_FS
87 static int __init fail_futex_debugfs(void)
89 umode_t mode = S_IFREG | S_IRUSR | S_IWUSR;
92 dir = fault_create_debugfs_attr("fail_futex", NULL,
97 debugfs_create_bool("ignore-private", mode, dir,
98 &fail_futex.ignore_private);
102 late_initcall(fail_futex_debugfs);
104 #endif /* CONFIG_FAULT_INJECTION_DEBUG_FS */
106 #endif /* CONFIG_FAIL_FUTEX */
109 * futex_hash - Return the hash bucket in the global hash
110 * @key: Pointer to the futex key for which the hash is calculated
112 * We hash on the keys returned from get_futex_key (see below) and return the
113 * corresponding hash bucket in the global hash.
115 struct futex_hash_bucket *futex_hash(union futex_key *key)
117 u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
120 return &futex_queues[hash & (futex_hashsize - 1)];
125 * futex_setup_timer - set up the sleeping hrtimer.
126 * @time: ptr to the given timeout value
127 * @timeout: the hrtimer_sleeper structure to be set up
128 * @flags: futex flags
129 * @range_ns: optional range in ns
131 * Return: Initialized hrtimer_sleeper structure or NULL if no timeout
134 struct hrtimer_sleeper *
135 futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout,
136 int flags, u64 range_ns)
141 hrtimer_init_sleeper_on_stack(timeout, (flags & FLAGS_CLOCKRT) ?
142 CLOCK_REALTIME : CLOCK_MONOTONIC,
145 * If range_ns is 0, calling hrtimer_set_expires_range_ns() is
146 * effectively the same as calling hrtimer_set_expires().
148 hrtimer_set_expires_range_ns(&timeout->timer, *time, range_ns);
154 * Generate a machine wide unique identifier for this inode.
156 * This relies on u64 not wrapping in the life-time of the machine; which with
157 * 1ns resolution means almost 585 years.
159 * This further relies on the fact that a well formed program will not unmap
160 * the file while it has a (shared) futex waiting on it. This mapping will have
161 * a file reference which pins the mount and inode.
163 * If for some reason an inode gets evicted and read back in again, it will get
164 * a new sequence number and will _NOT_ match, even though it is the exact same
167 * It is important that futex_match() will never have a false-positive, esp.
168 * for PI futexes that can mess up the state. The above argues that false-negatives
169 * are only possible for malformed programs.
171 static u64 get_inode_sequence_number(struct inode *inode)
173 static atomic64_t i_seq;
176 /* Does the inode already have a sequence number? */
177 old = atomic64_read(&inode->i_sequence);
182 u64 new = atomic64_add_return(1, &i_seq);
183 if (WARN_ON_ONCE(!new))
186 old = atomic64_cmpxchg_relaxed(&inode->i_sequence, 0, new);
194 * get_futex_key() - Get parameters which are the keys for a futex
195 * @uaddr: virtual address of the futex
197 * @key: address where result is stored.
198 * @rw: mapping needs to be read/write (values: FUTEX_READ,
201 * Return: a negative error code or 0
203 * The key words are stored in @key on success.
205 * For shared mappings (when @fshared), the key is:
207 * ( inode->i_sequence, page->index, offset_within_page )
209 * [ also see get_inode_sequence_number() ]
211 * For private mappings (or when !@fshared), the key is:
213 * ( current->mm, address, 0 )
215 * This allows (cross process, where applicable) identification of the futex
216 * without keeping the page pinned for the duration of the FUTEX_WAIT.
218 * lock_page() might sleep, the caller should not hold a spinlock.
220 int get_futex_key(u32 __user *uaddr, unsigned int flags, union futex_key *key,
221 enum futex_access rw)
223 unsigned long address = (unsigned long)uaddr;
224 struct mm_struct *mm = current->mm;
227 struct address_space *mapping;
231 fshared = flags & FLAGS_SHARED;
234 * The futex address must be "naturally" aligned.
236 key->both.offset = address % PAGE_SIZE;
237 if (unlikely((address % sizeof(u32)) != 0))
239 address -= key->both.offset;
241 if (unlikely(!access_ok(uaddr, sizeof(u32))))
244 if (unlikely(should_fail_futex(fshared)))
248 * PROCESS_PRIVATE futexes are fast.
249 * As the mm cannot disappear under us and the 'key' only needs
250 * virtual address, we dont even have to find the underlying vma.
251 * Note : We do have to check 'uaddr' is a valid user address,
252 * but access_ok() should be faster than find_vma()
256 * On no-MMU, shared futexes are treated as private, therefore
257 * we must not include the current process in the key. Since
258 * there is only one address space, the address is a unique key
261 if (IS_ENABLED(CONFIG_MMU))
262 key->private.mm = mm;
264 key->private.mm = NULL;
266 key->private.address = address;
271 /* Ignore any VERIFY_READ mapping (futex common case) */
272 if (unlikely(should_fail_futex(true)))
275 err = get_user_pages_fast(address, 1, FOLL_WRITE, &page);
277 * If write access is not required (eg. FUTEX_WAIT), try
278 * and get read-only access.
280 if (err == -EFAULT && rw == FUTEX_READ) {
281 err = get_user_pages_fast(address, 1, 0, &page);
290 * The treatment of mapping from this point on is critical. The folio
291 * lock protects many things but in this context the folio lock
292 * stabilizes mapping, prevents inode freeing in the shared
293 * file-backed region case and guards against movement to swap cache.
295 * Strictly speaking the folio lock is not needed in all cases being
296 * considered here and folio lock forces unnecessarily serialization.
297 * From this point on, mapping will be re-verified if necessary and
298 * folio lock will be acquired only if it is unavoidable
300 * Mapping checks require the folio so it is looked up now. For
301 * anonymous pages, it does not matter if the folio is split
302 * in the future as the key is based on the address. For
303 * filesystem-backed pages, the precise page is required as the
304 * index of the page determines the key.
306 folio = page_folio(page);
307 mapping = READ_ONCE(folio->mapping);
310 * If folio->mapping is NULL, then it cannot be an anonymous
311 * page; but it might be the ZERO_PAGE or in the gate area or
312 * in a special mapping (all cases which we are happy to fail);
313 * or it may have been a good file page when get_user_pages_fast
314 * found it, but truncated or holepunched or subjected to
315 * invalidate_complete_page2 before we got the folio lock (also
316 * cases which we are happy to fail). And we hold a reference,
317 * so refcount care in invalidate_inode_page's remove_mapping
318 * prevents drop_caches from setting mapping to NULL beneath us.
320 * The case we do have to guard against is when memory pressure made
321 * shmem_writepage move it from filecache to swapcache beneath us:
322 * an unlikely race, but we do need to retry for folio->mapping.
324 if (unlikely(!mapping)) {
328 * Folio lock is required to identify which special case above
329 * applies. If this is really a shmem page then the folio lock
330 * will prevent unexpected transitions.
333 shmem_swizzled = folio_test_swapcache(folio) || folio->mapping;
344 * Private mappings are handled in a simple way.
346 * If the futex key is stored in anonymous memory, then the associated
347 * object is the mm which is implicitly pinned by the calling process.
349 * NOTE: When userspace waits on a MAP_SHARED mapping, even if
350 * it's a read-only handle, it's expected that futexes attach to
351 * the object not the particular process.
353 if (folio_test_anon(folio)) {
355 * A RO anonymous page will never change and thus doesn't make
356 * sense for futex operations.
358 if (unlikely(should_fail_futex(true)) || ro) {
363 key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
364 key->private.mm = mm;
365 key->private.address = address;
371 * The associated futex object in this case is the inode and
372 * the folio->mapping must be traversed. Ordinarily this should
373 * be stabilised under folio lock but it's not strictly
374 * necessary in this case as we just want to pin the inode, not
375 * update i_pages or anything like that.
377 * The RCU read lock is taken as the inode is finally freed
378 * under RCU. If the mapping still matches expectations then the
379 * mapping->host can be safely accessed as being a valid inode.
383 if (READ_ONCE(folio->mapping) != mapping) {
390 inode = READ_ONCE(mapping->host);
398 key->both.offset |= FUT_OFF_INODE; /* inode-based key */
399 key->shared.i_seq = get_inode_sequence_number(inode);
400 key->shared.pgoff = folio->index + folio_page_idx(folio, page);
410 * fault_in_user_writeable() - Fault in user address and verify RW access
411 * @uaddr: pointer to faulting user space address
413 * Slow path to fixup the fault we just took in the atomic write
416 * We have no generic implementation of a non-destructive write to the
417 * user address. We know that we faulted in the atomic pagefault
418 * disabled section so we can as well avoid the #PF overhead by
419 * calling get_user_pages() right away.
421 int fault_in_user_writeable(u32 __user *uaddr)
423 struct mm_struct *mm = current->mm;
427 ret = fixup_user_fault(mm, (unsigned long)uaddr,
428 FAULT_FLAG_WRITE, NULL);
429 mmap_read_unlock(mm);
431 return ret < 0 ? ret : 0;
435 * futex_top_waiter() - Return the highest priority waiter on a futex
436 * @hb: the hash bucket the futex_q's reside in
437 * @key: the futex key (to distinguish it from other futex futex_q's)
439 * Must be called with the hb lock held.
441 struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb, union futex_key *key)
443 struct futex_q *this;
445 plist_for_each_entry(this, &hb->chain, list) {
446 if (futex_match(&this->key, key))
452 int futex_cmpxchg_value_locked(u32 *curval, u32 __user *uaddr, u32 uval, u32 newval)
457 ret = futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval);
463 int futex_get_value_locked(u32 *dest, u32 __user *from)
468 ret = __get_user(*dest, from);
471 return ret ? -EFAULT : 0;
475 * wait_for_owner_exiting - Block until the owner has exited
476 * @ret: owner's current futex lock status
477 * @exiting: Pointer to the exiting task
479 * Caller must hold a refcount on @exiting.
481 void wait_for_owner_exiting(int ret, struct task_struct *exiting)
484 WARN_ON_ONCE(exiting);
488 if (WARN_ON_ONCE(ret == -EBUSY && !exiting))
491 mutex_lock(&exiting->futex_exit_mutex);
493 * No point in doing state checking here. If the waiter got here
494 * while the task was in exec()->exec_futex_release() then it can
495 * have any FUTEX_STATE_* value when the waiter has acquired the
496 * mutex. OK, if running, EXITING or DEAD if it reached exit()
497 * already. Highly unlikely and not a problem. Just one more round
498 * through the futex maze.
500 mutex_unlock(&exiting->futex_exit_mutex);
502 put_task_struct(exiting);
506 * __futex_unqueue() - Remove the futex_q from its futex_hash_bucket
507 * @q: The futex_q to unqueue
509 * The q->lock_ptr must not be NULL and must be held by the caller.
511 void __futex_unqueue(struct futex_q *q)
513 struct futex_hash_bucket *hb;
515 if (WARN_ON_SMP(!q->lock_ptr) || WARN_ON(plist_node_empty(&q->list)))
517 lockdep_assert_held(q->lock_ptr);
519 hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock);
520 plist_del(&q->list, &hb->chain);
521 futex_hb_waiters_dec(hb);
524 /* The key must be already stored in q->key. */
525 struct futex_hash_bucket *futex_q_lock(struct futex_q *q)
526 __acquires(&hb->lock)
528 struct futex_hash_bucket *hb;
530 hb = futex_hash(&q->key);
533 * Increment the counter before taking the lock so that
534 * a potential waker won't miss a to-be-slept task that is
535 * waiting for the spinlock. This is safe as all futex_q_lock()
536 * users end up calling futex_queue(). Similarly, for housekeeping,
537 * decrement the counter at futex_q_unlock() when some error has
538 * occurred and we don't end up adding the task to the list.
540 futex_hb_waiters_inc(hb); /* implies smp_mb(); (A) */
542 q->lock_ptr = &hb->lock;
544 spin_lock(&hb->lock);
548 void futex_q_unlock(struct futex_hash_bucket *hb)
549 __releases(&hb->lock)
551 spin_unlock(&hb->lock);
552 futex_hb_waiters_dec(hb);
555 void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)
560 * The priority used to register this element is
561 * - either the real thread-priority for the real-time threads
562 * (i.e. threads with a priority lower than MAX_RT_PRIO)
563 * - or MAX_RT_PRIO for non-RT threads.
564 * Thus, all RT-threads are woken first in priority order, and
565 * the others are woken last, in FIFO order.
567 prio = min(current->normal_prio, MAX_RT_PRIO);
569 plist_node_init(&q->list, prio);
570 plist_add(&q->list, &hb->chain);
575 * futex_unqueue() - Remove the futex_q from its futex_hash_bucket
576 * @q: The futex_q to unqueue
578 * The q->lock_ptr must not be held by the caller. A call to futex_unqueue() must
579 * be paired with exactly one earlier call to futex_queue().
582 * - 1 - if the futex_q was still queued (and we removed unqueued it);
583 * - 0 - if the futex_q was already removed by the waking thread
585 int futex_unqueue(struct futex_q *q)
587 spinlock_t *lock_ptr;
590 /* In the common case we don't take the spinlock, which is nice. */
593 * q->lock_ptr can change between this read and the following spin_lock.
594 * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
595 * optimizing lock_ptr out of the logic below.
597 lock_ptr = READ_ONCE(q->lock_ptr);
598 if (lock_ptr != NULL) {
601 * q->lock_ptr can change between reading it and
602 * spin_lock(), causing us to take the wrong lock. This
603 * corrects the race condition.
605 * Reasoning goes like this: if we have the wrong lock,
606 * q->lock_ptr must have changed (maybe several times)
607 * between reading it and the spin_lock(). It can
608 * change again after the spin_lock() but only if it was
609 * already changed before the spin_lock(). It cannot,
610 * however, change back to the original value. Therefore
611 * we can detect whether we acquired the correct lock.
613 if (unlikely(lock_ptr != q->lock_ptr)) {
614 spin_unlock(lock_ptr);
621 spin_unlock(lock_ptr);
629 * PI futexes can not be requeued and must remove themselves from the
630 * hash bucket. The hash bucket lock (i.e. lock_ptr) is held.
632 void futex_unqueue_pi(struct futex_q *q)
636 BUG_ON(!q->pi_state);
637 put_pi_state(q->pi_state);
641 /* Constants for the pending_op argument of handle_futex_death */
642 #define HANDLE_DEATH_PENDING true
643 #define HANDLE_DEATH_LIST false
646 * Process a futex-list entry, check whether it's owned by the
647 * dying task, and do notification if so:
649 static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
650 bool pi, bool pending_op)
652 u32 uval, nval, mval;
656 /* Futex address must be 32bit aligned */
657 if ((((unsigned long)uaddr) % sizeof(*uaddr)) != 0)
661 if (get_user(uval, uaddr))
665 * Special case for regular (non PI) futexes. The unlock path in
666 * user space has two race scenarios:
668 * 1. The unlock path releases the user space futex value and
669 * before it can execute the futex() syscall to wake up
670 * waiters it is killed.
672 * 2. A woken up waiter is killed before it can acquire the
673 * futex in user space.
675 * In the second case, the wake up notification could be generated
676 * by the unlock path in user space after setting the futex value
677 * to zero or by the kernel after setting the OWNER_DIED bit below.
679 * In both cases the TID validation below prevents a wakeup of
680 * potential waiters which can cause these waiters to block
683 * In both cases the following conditions are met:
685 * 1) task->robust_list->list_op_pending != NULL
686 * @pending_op == true
687 * 2) The owner part of user space futex value == 0
688 * 3) Regular futex: @pi == false
690 * If these conditions are met, it is safe to attempt waking up a
691 * potential waiter without touching the user space futex value and
692 * trying to set the OWNER_DIED bit. If the futex value is zero,
693 * the rest of the user space mutex state is consistent, so a woken
694 * waiter will just take over the uncontended futex. Setting the
695 * OWNER_DIED bit would create inconsistent state and malfunction
696 * of the user space owner died handling. Otherwise, the OWNER_DIED
697 * bit is already set, and the woken waiter is expected to deal with
700 owner = uval & FUTEX_TID_MASK;
702 if (pending_op && !pi && !owner) {
703 futex_wake(uaddr, FLAGS_SIZE_32 | FLAGS_SHARED, 1,
704 FUTEX_BITSET_MATCH_ANY);
708 if (owner != task_pid_vnr(curr))
712 * Ok, this dying thread is truly holding a futex
713 * of interest. Set the OWNER_DIED bit atomically
714 * via cmpxchg, and if the value had FUTEX_WAITERS
715 * set, wake up a waiter (if any). (We have to do a
716 * futex_wake() even if OWNER_DIED is already set -
717 * to handle the rare but possible case of recursive
718 * thread-death.) The rest of the cleanup is done in
721 mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
724 * We are not holding a lock here, but we want to have
725 * the pagefault_disable/enable() protection because
726 * we want to handle the fault gracefully. If the
727 * access fails we try to fault in the futex with R/W
728 * verification via get_user_pages. get_user() above
729 * does not guarantee R/W access. If that fails we
730 * give up and leave the futex locked.
732 if ((err = futex_cmpxchg_value_locked(&nval, uaddr, uval, mval))) {
735 if (fault_in_user_writeable(uaddr))
753 * Wake robust non-PI futexes here. The wakeup of
754 * PI futexes happens in exit_pi_state():
756 if (!pi && (uval & FUTEX_WAITERS)) {
757 futex_wake(uaddr, FLAGS_SIZE_32 | FLAGS_SHARED, 1,
758 FUTEX_BITSET_MATCH_ANY);
765 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
767 static inline int fetch_robust_entry(struct robust_list __user **entry,
768 struct robust_list __user * __user *head,
771 unsigned long uentry;
773 if (get_user(uentry, (unsigned long __user *)head))
776 *entry = (void __user *)(uentry & ~1UL);
783 * Walk curr->robust_list (very carefully, it's a userspace list!)
784 * and mark any locks found there dead, and notify any waiters.
786 * We silently return on any sign of list-walking problem.
788 static void exit_robust_list(struct task_struct *curr)
790 struct robust_list_head __user *head = curr->robust_list;
791 struct robust_list __user *entry, *next_entry, *pending;
792 unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
793 unsigned int next_pi;
794 unsigned long futex_offset;
798 * Fetch the list head (which was registered earlier, via
799 * sys_set_robust_list()):
801 if (fetch_robust_entry(&entry, &head->list.next, &pi))
804 * Fetch the relative futex offset:
806 if (get_user(futex_offset, &head->futex_offset))
809 * Fetch any possibly pending lock-add first, and handle it
812 if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
815 next_entry = NULL; /* avoid warning with gcc */
816 while (entry != &head->list) {
818 * Fetch the next entry in the list before calling
819 * handle_futex_death:
821 rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
823 * A pending lock might already be on the list, so
824 * don't process it twice:
826 if (entry != pending) {
827 if (handle_futex_death((void __user *)entry + futex_offset,
828 curr, pi, HANDLE_DEATH_LIST))
836 * Avoid excessively long or circular lists:
845 handle_futex_death((void __user *)pending + futex_offset,
846 curr, pip, HANDLE_DEATH_PENDING);
851 static void __user *futex_uaddr(struct robust_list __user *entry,
852 compat_long_t futex_offset)
854 compat_uptr_t base = ptr_to_compat(entry);
855 void __user *uaddr = compat_ptr(base + futex_offset);
861 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
864 compat_fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
865 compat_uptr_t __user *head, unsigned int *pi)
867 if (get_user(*uentry, head))
870 *entry = compat_ptr((*uentry) & ~1);
871 *pi = (unsigned int)(*uentry) & 1;
877 * Walk curr->robust_list (very carefully, it's a userspace list!)
878 * and mark any locks found there dead, and notify any waiters.
880 * We silently return on any sign of list-walking problem.
882 static void compat_exit_robust_list(struct task_struct *curr)
884 struct compat_robust_list_head __user *head = curr->compat_robust_list;
885 struct robust_list __user *entry, *next_entry, *pending;
886 unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
887 unsigned int next_pi;
888 compat_uptr_t uentry, next_uentry, upending;
889 compat_long_t futex_offset;
893 * Fetch the list head (which was registered earlier, via
894 * sys_set_robust_list()):
896 if (compat_fetch_robust_entry(&uentry, &entry, &head->list.next, &pi))
899 * Fetch the relative futex offset:
901 if (get_user(futex_offset, &head->futex_offset))
904 * Fetch any possibly pending lock-add first, and handle it
907 if (compat_fetch_robust_entry(&upending, &pending,
908 &head->list_op_pending, &pip))
911 next_entry = NULL; /* avoid warning with gcc */
912 while (entry != (struct robust_list __user *) &head->list) {
914 * Fetch the next entry in the list before calling
915 * handle_futex_death:
917 rc = compat_fetch_robust_entry(&next_uentry, &next_entry,
918 (compat_uptr_t __user *)&entry->next, &next_pi);
920 * A pending lock might already be on the list, so
921 * dont process it twice:
923 if (entry != pending) {
924 void __user *uaddr = futex_uaddr(entry, futex_offset);
926 if (handle_futex_death(uaddr, curr, pi,
932 uentry = next_uentry;
936 * Avoid excessively long or circular lists:
944 void __user *uaddr = futex_uaddr(pending, futex_offset);
946 handle_futex_death(uaddr, curr, pip, HANDLE_DEATH_PENDING);
951 #ifdef CONFIG_FUTEX_PI
954 * This task is holding PI mutexes at exit time => bad.
955 * Kernel cleans up PI-state, but userspace is likely hosed.
956 * (Robust-futex cleanup is separate and might save the day for userspace.)
958 static void exit_pi_state_list(struct task_struct *curr)
960 struct list_head *next, *head = &curr->pi_state_list;
961 struct futex_pi_state *pi_state;
962 struct futex_hash_bucket *hb;
963 union futex_key key = FUTEX_KEY_INIT;
966 * We are a ZOMBIE and nobody can enqueue itself on
967 * pi_state_list anymore, but we have to be careful
968 * versus waiters unqueueing themselves:
970 raw_spin_lock_irq(&curr->pi_lock);
971 while (!list_empty(head)) {
973 pi_state = list_entry(next, struct futex_pi_state, list);
975 hb = futex_hash(&key);
978 * We can race against put_pi_state() removing itself from the
979 * list (a waiter going away). put_pi_state() will first
980 * decrement the reference count and then modify the list, so
981 * its possible to see the list entry but fail this reference
984 * In that case; drop the locks to let put_pi_state() make
985 * progress and retry the loop.
987 if (!refcount_inc_not_zero(&pi_state->refcount)) {
988 raw_spin_unlock_irq(&curr->pi_lock);
990 raw_spin_lock_irq(&curr->pi_lock);
993 raw_spin_unlock_irq(&curr->pi_lock);
995 spin_lock(&hb->lock);
996 raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
997 raw_spin_lock(&curr->pi_lock);
999 * We dropped the pi-lock, so re-check whether this
1000 * task still owns the PI-state:
1002 if (head->next != next) {
1003 /* retain curr->pi_lock for the loop invariant */
1004 raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
1005 spin_unlock(&hb->lock);
1006 put_pi_state(pi_state);
1010 WARN_ON(pi_state->owner != curr);
1011 WARN_ON(list_empty(&pi_state->list));
1012 list_del_init(&pi_state->list);
1013 pi_state->owner = NULL;
1015 raw_spin_unlock(&curr->pi_lock);
1016 raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
1017 spin_unlock(&hb->lock);
1019 rt_mutex_futex_unlock(&pi_state->pi_mutex);
1020 put_pi_state(pi_state);
1022 raw_spin_lock_irq(&curr->pi_lock);
1024 raw_spin_unlock_irq(&curr->pi_lock);
1027 static inline void exit_pi_state_list(struct task_struct *curr) { }
1030 static void futex_cleanup(struct task_struct *tsk)
1032 if (unlikely(tsk->robust_list)) {
1033 exit_robust_list(tsk);
1034 tsk->robust_list = NULL;
1037 #ifdef CONFIG_COMPAT
1038 if (unlikely(tsk->compat_robust_list)) {
1039 compat_exit_robust_list(tsk);
1040 tsk->compat_robust_list = NULL;
1044 if (unlikely(!list_empty(&tsk->pi_state_list)))
1045 exit_pi_state_list(tsk);
1049 * futex_exit_recursive - Set the tasks futex state to FUTEX_STATE_DEAD
1050 * @tsk: task to set the state on
1052 * Set the futex exit state of the task lockless. The futex waiter code
1053 * observes that state when a task is exiting and loops until the task has
1054 * actually finished the futex cleanup. The worst case for this is that the
1055 * waiter runs through the wait loop until the state becomes visible.
1057 * This is called from the recursive fault handling path in make_task_dead().
1059 * This is best effort. Either the futex exit code has run already or
1060 * not. If the OWNER_DIED bit has been set on the futex then the waiter can
1061 * take it over. If not, the problem is pushed back to user space. If the
1062 * futex exit code did not run yet, then an already queued waiter might
1063 * block forever, but there is nothing which can be done about that.
1065 void futex_exit_recursive(struct task_struct *tsk)
1067 /* If the state is FUTEX_STATE_EXITING then futex_exit_mutex is held */
1068 if (tsk->futex_state == FUTEX_STATE_EXITING)
1069 mutex_unlock(&tsk->futex_exit_mutex);
1070 tsk->futex_state = FUTEX_STATE_DEAD;
1073 static void futex_cleanup_begin(struct task_struct *tsk)
1076 * Prevent various race issues against a concurrent incoming waiter
1077 * including live locks by forcing the waiter to block on
1078 * tsk->futex_exit_mutex when it observes FUTEX_STATE_EXITING in
1079 * attach_to_pi_owner().
1081 mutex_lock(&tsk->futex_exit_mutex);
1084 * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
1086 * This ensures that all subsequent checks of tsk->futex_state in
1087 * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
1088 * tsk->pi_lock held.
1090 * It guarantees also that a pi_state which was queued right before
1091 * the state change under tsk->pi_lock by a concurrent waiter must
1092 * be observed in exit_pi_state_list().
1094 raw_spin_lock_irq(&tsk->pi_lock);
1095 tsk->futex_state = FUTEX_STATE_EXITING;
1096 raw_spin_unlock_irq(&tsk->pi_lock);
1099 static void futex_cleanup_end(struct task_struct *tsk, int state)
1102 * Lockless store. The only side effect is that an observer might
1103 * take another loop until it becomes visible.
1105 tsk->futex_state = state;
1107 * Drop the exit protection. This unblocks waiters which observed
1108 * FUTEX_STATE_EXITING to reevaluate the state.
1110 mutex_unlock(&tsk->futex_exit_mutex);
1113 void futex_exec_release(struct task_struct *tsk)
1116 * The state handling is done for consistency, but in the case of
1117 * exec() there is no way to prevent further damage as the PID stays
1118 * the same. But for the unlikely and arguably buggy case that a
1119 * futex is held on exec(), this provides at least as much state
1120 * consistency protection which is possible.
1122 futex_cleanup_begin(tsk);
1125 * Reset the state to FUTEX_STATE_OK. The task is alive and about
1126 * exec a new binary.
1128 futex_cleanup_end(tsk, FUTEX_STATE_OK);
1131 void futex_exit_release(struct task_struct *tsk)
1133 futex_cleanup_begin(tsk);
1135 futex_cleanup_end(tsk, FUTEX_STATE_DEAD);
1138 static int __init futex_init(void)
1140 unsigned int futex_shift;
1143 #if CONFIG_BASE_SMALL
1144 futex_hashsize = 16;
1146 futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
1149 futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
1150 futex_hashsize, 0, 0,
1152 futex_hashsize, futex_hashsize);
1153 futex_hashsize = 1UL << futex_shift;
1155 for (i = 0; i < futex_hashsize; i++) {
1156 atomic_set(&futex_queues[i].waiters, 0);
1157 plist_head_init(&futex_queues[i].chain);
1158 spin_lock_init(&futex_queues[i].lock);
1163 core_initcall(futex_init);