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TOMOYO Linux Cross Reference
Linux/kernel/locking/mutex.c

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  1 /*
  2  * kernel/locking/mutex.c
  3  *
  4  * Mutexes: blocking mutual exclusion locks
  5  *
  6  * Started by Ingo Molnar:
  7  *
  8  *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
  9  *
 10  * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and
 11  * David Howells for suggestions and improvements.
 12  *
 13  *  - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline
 14  *    from the -rt tree, where it was originally implemented for rtmutexes
 15  *    by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale
 16  *    and Sven Dietrich.
 17  *
 18  * Also see Documentation/locking/mutex-design.txt.
 19  */
 20 #include <linux/mutex.h>
 21 #include <linux/ww_mutex.h>
 22 #include <linux/sched/signal.h>
 23 #include <linux/sched/rt.h>
 24 #include <linux/sched/wake_q.h>
 25 #include <linux/sched/debug.h>
 26 #include <linux/export.h>
 27 #include <linux/spinlock.h>
 28 #include <linux/interrupt.h>
 29 #include <linux/debug_locks.h>
 30 #include <linux/osq_lock.h>
 31 
 32 #ifdef CONFIG_DEBUG_MUTEXES
 33 # include "mutex-debug.h"
 34 #else
 35 # include "mutex.h"
 36 #endif
 37 
 38 void
 39 __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
 40 {
 41         atomic_long_set(&lock->owner, 0);
 42         spin_lock_init(&lock->wait_lock);
 43         INIT_LIST_HEAD(&lock->wait_list);
 44 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
 45         osq_lock_init(&lock->osq);
 46 #endif
 47 
 48         debug_mutex_init(lock, name, key);
 49 }
 50 EXPORT_SYMBOL(__mutex_init);
 51 
 52 /*
 53  * @owner: contains: 'struct task_struct *' to the current lock owner,
 54  * NULL means not owned. Since task_struct pointers are aligned at
 55  * at least L1_CACHE_BYTES, we have low bits to store extra state.
 56  *
 57  * Bit0 indicates a non-empty waiter list; unlock must issue a wakeup.
 58  * Bit1 indicates unlock needs to hand the lock to the top-waiter
 59  * Bit2 indicates handoff has been done and we're waiting for pickup.
 60  */
 61 #define MUTEX_FLAG_WAITERS      0x01
 62 #define MUTEX_FLAG_HANDOFF      0x02
 63 #define MUTEX_FLAG_PICKUP       0x04
 64 
 65 #define MUTEX_FLAGS             0x07
 66 
 67 static inline struct task_struct *__owner_task(unsigned long owner)
 68 {
 69         return (struct task_struct *)(owner & ~MUTEX_FLAGS);
 70 }
 71 
 72 static inline unsigned long __owner_flags(unsigned long owner)
 73 {
 74         return owner & MUTEX_FLAGS;
 75 }
 76 
 77 /*
 78  * Trylock variant that retuns the owning task on failure.
 79  */
 80 static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock)
 81 {
 82         unsigned long owner, curr = (unsigned long)current;
 83 
 84         owner = atomic_long_read(&lock->owner);
 85         for (;;) { /* must loop, can race against a flag */
 86                 unsigned long old, flags = __owner_flags(owner);
 87                 unsigned long task = owner & ~MUTEX_FLAGS;
 88 
 89                 if (task) {
 90                         if (likely(task != curr))
 91                                 break;
 92 
 93                         if (likely(!(flags & MUTEX_FLAG_PICKUP)))
 94                                 break;
 95 
 96                         flags &= ~MUTEX_FLAG_PICKUP;
 97                 } else {
 98 #ifdef CONFIG_DEBUG_MUTEXES
 99                         DEBUG_LOCKS_WARN_ON(flags & MUTEX_FLAG_PICKUP);
100 #endif
101                 }
102 
103                 /*
104                  * We set the HANDOFF bit, we must make sure it doesn't live
105                  * past the point where we acquire it. This would be possible
106                  * if we (accidentally) set the bit on an unlocked mutex.
107                  */
108                 flags &= ~MUTEX_FLAG_HANDOFF;
109 
110                 old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);
111                 if (old == owner)
112                         return NULL;
113 
114                 owner = old;
115         }
116 
117         return __owner_task(owner);
118 }
119 
120 /*
121  * Actual trylock that will work on any unlocked state.
122  */
123 static inline bool __mutex_trylock(struct mutex *lock)
124 {
125         return !__mutex_trylock_or_owner(lock);
126 }
127 
128 #ifndef CONFIG_DEBUG_LOCK_ALLOC
129 /*
130  * Lockdep annotations are contained to the slow paths for simplicity.
131  * There is nothing that would stop spreading the lockdep annotations outwards
132  * except more code.
133  */
134 
135 /*
136  * Optimistic trylock that only works in the uncontended case. Make sure to
137  * follow with a __mutex_trylock() before failing.
138  */
139 static __always_inline bool __mutex_trylock_fast(struct mutex *lock)
140 {
141         unsigned long curr = (unsigned long)current;
142 
143         if (!atomic_long_cmpxchg_acquire(&lock->owner, 0UL, curr))
144                 return true;
145 
146         return false;
147 }
148 
149 static __always_inline bool __mutex_unlock_fast(struct mutex *lock)
150 {
151         unsigned long curr = (unsigned long)current;
152 
153         if (atomic_long_cmpxchg_release(&lock->owner, curr, 0UL) == curr)
154                 return true;
155 
156         return false;
157 }
158 #endif
159 
160 static inline void __mutex_set_flag(struct mutex *lock, unsigned long flag)
161 {
162         atomic_long_or(flag, &lock->owner);
163 }
164 
165 static inline void __mutex_clear_flag(struct mutex *lock, unsigned long flag)
166 {
167         atomic_long_andnot(flag, &lock->owner);
168 }
169 
170 static inline bool __mutex_waiter_is_first(struct mutex *lock, struct mutex_waiter *waiter)
171 {
172         return list_first_entry(&lock->wait_list, struct mutex_waiter, list) == waiter;
173 }
174 
175 /*
176  * Give up ownership to a specific task, when @task = NULL, this is equivalent
177  * to a regular unlock. Sets PICKUP on a handoff, clears HANDOF, preserves
178  * WAITERS. Provides RELEASE semantics like a regular unlock, the
179  * __mutex_trylock() provides a matching ACQUIRE semantics for the handoff.
180  */
181 static void __mutex_handoff(struct mutex *lock, struct task_struct *task)
182 {
183         unsigned long owner = atomic_long_read(&lock->owner);
184 
185         for (;;) {
186                 unsigned long old, new;
187 
188 #ifdef CONFIG_DEBUG_MUTEXES
189                 DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
190                 DEBUG_LOCKS_WARN_ON(owner & MUTEX_FLAG_PICKUP);
191 #endif
192 
193                 new = (owner & MUTEX_FLAG_WAITERS);
194                 new |= (unsigned long)task;
195                 if (task)
196                         new |= MUTEX_FLAG_PICKUP;
197 
198                 old = atomic_long_cmpxchg_release(&lock->owner, owner, new);
199                 if (old == owner)
200                         break;
201 
202                 owner = old;
203         }
204 }
205 
206 #ifndef CONFIG_DEBUG_LOCK_ALLOC
207 /*
208  * We split the mutex lock/unlock logic into separate fastpath and
209  * slowpath functions, to reduce the register pressure on the fastpath.
210  * We also put the fastpath first in the kernel image, to make sure the
211  * branch is predicted by the CPU as default-untaken.
212  */
213 static void __sched __mutex_lock_slowpath(struct mutex *lock);
214 
215 /**
216  * mutex_lock - acquire the mutex
217  * @lock: the mutex to be acquired
218  *
219  * Lock the mutex exclusively for this task. If the mutex is not
220  * available right now, it will sleep until it can get it.
221  *
222  * The mutex must later on be released by the same task that
223  * acquired it. Recursive locking is not allowed. The task
224  * may not exit without first unlocking the mutex. Also, kernel
225  * memory where the mutex resides must not be freed with
226  * the mutex still locked. The mutex must first be initialized
227  * (or statically defined) before it can be locked. memset()-ing
228  * the mutex to 0 is not allowed.
229  *
230  * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
231  *   checks that will enforce the restrictions and will also do
232  *   deadlock debugging. )
233  *
234  * This function is similar to (but not equivalent to) down().
235  */
236 void __sched mutex_lock(struct mutex *lock)
237 {
238         might_sleep();
239 
240         if (!__mutex_trylock_fast(lock))
241                 __mutex_lock_slowpath(lock);
242 }
243 EXPORT_SYMBOL(mutex_lock);
244 #endif
245 
246 static __always_inline void
247 ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
248 {
249 #ifdef CONFIG_DEBUG_MUTEXES
250         /*
251          * If this WARN_ON triggers, you used ww_mutex_lock to acquire,
252          * but released with a normal mutex_unlock in this call.
253          *
254          * This should never happen, always use ww_mutex_unlock.
255          */
256         DEBUG_LOCKS_WARN_ON(ww->ctx);
257 
258         /*
259          * Not quite done after calling ww_acquire_done() ?
260          */
261         DEBUG_LOCKS_WARN_ON(ww_ctx->done_acquire);
262 
263         if (ww_ctx->contending_lock) {
264                 /*
265                  * After -EDEADLK you tried to
266                  * acquire a different ww_mutex? Bad!
267                  */
268                 DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock != ww);
269 
270                 /*
271                  * You called ww_mutex_lock after receiving -EDEADLK,
272                  * but 'forgot' to unlock everything else first?
273                  */
274                 DEBUG_LOCKS_WARN_ON(ww_ctx->acquired > 0);
275                 ww_ctx->contending_lock = NULL;
276         }
277 
278         /*
279          * Naughty, using a different class will lead to undefined behavior!
280          */
281         DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class);
282 #endif
283         ww_ctx->acquired++;
284 }
285 
286 static inline bool __sched
287 __ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
288 {
289         return a->stamp - b->stamp <= LONG_MAX &&
290                (a->stamp != b->stamp || a > b);
291 }
292 
293 /*
294  * Wake up any waiters that may have to back off when the lock is held by the
295  * given context.
296  *
297  * Due to the invariants on the wait list, this can only affect the first
298  * waiter with a context.
299  *
300  * The current task must not be on the wait list.
301  */
302 static void __sched
303 __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
304 {
305         struct mutex_waiter *cur;
306 
307         lockdep_assert_held(&lock->wait_lock);
308 
309         list_for_each_entry(cur, &lock->wait_list, list) {
310                 if (!cur->ww_ctx)
311                         continue;
312 
313                 if (cur->ww_ctx->acquired > 0 &&
314                     __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {
315                         debug_mutex_wake_waiter(lock, cur);
316                         wake_up_process(cur->task);
317                 }
318 
319                 break;
320         }
321 }
322 
323 /*
324  * After acquiring lock with fastpath or when we lost out in contested
325  * slowpath, set ctx and wake up any waiters so they can recheck.
326  */
327 static __always_inline void
328 ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
329 {
330         ww_mutex_lock_acquired(lock, ctx);
331 
332         lock->ctx = ctx;
333 
334         /*
335          * The lock->ctx update should be visible on all cores before
336          * the atomic read is done, otherwise contended waiters might be
337          * missed. The contended waiters will either see ww_ctx == NULL
338          * and keep spinning, or it will acquire wait_lock, add itself
339          * to waiter list and sleep.
340          */
341         smp_mb(); /* ^^^ */
342 
343         /*
344          * Check if lock is contended, if not there is nobody to wake up
345          */
346         if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAITERS)))
347                 return;
348 
349         /*
350          * Uh oh, we raced in fastpath, wake up everyone in this case,
351          * so they can see the new lock->ctx.
352          */
353         spin_lock(&lock->base.wait_lock);
354         __ww_mutex_wakeup_for_backoff(&lock->base, ctx);
355         spin_unlock(&lock->base.wait_lock);
356 }
357 
358 /*
359  * After acquiring lock in the slowpath set ctx.
360  *
361  * Unlike for the fast path, the caller ensures that waiters are woken up where
362  * necessary.
363  *
364  * Callers must hold the mutex wait_lock.
365  */
366 static __always_inline void
367 ww_mutex_set_context_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
368 {
369         ww_mutex_lock_acquired(lock, ctx);
370         lock->ctx = ctx;
371 }
372 
373 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
374 
375 static inline
376 bool ww_mutex_spin_on_owner(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
377                             struct mutex_waiter *waiter)
378 {
379         struct ww_mutex *ww;
380 
381         ww = container_of(lock, struct ww_mutex, base);
382 
383         /*
384          * If ww->ctx is set the contents are undefined, only
385          * by acquiring wait_lock there is a guarantee that
386          * they are not invalid when reading.
387          *
388          * As such, when deadlock detection needs to be
389          * performed the optimistic spinning cannot be done.
390          *
391          * Check this in every inner iteration because we may
392          * be racing against another thread's ww_mutex_lock.
393          */
394         if (ww_ctx->acquired > 0 && READ_ONCE(ww->ctx))
395                 return false;
396 
397         /*
398          * If we aren't on the wait list yet, cancel the spin
399          * if there are waiters. We want  to avoid stealing the
400          * lock from a waiter with an earlier stamp, since the
401          * other thread may already own a lock that we also
402          * need.
403          */
404         if (!waiter && (atomic_long_read(&lock->owner) & MUTEX_FLAG_WAITERS))
405                 return false;
406 
407         /*
408          * Similarly, stop spinning if we are no longer the
409          * first waiter.
410          */
411         if (waiter && !__mutex_waiter_is_first(lock, waiter))
412                 return false;
413 
414         return true;
415 }
416 
417 /*
418  * Look out! "owner" is an entirely speculative pointer access and not
419  * reliable.
420  *
421  * "noinline" so that this function shows up on perf profiles.
422  */
423 static noinline
424 bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
425                          struct ww_acquire_ctx *ww_ctx, struct mutex_waiter *waiter)
426 {
427         bool ret = true;
428 
429         rcu_read_lock();
430         while (__mutex_owner(lock) == owner) {
431                 /*
432                  * Ensure we emit the owner->on_cpu, dereference _after_
433                  * checking lock->owner still matches owner. If that fails,
434                  * owner might point to freed memory. If it still matches,
435                  * the rcu_read_lock() ensures the memory stays valid.
436                  */
437                 barrier();
438 
439                 /*
440                  * Use vcpu_is_preempted to detect lock holder preemption issue.
441                  */
442                 if (!owner->on_cpu || need_resched() ||
443                                 vcpu_is_preempted(task_cpu(owner))) {
444                         ret = false;
445                         break;
446                 }
447 
448                 if (ww_ctx && !ww_mutex_spin_on_owner(lock, ww_ctx, waiter)) {
449                         ret = false;
450                         break;
451                 }
452 
453                 cpu_relax();
454         }
455         rcu_read_unlock();
456 
457         return ret;
458 }
459 
460 /*
461  * Initial check for entering the mutex spinning loop
462  */
463 static inline int mutex_can_spin_on_owner(struct mutex *lock)
464 {
465         struct task_struct *owner;
466         int retval = 1;
467 
468         if (need_resched())
469                 return 0;
470 
471         rcu_read_lock();
472         owner = __mutex_owner(lock);
473 
474         /*
475          * As lock holder preemption issue, we both skip spinning if task is not
476          * on cpu or its cpu is preempted
477          */
478         if (owner)
479                 retval = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
480         rcu_read_unlock();
481 
482         /*
483          * If lock->owner is not set, the mutex has been released. Return true
484          * such that we'll trylock in the spin path, which is a faster option
485          * than the blocking slow path.
486          */
487         return retval;
488 }
489 
490 /*
491  * Optimistic spinning.
492  *
493  * We try to spin for acquisition when we find that the lock owner
494  * is currently running on a (different) CPU and while we don't
495  * need to reschedule. The rationale is that if the lock owner is
496  * running, it is likely to release the lock soon.
497  *
498  * The mutex spinners are queued up using MCS lock so that only one
499  * spinner can compete for the mutex. However, if mutex spinning isn't
500  * going to happen, there is no point in going through the lock/unlock
501  * overhead.
502  *
503  * Returns true when the lock was taken, otherwise false, indicating
504  * that we need to jump to the slowpath and sleep.
505  *
506  * The waiter flag is set to true if the spinner is a waiter in the wait
507  * queue. The waiter-spinner will spin on the lock directly and concurrently
508  * with the spinner at the head of the OSQ, if present, until the owner is
509  * changed to itself.
510  */
511 static __always_inline bool
512 mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
513                       const bool use_ww_ctx, struct mutex_waiter *waiter)
514 {
515         if (!waiter) {
516                 /*
517                  * The purpose of the mutex_can_spin_on_owner() function is
518                  * to eliminate the overhead of osq_lock() and osq_unlock()
519                  * in case spinning isn't possible. As a waiter-spinner
520                  * is not going to take OSQ lock anyway, there is no need
521                  * to call mutex_can_spin_on_owner().
522                  */
523                 if (!mutex_can_spin_on_owner(lock))
524                         goto fail;
525 
526                 /*
527                  * In order to avoid a stampede of mutex spinners trying to
528                  * acquire the mutex all at once, the spinners need to take a
529                  * MCS (queued) lock first before spinning on the owner field.
530                  */
531                 if (!osq_lock(&lock->osq))
532                         goto fail;
533         }
534 
535         for (;;) {
536                 struct task_struct *owner;
537 
538                 /* Try to acquire the mutex... */
539                 owner = __mutex_trylock_or_owner(lock);
540                 if (!owner)
541                         break;
542 
543                 /*
544                  * There's an owner, wait for it to either
545                  * release the lock or go to sleep.
546                  */
547                 if (!mutex_spin_on_owner(lock, owner, ww_ctx, waiter))
548                         goto fail_unlock;
549 
550                 /*
551                  * The cpu_relax() call is a compiler barrier which forces
552                  * everything in this loop to be re-loaded. We don't need
553                  * memory barriers as we'll eventually observe the right
554                  * values at the cost of a few extra spins.
555                  */
556                 cpu_relax();
557         }
558 
559         if (!waiter)
560                 osq_unlock(&lock->osq);
561 
562         return true;
563 
564 
565 fail_unlock:
566         if (!waiter)
567                 osq_unlock(&lock->osq);
568 
569 fail:
570         /*
571          * If we fell out of the spin path because of need_resched(),
572          * reschedule now, before we try-lock the mutex. This avoids getting
573          * scheduled out right after we obtained the mutex.
574          */
575         if (need_resched()) {
576                 /*
577                  * We _should_ have TASK_RUNNING here, but just in case
578                  * we do not, make it so, otherwise we might get stuck.
579                  */
580                 __set_current_state(TASK_RUNNING);
581                 schedule_preempt_disabled();
582         }
583 
584         return false;
585 }
586 #else
587 static __always_inline bool
588 mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
589                       const bool use_ww_ctx, struct mutex_waiter *waiter)
590 {
591         return false;
592 }
593 #endif
594 
595 static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip);
596 
597 /**
598  * mutex_unlock - release the mutex
599  * @lock: the mutex to be released
600  *
601  * Unlock a mutex that has been locked by this task previously.
602  *
603  * This function must not be used in interrupt context. Unlocking
604  * of a not locked mutex is not allowed.
605  *
606  * This function is similar to (but not equivalent to) up().
607  */
608 void __sched mutex_unlock(struct mutex *lock)
609 {
610 #ifndef CONFIG_DEBUG_LOCK_ALLOC
611         if (__mutex_unlock_fast(lock))
612                 return;
613 #endif
614         __mutex_unlock_slowpath(lock, _RET_IP_);
615 }
616 EXPORT_SYMBOL(mutex_unlock);
617 
618 /**
619  * ww_mutex_unlock - release the w/w mutex
620  * @lock: the mutex to be released
621  *
622  * Unlock a mutex that has been locked by this task previously with any of the
623  * ww_mutex_lock* functions (with or without an acquire context). It is
624  * forbidden to release the locks after releasing the acquire context.
625  *
626  * This function must not be used in interrupt context. Unlocking
627  * of a unlocked mutex is not allowed.
628  */
629 void __sched ww_mutex_unlock(struct ww_mutex *lock)
630 {
631         /*
632          * The unlocking fastpath is the 0->1 transition from 'locked'
633          * into 'unlocked' state:
634          */
635         if (lock->ctx) {
636 #ifdef CONFIG_DEBUG_MUTEXES
637                 DEBUG_LOCKS_WARN_ON(!lock->ctx->acquired);
638 #endif
639                 if (lock->ctx->acquired > 0)
640                         lock->ctx->acquired--;
641                 lock->ctx = NULL;
642         }
643 
644         mutex_unlock(&lock->base);
645 }
646 EXPORT_SYMBOL(ww_mutex_unlock);
647 
648 static inline int __sched
649 __ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
650                             struct ww_acquire_ctx *ctx)
651 {
652         struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
653         struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);
654         struct mutex_waiter *cur;
655 
656         if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
657                 goto deadlock;
658 
659         /*
660          * If there is a waiter in front of us that has a context, then its
661          * stamp is earlier than ours and we must back off.
662          */
663         cur = waiter;
664         list_for_each_entry_continue_reverse(cur, &lock->wait_list, list) {
665                 if (cur->ww_ctx)
666                         goto deadlock;
667         }
668 
669         return 0;
670 
671 deadlock:
672 #ifdef CONFIG_DEBUG_MUTEXES
673         DEBUG_LOCKS_WARN_ON(ctx->contending_lock);
674         ctx->contending_lock = ww;
675 #endif
676         return -EDEADLK;
677 }
678 
679 static inline int __sched
680 __ww_mutex_add_waiter(struct mutex_waiter *waiter,
681                       struct mutex *lock,
682                       struct ww_acquire_ctx *ww_ctx)
683 {
684         struct mutex_waiter *cur;
685         struct list_head *pos;
686 
687         if (!ww_ctx) {
688                 list_add_tail(&waiter->list, &lock->wait_list);
689                 return 0;
690         }
691 
692         /*
693          * Add the waiter before the first waiter with a higher stamp.
694          * Waiters without a context are skipped to avoid starving
695          * them.
696          */
697         pos = &lock->wait_list;
698         list_for_each_entry_reverse(cur, &lock->wait_list, list) {
699                 if (!cur->ww_ctx)
700                         continue;
701 
702                 if (__ww_ctx_stamp_after(ww_ctx, cur->ww_ctx)) {
703                         /* Back off immediately if necessary. */
704                         if (ww_ctx->acquired > 0) {
705 #ifdef CONFIG_DEBUG_MUTEXES
706                                 struct ww_mutex *ww;
707 
708                                 ww = container_of(lock, struct ww_mutex, base);
709                                 DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
710                                 ww_ctx->contending_lock = ww;
711 #endif
712                                 return -EDEADLK;
713                         }
714 
715                         break;
716                 }
717 
718                 pos = &cur->list;
719 
720                 /*
721                  * Wake up the waiter so that it gets a chance to back
722                  * off.
723                  */
724                 if (cur->ww_ctx->acquired > 0) {
725                         debug_mutex_wake_waiter(lock, cur);
726                         wake_up_process(cur->task);
727                 }
728         }
729 
730         list_add_tail(&waiter->list, pos);
731         return 0;
732 }
733 
734 /*
735  * Lock a mutex (possibly interruptible), slowpath:
736  */
737 static __always_inline int __sched
738 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
739                     struct lockdep_map *nest_lock, unsigned long ip,
740                     struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
741 {
742         struct mutex_waiter waiter;
743         bool first = false;
744         struct ww_mutex *ww;
745         int ret;
746 
747         might_sleep();
748 
749         ww = container_of(lock, struct ww_mutex, base);
750         if (use_ww_ctx && ww_ctx) {
751                 if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
752                         return -EALREADY;
753         }
754 
755         preempt_disable();
756         mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
757 
758         if (__mutex_trylock(lock) ||
759             mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, NULL)) {
760                 /* got the lock, yay! */
761                 lock_acquired(&lock->dep_map, ip);
762                 if (use_ww_ctx && ww_ctx)
763                         ww_mutex_set_context_fastpath(ww, ww_ctx);
764                 preempt_enable();
765                 return 0;
766         }
767 
768         spin_lock(&lock->wait_lock);
769         /*
770          * After waiting to acquire the wait_lock, try again.
771          */
772         if (__mutex_trylock(lock)) {
773                 if (use_ww_ctx && ww_ctx)
774                         __ww_mutex_wakeup_for_backoff(lock, ww_ctx);
775 
776                 goto skip_wait;
777         }
778 
779         debug_mutex_lock_common(lock, &waiter);
780         debug_mutex_add_waiter(lock, &waiter, current);
781 
782         lock_contended(&lock->dep_map, ip);
783 
784         if (!use_ww_ctx) {
785                 /* add waiting tasks to the end of the waitqueue (FIFO): */
786                 list_add_tail(&waiter.list, &lock->wait_list);
787 
788 #ifdef CONFIG_DEBUG_MUTEXES
789                 waiter.ww_ctx = MUTEX_POISON_WW_CTX;
790 #endif
791         } else {
792                 /* Add in stamp order, waking up waiters that must back off. */
793                 ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
794                 if (ret)
795                         goto err_early_backoff;
796 
797                 waiter.ww_ctx = ww_ctx;
798         }
799 
800         waiter.task = current;
801 
802         if (__mutex_waiter_is_first(lock, &waiter))
803                 __mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
804 
805         set_current_state(state);
806         for (;;) {
807                 /*
808                  * Once we hold wait_lock, we're serialized against
809                  * mutex_unlock() handing the lock off to us, do a trylock
810                  * before testing the error conditions to make sure we pick up
811                  * the handoff.
812                  */
813                 if (__mutex_trylock(lock))
814                         goto acquired;
815 
816                 /*
817                  * Check for signals and wound conditions while holding
818                  * wait_lock. This ensures the lock cancellation is ordered
819                  * against mutex_unlock() and wake-ups do not go missing.
820                  */
821                 if (unlikely(signal_pending_state(state, current))) {
822                         ret = -EINTR;
823                         goto err;
824                 }
825 
826                 if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
827                         ret = __ww_mutex_lock_check_stamp(lock, &waiter, ww_ctx);
828                         if (ret)
829                                 goto err;
830                 }
831 
832                 spin_unlock(&lock->wait_lock);
833                 schedule_preempt_disabled();
834 
835                 /*
836                  * ww_mutex needs to always recheck its position since its waiter
837                  * list is not FIFO ordered.
838                  */
839                 if ((use_ww_ctx && ww_ctx) || !first) {
840                         first = __mutex_waiter_is_first(lock, &waiter);
841                         if (first)
842                                 __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
843                 }
844 
845                 set_current_state(state);
846                 /*
847                  * Here we order against unlock; we must either see it change
848                  * state back to RUNNING and fall through the next schedule(),
849                  * or we must see its unlock and acquire.
850                  */
851                 if (__mutex_trylock(lock) ||
852                     (first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, &waiter)))
853                         break;
854 
855                 spin_lock(&lock->wait_lock);
856         }
857         spin_lock(&lock->wait_lock);
858 acquired:
859         __set_current_state(TASK_RUNNING);
860 
861         mutex_remove_waiter(lock, &waiter, current);
862         if (likely(list_empty(&lock->wait_list)))
863                 __mutex_clear_flag(lock, MUTEX_FLAGS);
864 
865         debug_mutex_free_waiter(&waiter);
866 
867 skip_wait:
868         /* got the lock - cleanup and rejoice! */
869         lock_acquired(&lock->dep_map, ip);
870 
871         if (use_ww_ctx && ww_ctx)
872                 ww_mutex_set_context_slowpath(ww, ww_ctx);
873 
874         spin_unlock(&lock->wait_lock);
875         preempt_enable();
876         return 0;
877 
878 err:
879         __set_current_state(TASK_RUNNING);
880         mutex_remove_waiter(lock, &waiter, current);
881 err_early_backoff:
882         spin_unlock(&lock->wait_lock);
883         debug_mutex_free_waiter(&waiter);
884         mutex_release(&lock->dep_map, 1, ip);
885         preempt_enable();
886         return ret;
887 }
888 
889 static int __sched
890 __mutex_lock(struct mutex *lock, long state, unsigned int subclass,
891              struct lockdep_map *nest_lock, unsigned long ip)
892 {
893         return __mutex_lock_common(lock, state, subclass, nest_lock, ip, NULL, false);
894 }
895 
896 static int __sched
897 __ww_mutex_lock(struct mutex *lock, long state, unsigned int subclass,
898                 struct lockdep_map *nest_lock, unsigned long ip,
899                 struct ww_acquire_ctx *ww_ctx)
900 {
901         return __mutex_lock_common(lock, state, subclass, nest_lock, ip, ww_ctx, true);
902 }
903 
904 #ifdef CONFIG_DEBUG_LOCK_ALLOC
905 void __sched
906 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
907 {
908         __mutex_lock(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_);
909 }
910 
911 EXPORT_SYMBOL_GPL(mutex_lock_nested);
912 
913 void __sched
914 _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
915 {
916         __mutex_lock(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_);
917 }
918 EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);
919 
920 int __sched
921 mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
922 {
923         return __mutex_lock(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
924 }
925 EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
926 
927 int __sched
928 mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
929 {
930         return __mutex_lock(lock, TASK_INTERRUPTIBLE, subclass, NULL, _RET_IP_);
931 }
932 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
933 
934 void __sched
935 mutex_lock_io_nested(struct mutex *lock, unsigned int subclass)
936 {
937         int token;
938 
939         might_sleep();
940 
941         token = io_schedule_prepare();
942         __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE,
943                             subclass, NULL, _RET_IP_, NULL, 0);
944         io_schedule_finish(token);
945 }
946 EXPORT_SYMBOL_GPL(mutex_lock_io_nested);
947 
948 static inline int
949 ww_mutex_deadlock_injection(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
950 {
951 #ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
952         unsigned tmp;
953 
954         if (ctx->deadlock_inject_countdown-- == 0) {
955                 tmp = ctx->deadlock_inject_interval;
956                 if (tmp > UINT_MAX/4)
957                         tmp = UINT_MAX;
958                 else
959                         tmp = tmp*2 + tmp + tmp/2;
960 
961                 ctx->deadlock_inject_interval = tmp;
962                 ctx->deadlock_inject_countdown = tmp;
963                 ctx->contending_lock = lock;
964 
965                 ww_mutex_unlock(lock);
966 
967                 return -EDEADLK;
968         }
969 #endif
970 
971         return 0;
972 }
973 
974 int __sched
975 ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
976 {
977         int ret;
978 
979         might_sleep();
980         ret =  __ww_mutex_lock(&lock->base, TASK_UNINTERRUPTIBLE,
981                                0, ctx ? &ctx->dep_map : NULL, _RET_IP_,
982                                ctx);
983         if (!ret && ctx && ctx->acquired > 1)
984                 return ww_mutex_deadlock_injection(lock, ctx);
985 
986         return ret;
987 }
988 EXPORT_SYMBOL_GPL(ww_mutex_lock);
989 
990 int __sched
991 ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
992 {
993         int ret;
994 
995         might_sleep();
996         ret = __ww_mutex_lock(&lock->base, TASK_INTERRUPTIBLE,
997                               0, ctx ? &ctx->dep_map : NULL, _RET_IP_,
998                               ctx);
999 
1000         if (!ret && ctx && ctx->acquired > 1)
1001                 return ww_mutex_deadlock_injection(lock, ctx);
1002 
1003         return ret;
1004 }
1005 EXPORT_SYMBOL_GPL(ww_mutex_lock_interruptible);
1006 
1007 #endif
1008 
1009 /*
1010  * Release the lock, slowpath:
1011  */
1012 static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip)
1013 {
1014         struct task_struct *next = NULL;
1015         DEFINE_WAKE_Q(wake_q);
1016         unsigned long owner;
1017 
1018         mutex_release(&lock->dep_map, 1, ip);
1019 
1020         /*
1021          * Release the lock before (potentially) taking the spinlock such that
1022          * other contenders can get on with things ASAP.
1023          *
1024          * Except when HANDOFF, in that case we must not clear the owner field,
1025          * but instead set it to the top waiter.
1026          */
1027         owner = atomic_long_read(&lock->owner);
1028         for (;;) {
1029                 unsigned long old;
1030 
1031 #ifdef CONFIG_DEBUG_MUTEXES
1032                 DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
1033                 DEBUG_LOCKS_WARN_ON(owner & MUTEX_FLAG_PICKUP);
1034 #endif
1035 
1036                 if (owner & MUTEX_FLAG_HANDOFF)
1037                         break;
1038 
1039                 old = atomic_long_cmpxchg_release(&lock->owner, owner,
1040                                                   __owner_flags(owner));
1041                 if (old == owner) {
1042                         if (owner & MUTEX_FLAG_WAITERS)
1043                                 break;
1044 
1045                         return;
1046                 }
1047 
1048                 owner = old;
1049         }
1050 
1051         spin_lock(&lock->wait_lock);
1052         debug_mutex_unlock(lock);
1053         if (!list_empty(&lock->wait_list)) {
1054                 /* get the first entry from the wait-list: */
1055                 struct mutex_waiter *waiter =
1056                         list_first_entry(&lock->wait_list,
1057                                          struct mutex_waiter, list);
1058 
1059                 next = waiter->task;
1060 
1061                 debug_mutex_wake_waiter(lock, waiter);
1062                 wake_q_add(&wake_q, next);
1063         }
1064 
1065         if (owner & MUTEX_FLAG_HANDOFF)
1066                 __mutex_handoff(lock, next);
1067 
1068         spin_unlock(&lock->wait_lock);
1069 
1070         wake_up_q(&wake_q);
1071 }
1072 
1073 #ifndef CONFIG_DEBUG_LOCK_ALLOC
1074 /*
1075  * Here come the less common (and hence less performance-critical) APIs:
1076  * mutex_lock_interruptible() and mutex_trylock().
1077  */
1078 static noinline int __sched
1079 __mutex_lock_killable_slowpath(struct mutex *lock);
1080 
1081 static noinline int __sched
1082 __mutex_lock_interruptible_slowpath(struct mutex *lock);
1083 
1084 /**
1085  * mutex_lock_interruptible - acquire the mutex, interruptible
1086  * @lock: the mutex to be acquired
1087  *
1088  * Lock the mutex like mutex_lock(), and return 0 if the mutex has
1089  * been acquired or sleep until the mutex becomes available. If a
1090  * signal arrives while waiting for the lock then this function
1091  * returns -EINTR.
1092  *
1093  * This function is similar to (but not equivalent to) down_interruptible().
1094  */
1095 int __sched mutex_lock_interruptible(struct mutex *lock)
1096 {
1097         might_sleep();
1098 
1099         if (__mutex_trylock_fast(lock))
1100                 return 0;
1101 
1102         return __mutex_lock_interruptible_slowpath(lock);
1103 }
1104 
1105 EXPORT_SYMBOL(mutex_lock_interruptible);
1106 
1107 int __sched mutex_lock_killable(struct mutex *lock)
1108 {
1109         might_sleep();
1110 
1111         if (__mutex_trylock_fast(lock))
1112                 return 0;
1113 
1114         return __mutex_lock_killable_slowpath(lock);
1115 }
1116 EXPORT_SYMBOL(mutex_lock_killable);
1117 
1118 void __sched mutex_lock_io(struct mutex *lock)
1119 {
1120         int token;
1121 
1122         token = io_schedule_prepare();
1123         mutex_lock(lock);
1124         io_schedule_finish(token);
1125 }
1126 EXPORT_SYMBOL_GPL(mutex_lock_io);
1127 
1128 static noinline void __sched
1129 __mutex_lock_slowpath(struct mutex *lock)
1130 {
1131         __mutex_lock(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
1132 }
1133 
1134 static noinline int __sched
1135 __mutex_lock_killable_slowpath(struct mutex *lock)
1136 {
1137         return __mutex_lock(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
1138 }
1139 
1140 static noinline int __sched
1141 __mutex_lock_interruptible_slowpath(struct mutex *lock)
1142 {
1143         return __mutex_lock(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
1144 }
1145 
1146 static noinline int __sched
1147 __ww_mutex_lock_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
1148 {
1149         return __ww_mutex_lock(&lock->base, TASK_UNINTERRUPTIBLE, 0, NULL,
1150                                _RET_IP_, ctx);
1151 }
1152 
1153 static noinline int __sched
1154 __ww_mutex_lock_interruptible_slowpath(struct ww_mutex *lock,
1155                                             struct ww_acquire_ctx *ctx)
1156 {
1157         return __ww_mutex_lock(&lock->base, TASK_INTERRUPTIBLE, 0, NULL,
1158                                _RET_IP_, ctx);
1159 }
1160 
1161 #endif
1162 
1163 /**
1164  * mutex_trylock - try to acquire the mutex, without waiting
1165  * @lock: the mutex to be acquired
1166  *
1167  * Try to acquire the mutex atomically. Returns 1 if the mutex
1168  * has been acquired successfully, and 0 on contention.
1169  *
1170  * NOTE: this function follows the spin_trylock() convention, so
1171  * it is negated from the down_trylock() return values! Be careful
1172  * about this when converting semaphore users to mutexes.
1173  *
1174  * This function must not be used in interrupt context. The
1175  * mutex must be released by the same task that acquired it.
1176  */
1177 int __sched mutex_trylock(struct mutex *lock)
1178 {
1179         bool locked = __mutex_trylock(lock);
1180 
1181         if (locked)
1182                 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
1183 
1184         return locked;
1185 }
1186 EXPORT_SYMBOL(mutex_trylock);
1187 
1188 #ifndef CONFIG_DEBUG_LOCK_ALLOC
1189 int __sched
1190 ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
1191 {
1192         might_sleep();
1193 
1194         if (__mutex_trylock_fast(&lock->base)) {
1195                 if (ctx)
1196                         ww_mutex_set_context_fastpath(lock, ctx);
1197                 return 0;
1198         }
1199 
1200         return __ww_mutex_lock_slowpath(lock, ctx);
1201 }
1202 EXPORT_SYMBOL(ww_mutex_lock);
1203 
1204 int __sched
1205 ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
1206 {
1207         might_sleep();
1208 
1209         if (__mutex_trylock_fast(&lock->base)) {
1210                 if (ctx)
1211                         ww_mutex_set_context_fastpath(lock, ctx);
1212                 return 0;
1213         }
1214 
1215         return __ww_mutex_lock_interruptible_slowpath(lock, ctx);
1216 }
1217 EXPORT_SYMBOL(ww_mutex_lock_interruptible);
1218 
1219 #endif
1220 
1221 /**
1222  * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
1223  * @cnt: the atomic which we are to dec
1224  * @lock: the mutex to return holding if we dec to 0
1225  *
1226  * return true and hold lock if we dec to 0, return false otherwise
1227  */
1228 int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
1229 {
1230         /* dec if we can't possibly hit 0 */
1231         if (atomic_add_unless(cnt, -1, 1))
1232                 return 0;
1233         /* we might hit 0, so take the lock */
1234         mutex_lock(lock);
1235         if (!atomic_dec_and_test(cnt)) {
1236                 /* when we actually did the dec, we didn't hit 0 */
1237                 mutex_unlock(lock);
1238                 return 0;
1239         }
1240         /* we hit 0, and we hold the lock */
1241         return 1;
1242 }
1243 EXPORT_SYMBOL(atomic_dec_and_mutex_lock);
1244 

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