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

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  1 /*
  2  * RT-Mutexes: simple blocking mutual exclusion locks with PI support
  3  *
  4  * started by Ingo Molnar and Thomas Gleixner.
  5  *
  6  *  Copyright (C) 2004-2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
  7  *  Copyright (C) 2005-2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>
  8  *  Copyright (C) 2005 Kihon Technologies Inc., Steven Rostedt
  9  *  Copyright (C) 2006 Esben Nielsen
 10  *
 11  *  See Documentation/locking/rt-mutex-design.txt for details.
 12  */
 13 #include <linux/spinlock.h>
 14 #include <linux/export.h>
 15 #include <linux/sched/signal.h>
 16 #include <linux/sched/rt.h>
 17 #include <linux/sched/deadline.h>
 18 #include <linux/sched/wake_q.h>
 19 #include <linux/sched/debug.h>
 20 #include <linux/timer.h>
 21 
 22 #include "rtmutex_common.h"
 23 
 24 /*
 25  * lock->owner state tracking:
 26  *
 27  * lock->owner holds the task_struct pointer of the owner. Bit 0
 28  * is used to keep track of the "lock has waiters" state.
 29  *
 30  * owner        bit0
 31  * NULL         0       lock is free (fast acquire possible)
 32  * NULL         1       lock is free and has waiters and the top waiter
 33  *                              is going to take the lock*
 34  * taskpointer  0       lock is held (fast release possible)
 35  * taskpointer  1       lock is held and has waiters**
 36  *
 37  * The fast atomic compare exchange based acquire and release is only
 38  * possible when bit 0 of lock->owner is 0.
 39  *
 40  * (*) It also can be a transitional state when grabbing the lock
 41  * with ->wait_lock is held. To prevent any fast path cmpxchg to the lock,
 42  * we need to set the bit0 before looking at the lock, and the owner may be
 43  * NULL in this small time, hence this can be a transitional state.
 44  *
 45  * (**) There is a small time when bit 0 is set but there are no
 46  * waiters. This can happen when grabbing the lock in the slow path.
 47  * To prevent a cmpxchg of the owner releasing the lock, we need to
 48  * set this bit before looking at the lock.
 49  */
 50 
 51 static void
 52 rt_mutex_set_owner(struct rt_mutex *lock, struct task_struct *owner)
 53 {
 54         unsigned long val = (unsigned long)owner;
 55 
 56         if (rt_mutex_has_waiters(lock))
 57                 val |= RT_MUTEX_HAS_WAITERS;
 58 
 59         lock->owner = (struct task_struct *)val;
 60 }
 61 
 62 static inline void clear_rt_mutex_waiters(struct rt_mutex *lock)
 63 {
 64         lock->owner = (struct task_struct *)
 65                         ((unsigned long)lock->owner & ~RT_MUTEX_HAS_WAITERS);
 66 }
 67 
 68 static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
 69 {
 70         unsigned long owner, *p = (unsigned long *) &lock->owner;
 71 
 72         if (rt_mutex_has_waiters(lock))
 73                 return;
 74 
 75         /*
 76          * The rbtree has no waiters enqueued, now make sure that the
 77          * lock->owner still has the waiters bit set, otherwise the
 78          * following can happen:
 79          *
 80          * CPU 0        CPU 1           CPU2
 81          * l->owner=T1
 82          *              rt_mutex_lock(l)
 83          *              lock(l->lock)
 84          *              l->owner = T1 | HAS_WAITERS;
 85          *              enqueue(T2)
 86          *              boost()
 87          *                unlock(l->lock)
 88          *              block()
 89          *
 90          *                              rt_mutex_lock(l)
 91          *                              lock(l->lock)
 92          *                              l->owner = T1 | HAS_WAITERS;
 93          *                              enqueue(T3)
 94          *                              boost()
 95          *                                unlock(l->lock)
 96          *                              block()
 97          *              signal(->T2)    signal(->T3)
 98          *              lock(l->lock)
 99          *              dequeue(T2)
100          *              deboost()
101          *                unlock(l->lock)
102          *                              lock(l->lock)
103          *                              dequeue(T3)
104          *                               ==> wait list is empty
105          *                              deboost()
106          *                               unlock(l->lock)
107          *              lock(l->lock)
108          *              fixup_rt_mutex_waiters()
109          *                if (wait_list_empty(l) {
110          *                  l->owner = owner
111          *                  owner = l->owner & ~HAS_WAITERS;
112          *                    ==> l->owner = T1
113          *                }
114          *                              lock(l->lock)
115          * rt_mutex_unlock(l)           fixup_rt_mutex_waiters()
116          *                                if (wait_list_empty(l) {
117          *                                  owner = l->owner & ~HAS_WAITERS;
118          * cmpxchg(l->owner, T1, NULL)
119          *  ===> Success (l->owner = NULL)
120          *
121          *                                  l->owner = owner
122          *                                    ==> l->owner = T1
123          *                                }
124          *
125          * With the check for the waiter bit in place T3 on CPU2 will not
126          * overwrite. All tasks fiddling with the waiters bit are
127          * serialized by l->lock, so nothing else can modify the waiters
128          * bit. If the bit is set then nothing can change l->owner either
129          * so the simple RMW is safe. The cmpxchg() will simply fail if it
130          * happens in the middle of the RMW because the waiters bit is
131          * still set.
132          */
133         owner = READ_ONCE(*p);
134         if (owner & RT_MUTEX_HAS_WAITERS)
135                 WRITE_ONCE(*p, owner & ~RT_MUTEX_HAS_WAITERS);
136 }
137 
138 /*
139  * We can speed up the acquire/release, if there's no debugging state to be
140  * set up.
141  */
142 #ifndef CONFIG_DEBUG_RT_MUTEXES
143 # define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
144 # define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
145 # define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
146 
147 /*
148  * Callers must hold the ->wait_lock -- which is the whole purpose as we force
149  * all future threads that attempt to [Rmw] the lock to the slowpath. As such
150  * relaxed semantics suffice.
151  */
152 static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
153 {
154         unsigned long owner, *p = (unsigned long *) &lock->owner;
155 
156         do {
157                 owner = *p;
158         } while (cmpxchg_relaxed(p, owner,
159                                  owner | RT_MUTEX_HAS_WAITERS) != owner);
160 }
161 
162 /*
163  * Safe fastpath aware unlock:
164  * 1) Clear the waiters bit
165  * 2) Drop lock->wait_lock
166  * 3) Try to unlock the lock with cmpxchg
167  */
168 static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock,
169                                         unsigned long flags)
170         __releases(lock->wait_lock)
171 {
172         struct task_struct *owner = rt_mutex_owner(lock);
173 
174         clear_rt_mutex_waiters(lock);
175         raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
176         /*
177          * If a new waiter comes in between the unlock and the cmpxchg
178          * we have two situations:
179          *
180          * unlock(wait_lock);
181          *                                      lock(wait_lock);
182          * cmpxchg(p, owner, 0) == owner
183          *                                      mark_rt_mutex_waiters(lock);
184          *                                      acquire(lock);
185          * or:
186          *
187          * unlock(wait_lock);
188          *                                      lock(wait_lock);
189          *                                      mark_rt_mutex_waiters(lock);
190          *
191          * cmpxchg(p, owner, 0) != owner
192          *                                      enqueue_waiter();
193          *                                      unlock(wait_lock);
194          * lock(wait_lock);
195          * wake waiter();
196          * unlock(wait_lock);
197          *                                      lock(wait_lock);
198          *                                      acquire(lock);
199          */
200         return rt_mutex_cmpxchg_release(lock, owner, NULL);
201 }
202 
203 #else
204 # define rt_mutex_cmpxchg_relaxed(l,c,n)        (0)
205 # define rt_mutex_cmpxchg_acquire(l,c,n)        (0)
206 # define rt_mutex_cmpxchg_release(l,c,n)        (0)
207 
208 static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
209 {
210         lock->owner = (struct task_struct *)
211                         ((unsigned long)lock->owner | RT_MUTEX_HAS_WAITERS);
212 }
213 
214 /*
215  * Simple slow path only version: lock->owner is protected by lock->wait_lock.
216  */
217 static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock,
218                                         unsigned long flags)
219         __releases(lock->wait_lock)
220 {
221         lock->owner = NULL;
222         raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
223         return true;
224 }
225 #endif
226 
227 /*
228  * Only use with rt_mutex_waiter_{less,equal}()
229  */
230 #define task_to_waiter(p)       \
231         &(struct rt_mutex_waiter){ .prio = (p)->prio, .deadline = (p)->dl.deadline }
232 
233 static inline int
234 rt_mutex_waiter_less(struct rt_mutex_waiter *left,
235                      struct rt_mutex_waiter *right)
236 {
237         if (left->prio < right->prio)
238                 return 1;
239 
240         /*
241          * If both waiters have dl_prio(), we check the deadlines of the
242          * associated tasks.
243          * If left waiter has a dl_prio(), and we didn't return 1 above,
244          * then right waiter has a dl_prio() too.
245          */
246         if (dl_prio(left->prio))
247                 return dl_time_before(left->deadline, right->deadline);
248 
249         return 0;
250 }
251 
252 static inline int
253 rt_mutex_waiter_equal(struct rt_mutex_waiter *left,
254                       struct rt_mutex_waiter *right)
255 {
256         if (left->prio != right->prio)
257                 return 0;
258 
259         /*
260          * If both waiters have dl_prio(), we check the deadlines of the
261          * associated tasks.
262          * If left waiter has a dl_prio(), and we didn't return 0 above,
263          * then right waiter has a dl_prio() too.
264          */
265         if (dl_prio(left->prio))
266                 return left->deadline == right->deadline;
267 
268         return 1;
269 }
270 
271 static void
272 rt_mutex_enqueue(struct rt_mutex *lock, struct rt_mutex_waiter *waiter)
273 {
274         struct rb_node **link = &lock->waiters.rb_node;
275         struct rb_node *parent = NULL;
276         struct rt_mutex_waiter *entry;
277         int leftmost = 1;
278 
279         while (*link) {
280                 parent = *link;
281                 entry = rb_entry(parent, struct rt_mutex_waiter, tree_entry);
282                 if (rt_mutex_waiter_less(waiter, entry)) {
283                         link = &parent->rb_left;
284                 } else {
285                         link = &parent->rb_right;
286                         leftmost = 0;
287                 }
288         }
289 
290         if (leftmost)
291                 lock->waiters_leftmost = &waiter->tree_entry;
292 
293         rb_link_node(&waiter->tree_entry, parent, link);
294         rb_insert_color(&waiter->tree_entry, &lock->waiters);
295 }
296 
297 static void
298 rt_mutex_dequeue(struct rt_mutex *lock, struct rt_mutex_waiter *waiter)
299 {
300         if (RB_EMPTY_NODE(&waiter->tree_entry))
301                 return;
302 
303         if (lock->waiters_leftmost == &waiter->tree_entry)
304                 lock->waiters_leftmost = rb_next(&waiter->tree_entry);
305 
306         rb_erase(&waiter->tree_entry, &lock->waiters);
307         RB_CLEAR_NODE(&waiter->tree_entry);
308 }
309 
310 static void
311 rt_mutex_enqueue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
312 {
313         struct rb_node **link = &task->pi_waiters.rb_node;
314         struct rb_node *parent = NULL;
315         struct rt_mutex_waiter *entry;
316         int leftmost = 1;
317 
318         while (*link) {
319                 parent = *link;
320                 entry = rb_entry(parent, struct rt_mutex_waiter, pi_tree_entry);
321                 if (rt_mutex_waiter_less(waiter, entry)) {
322                         link = &parent->rb_left;
323                 } else {
324                         link = &parent->rb_right;
325                         leftmost = 0;
326                 }
327         }
328 
329         if (leftmost)
330                 task->pi_waiters_leftmost = &waiter->pi_tree_entry;
331 
332         rb_link_node(&waiter->pi_tree_entry, parent, link);
333         rb_insert_color(&waiter->pi_tree_entry, &task->pi_waiters);
334 }
335 
336 static void
337 rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
338 {
339         if (RB_EMPTY_NODE(&waiter->pi_tree_entry))
340                 return;
341 
342         if (task->pi_waiters_leftmost == &waiter->pi_tree_entry)
343                 task->pi_waiters_leftmost = rb_next(&waiter->pi_tree_entry);
344 
345         rb_erase(&waiter->pi_tree_entry, &task->pi_waiters);
346         RB_CLEAR_NODE(&waiter->pi_tree_entry);
347 }
348 
349 static void rt_mutex_adjust_prio(struct task_struct *p)
350 {
351         struct task_struct *pi_task = NULL;
352 
353         lockdep_assert_held(&p->pi_lock);
354 
355         if (task_has_pi_waiters(p))
356                 pi_task = task_top_pi_waiter(p)->task;
357 
358         rt_mutex_setprio(p, pi_task);
359 }
360 
361 /*
362  * Deadlock detection is conditional:
363  *
364  * If CONFIG_DEBUG_RT_MUTEXES=n, deadlock detection is only conducted
365  * if the detect argument is == RT_MUTEX_FULL_CHAINWALK.
366  *
367  * If CONFIG_DEBUG_RT_MUTEXES=y, deadlock detection is always
368  * conducted independent of the detect argument.
369  *
370  * If the waiter argument is NULL this indicates the deboost path and
371  * deadlock detection is disabled independent of the detect argument
372  * and the config settings.
373  */
374 static bool rt_mutex_cond_detect_deadlock(struct rt_mutex_waiter *waiter,
375                                           enum rtmutex_chainwalk chwalk)
376 {
377         /*
378          * This is just a wrapper function for the following call,
379          * because debug_rt_mutex_detect_deadlock() smells like a magic
380          * debug feature and I wanted to keep the cond function in the
381          * main source file along with the comments instead of having
382          * two of the same in the headers.
383          */
384         return debug_rt_mutex_detect_deadlock(waiter, chwalk);
385 }
386 
387 /*
388  * Max number of times we'll walk the boosting chain:
389  */
390 int max_lock_depth = 1024;
391 
392 static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
393 {
394         return p->pi_blocked_on ? p->pi_blocked_on->lock : NULL;
395 }
396 
397 /*
398  * Adjust the priority chain. Also used for deadlock detection.
399  * Decreases task's usage by one - may thus free the task.
400  *
401  * @task:       the task owning the mutex (owner) for which a chain walk is
402  *              probably needed
403  * @chwalk:     do we have to carry out deadlock detection?
404  * @orig_lock:  the mutex (can be NULL if we are walking the chain to recheck
405  *              things for a task that has just got its priority adjusted, and
406  *              is waiting on a mutex)
407  * @next_lock:  the mutex on which the owner of @orig_lock was blocked before
408  *              we dropped its pi_lock. Is never dereferenced, only used for
409  *              comparison to detect lock chain changes.
410  * @orig_waiter: rt_mutex_waiter struct for the task that has just donated
411  *              its priority to the mutex owner (can be NULL in the case
412  *              depicted above or if the top waiter is gone away and we are
413  *              actually deboosting the owner)
414  * @top_task:   the current top waiter
415  *
416  * Returns 0 or -EDEADLK.
417  *
418  * Chain walk basics and protection scope
419  *
420  * [R] refcount on task
421  * [P] task->pi_lock held
422  * [L] rtmutex->wait_lock held
423  *
424  * Step Description                             Protected by
425  *      function arguments:
426  *      @task                                   [R]
427  *      @orig_lock if != NULL                   @top_task is blocked on it
428  *      @next_lock                              Unprotected. Cannot be
429  *                                              dereferenced. Only used for
430  *                                              comparison.
431  *      @orig_waiter if != NULL                 @top_task is blocked on it
432  *      @top_task                               current, or in case of proxy
433  *                                              locking protected by calling
434  *                                              code
435  *      again:
436  *        loop_sanity_check();
437  *      retry:
438  * [1]    lock(task->pi_lock);                  [R] acquire [P]
439  * [2]    waiter = task->pi_blocked_on;         [P]
440  * [3]    check_exit_conditions_1();            [P]
441  * [4]    lock = waiter->lock;                  [P]
442  * [5]    if (!try_lock(lock->wait_lock)) {     [P] try to acquire [L]
443  *          unlock(task->pi_lock);              release [P]
444  *          goto retry;
445  *        }
446  * [6]    check_exit_conditions_2();            [P] + [L]
447  * [7]    requeue_lock_waiter(lock, waiter);    [P] + [L]
448  * [8]    unlock(task->pi_lock);                release [P]
449  *        put_task_struct(task);                release [R]
450  * [9]    check_exit_conditions_3();            [L]
451  * [10]   task = owner(lock);                   [L]
452  *        get_task_struct(task);                [L] acquire [R]
453  *        lock(task->pi_lock);                  [L] acquire [P]
454  * [11]   requeue_pi_waiter(tsk, waiters(lock));[P] + [L]
455  * [12]   check_exit_conditions_4();            [P] + [L]
456  * [13]   unlock(task->pi_lock);                release [P]
457  *        unlock(lock->wait_lock);              release [L]
458  *        goto again;
459  */
460 static int rt_mutex_adjust_prio_chain(struct task_struct *task,
461                                       enum rtmutex_chainwalk chwalk,
462                                       struct rt_mutex *orig_lock,
463                                       struct rt_mutex *next_lock,
464                                       struct rt_mutex_waiter *orig_waiter,
465                                       struct task_struct *top_task)
466 {
467         struct rt_mutex_waiter *waiter, *top_waiter = orig_waiter;
468         struct rt_mutex_waiter *prerequeue_top_waiter;
469         int ret = 0, depth = 0;
470         struct rt_mutex *lock;
471         bool detect_deadlock;
472         bool requeue = true;
473 
474         detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter, chwalk);
475 
476         /*
477          * The (de)boosting is a step by step approach with a lot of
478          * pitfalls. We want this to be preemptible and we want hold a
479          * maximum of two locks per step. So we have to check
480          * carefully whether things change under us.
481          */
482  again:
483         /*
484          * We limit the lock chain length for each invocation.
485          */
486         if (++depth > max_lock_depth) {
487                 static int prev_max;
488 
489                 /*
490                  * Print this only once. If the admin changes the limit,
491                  * print a new message when reaching the limit again.
492                  */
493                 if (prev_max != max_lock_depth) {
494                         prev_max = max_lock_depth;
495                         printk(KERN_WARNING "Maximum lock depth %d reached "
496                                "task: %s (%d)\n", max_lock_depth,
497                                top_task->comm, task_pid_nr(top_task));
498                 }
499                 put_task_struct(task);
500 
501                 return -EDEADLK;
502         }
503 
504         /*
505          * We are fully preemptible here and only hold the refcount on
506          * @task. So everything can have changed under us since the
507          * caller or our own code below (goto retry/again) dropped all
508          * locks.
509          */
510  retry:
511         /*
512          * [1] Task cannot go away as we did a get_task() before !
513          */
514         raw_spin_lock_irq(&task->pi_lock);
515 
516         /*
517          * [2] Get the waiter on which @task is blocked on.
518          */
519         waiter = task->pi_blocked_on;
520 
521         /*
522          * [3] check_exit_conditions_1() protected by task->pi_lock.
523          */
524 
525         /*
526          * Check whether the end of the boosting chain has been
527          * reached or the state of the chain has changed while we
528          * dropped the locks.
529          */
530         if (!waiter)
531                 goto out_unlock_pi;
532 
533         /*
534          * Check the orig_waiter state. After we dropped the locks,
535          * the previous owner of the lock might have released the lock.
536          */
537         if (orig_waiter && !rt_mutex_owner(orig_lock))
538                 goto out_unlock_pi;
539 
540         /*
541          * We dropped all locks after taking a refcount on @task, so
542          * the task might have moved on in the lock chain or even left
543          * the chain completely and blocks now on an unrelated lock or
544          * on @orig_lock.
545          *
546          * We stored the lock on which @task was blocked in @next_lock,
547          * so we can detect the chain change.
548          */
549         if (next_lock != waiter->lock)
550                 goto out_unlock_pi;
551 
552         /*
553          * Drop out, when the task has no waiters. Note,
554          * top_waiter can be NULL, when we are in the deboosting
555          * mode!
556          */
557         if (top_waiter) {
558                 if (!task_has_pi_waiters(task))
559                         goto out_unlock_pi;
560                 /*
561                  * If deadlock detection is off, we stop here if we
562                  * are not the top pi waiter of the task. If deadlock
563                  * detection is enabled we continue, but stop the
564                  * requeueing in the chain walk.
565                  */
566                 if (top_waiter != task_top_pi_waiter(task)) {
567                         if (!detect_deadlock)
568                                 goto out_unlock_pi;
569                         else
570                                 requeue = false;
571                 }
572         }
573 
574         /*
575          * If the waiter priority is the same as the task priority
576          * then there is no further priority adjustment necessary.  If
577          * deadlock detection is off, we stop the chain walk. If its
578          * enabled we continue, but stop the requeueing in the chain
579          * walk.
580          */
581         if (rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
582                 if (!detect_deadlock)
583                         goto out_unlock_pi;
584                 else
585                         requeue = false;
586         }
587 
588         /*
589          * [4] Get the next lock
590          */
591         lock = waiter->lock;
592         /*
593          * [5] We need to trylock here as we are holding task->pi_lock,
594          * which is the reverse lock order versus the other rtmutex
595          * operations.
596          */
597         if (!raw_spin_trylock(&lock->wait_lock)) {
598                 raw_spin_unlock_irq(&task->pi_lock);
599                 cpu_relax();
600                 goto retry;
601         }
602 
603         /*
604          * [6] check_exit_conditions_2() protected by task->pi_lock and
605          * lock->wait_lock.
606          *
607          * Deadlock detection. If the lock is the same as the original
608          * lock which caused us to walk the lock chain or if the
609          * current lock is owned by the task which initiated the chain
610          * walk, we detected a deadlock.
611          */
612         if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
613                 debug_rt_mutex_deadlock(chwalk, orig_waiter, lock);
614                 raw_spin_unlock(&lock->wait_lock);
615                 ret = -EDEADLK;
616                 goto out_unlock_pi;
617         }
618 
619         /*
620          * If we just follow the lock chain for deadlock detection, no
621          * need to do all the requeue operations. To avoid a truckload
622          * of conditionals around the various places below, just do the
623          * minimum chain walk checks.
624          */
625         if (!requeue) {
626                 /*
627                  * No requeue[7] here. Just release @task [8]
628                  */
629                 raw_spin_unlock(&task->pi_lock);
630                 put_task_struct(task);
631 
632                 /*
633                  * [9] check_exit_conditions_3 protected by lock->wait_lock.
634                  * If there is no owner of the lock, end of chain.
635                  */
636                 if (!rt_mutex_owner(lock)) {
637                         raw_spin_unlock_irq(&lock->wait_lock);
638                         return 0;
639                 }
640 
641                 /* [10] Grab the next task, i.e. owner of @lock */
642                 task = rt_mutex_owner(lock);
643                 get_task_struct(task);
644                 raw_spin_lock(&task->pi_lock);
645 
646                 /*
647                  * No requeue [11] here. We just do deadlock detection.
648                  *
649                  * [12] Store whether owner is blocked
650                  * itself. Decision is made after dropping the locks
651                  */
652                 next_lock = task_blocked_on_lock(task);
653                 /*
654                  * Get the top waiter for the next iteration
655                  */
656                 top_waiter = rt_mutex_top_waiter(lock);
657 
658                 /* [13] Drop locks */
659                 raw_spin_unlock(&task->pi_lock);
660                 raw_spin_unlock_irq(&lock->wait_lock);
661 
662                 /* If owner is not blocked, end of chain. */
663                 if (!next_lock)
664                         goto out_put_task;
665                 goto again;
666         }
667 
668         /*
669          * Store the current top waiter before doing the requeue
670          * operation on @lock. We need it for the boost/deboost
671          * decision below.
672          */
673         prerequeue_top_waiter = rt_mutex_top_waiter(lock);
674 
675         /* [7] Requeue the waiter in the lock waiter tree. */
676         rt_mutex_dequeue(lock, waiter);
677 
678         /*
679          * Update the waiter prio fields now that we're dequeued.
680          *
681          * These values can have changed through either:
682          *
683          *   sys_sched_set_scheduler() / sys_sched_setattr()
684          *
685          * or
686          *
687          *   DL CBS enforcement advancing the effective deadline.
688          *
689          * Even though pi_waiters also uses these fields, and that tree is only
690          * updated in [11], we can do this here, since we hold [L], which
691          * serializes all pi_waiters access and rb_erase() does not care about
692          * the values of the node being removed.
693          */
694         waiter->prio = task->prio;
695         waiter->deadline = task->dl.deadline;
696 
697         rt_mutex_enqueue(lock, waiter);
698 
699         /* [8] Release the task */
700         raw_spin_unlock(&task->pi_lock);
701         put_task_struct(task);
702 
703         /*
704          * [9] check_exit_conditions_3 protected by lock->wait_lock.
705          *
706          * We must abort the chain walk if there is no lock owner even
707          * in the dead lock detection case, as we have nothing to
708          * follow here. This is the end of the chain we are walking.
709          */
710         if (!rt_mutex_owner(lock)) {
711                 /*
712                  * If the requeue [7] above changed the top waiter,
713                  * then we need to wake the new top waiter up to try
714                  * to get the lock.
715                  */
716                 if (prerequeue_top_waiter != rt_mutex_top_waiter(lock))
717                         wake_up_process(rt_mutex_top_waiter(lock)->task);
718                 raw_spin_unlock_irq(&lock->wait_lock);
719                 return 0;
720         }
721 
722         /* [10] Grab the next task, i.e. the owner of @lock */
723         task = rt_mutex_owner(lock);
724         get_task_struct(task);
725         raw_spin_lock(&task->pi_lock);
726 
727         /* [11] requeue the pi waiters if necessary */
728         if (waiter == rt_mutex_top_waiter(lock)) {
729                 /*
730                  * The waiter became the new top (highest priority)
731                  * waiter on the lock. Replace the previous top waiter
732                  * in the owner tasks pi waiters tree with this waiter
733                  * and adjust the priority of the owner.
734                  */
735                 rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
736                 rt_mutex_enqueue_pi(task, waiter);
737                 rt_mutex_adjust_prio(task);
738 
739         } else if (prerequeue_top_waiter == waiter) {
740                 /*
741                  * The waiter was the top waiter on the lock, but is
742                  * no longer the top prority waiter. Replace waiter in
743                  * the owner tasks pi waiters tree with the new top
744                  * (highest priority) waiter and adjust the priority
745                  * of the owner.
746                  * The new top waiter is stored in @waiter so that
747                  * @waiter == @top_waiter evaluates to true below and
748                  * we continue to deboost the rest of the chain.
749                  */
750                 rt_mutex_dequeue_pi(task, waiter);
751                 waiter = rt_mutex_top_waiter(lock);
752                 rt_mutex_enqueue_pi(task, waiter);
753                 rt_mutex_adjust_prio(task);
754         } else {
755                 /*
756                  * Nothing changed. No need to do any priority
757                  * adjustment.
758                  */
759         }
760 
761         /*
762          * [12] check_exit_conditions_4() protected by task->pi_lock
763          * and lock->wait_lock. The actual decisions are made after we
764          * dropped the locks.
765          *
766          * Check whether the task which owns the current lock is pi
767          * blocked itself. If yes we store a pointer to the lock for
768          * the lock chain change detection above. After we dropped
769          * task->pi_lock next_lock cannot be dereferenced anymore.
770          */
771         next_lock = task_blocked_on_lock(task);
772         /*
773          * Store the top waiter of @lock for the end of chain walk
774          * decision below.
775          */
776         top_waiter = rt_mutex_top_waiter(lock);
777 
778         /* [13] Drop the locks */
779         raw_spin_unlock(&task->pi_lock);
780         raw_spin_unlock_irq(&lock->wait_lock);
781 
782         /*
783          * Make the actual exit decisions [12], based on the stored
784          * values.
785          *
786          * We reached the end of the lock chain. Stop right here. No
787          * point to go back just to figure that out.
788          */
789         if (!next_lock)
790                 goto out_put_task;
791 
792         /*
793          * If the current waiter is not the top waiter on the lock,
794          * then we can stop the chain walk here if we are not in full
795          * deadlock detection mode.
796          */
797         if (!detect_deadlock && waiter != top_waiter)
798                 goto out_put_task;
799 
800         goto again;
801 
802  out_unlock_pi:
803         raw_spin_unlock_irq(&task->pi_lock);
804  out_put_task:
805         put_task_struct(task);
806 
807         return ret;
808 }
809 
810 /*
811  * Try to take an rt-mutex
812  *
813  * Must be called with lock->wait_lock held and interrupts disabled
814  *
815  * @lock:   The lock to be acquired.
816  * @task:   The task which wants to acquire the lock
817  * @waiter: The waiter that is queued to the lock's wait tree if the
818  *          callsite called task_blocked_on_lock(), otherwise NULL
819  */
820 static int try_to_take_rt_mutex(struct rt_mutex *lock, struct task_struct *task,
821                                 struct rt_mutex_waiter *waiter)
822 {
823         lockdep_assert_held(&lock->wait_lock);
824 
825         /*
826          * Before testing whether we can acquire @lock, we set the
827          * RT_MUTEX_HAS_WAITERS bit in @lock->owner. This forces all
828          * other tasks which try to modify @lock into the slow path
829          * and they serialize on @lock->wait_lock.
830          *
831          * The RT_MUTEX_HAS_WAITERS bit can have a transitional state
832          * as explained at the top of this file if and only if:
833          *
834          * - There is a lock owner. The caller must fixup the
835          *   transient state if it does a trylock or leaves the lock
836          *   function due to a signal or timeout.
837          *
838          * - @task acquires the lock and there are no other
839          *   waiters. This is undone in rt_mutex_set_owner(@task) at
840          *   the end of this function.
841          */
842         mark_rt_mutex_waiters(lock);
843 
844         /*
845          * If @lock has an owner, give up.
846          */
847         if (rt_mutex_owner(lock))
848                 return 0;
849 
850         /*
851          * If @waiter != NULL, @task has already enqueued the waiter
852          * into @lock waiter tree. If @waiter == NULL then this is a
853          * trylock attempt.
854          */
855         if (waiter) {
856                 /*
857                  * If waiter is not the highest priority waiter of
858                  * @lock, give up.
859                  */
860                 if (waiter != rt_mutex_top_waiter(lock))
861                         return 0;
862 
863                 /*
864                  * We can acquire the lock. Remove the waiter from the
865                  * lock waiters tree.
866                  */
867                 rt_mutex_dequeue(lock, waiter);
868 
869         } else {
870                 /*
871                  * If the lock has waiters already we check whether @task is
872                  * eligible to take over the lock.
873                  *
874                  * If there are no other waiters, @task can acquire
875                  * the lock.  @task->pi_blocked_on is NULL, so it does
876                  * not need to be dequeued.
877                  */
878                 if (rt_mutex_has_waiters(lock)) {
879                         /*
880                          * If @task->prio is greater than or equal to
881                          * the top waiter priority (kernel view),
882                          * @task lost.
883                          */
884                         if (!rt_mutex_waiter_less(task_to_waiter(task),
885                                                   rt_mutex_top_waiter(lock)))
886                                 return 0;
887 
888                         /*
889                          * The current top waiter stays enqueued. We
890                          * don't have to change anything in the lock
891                          * waiters order.
892                          */
893                 } else {
894                         /*
895                          * No waiters. Take the lock without the
896                          * pi_lock dance.@task->pi_blocked_on is NULL
897                          * and we have no waiters to enqueue in @task
898                          * pi waiters tree.
899                          */
900                         goto takeit;
901                 }
902         }
903 
904         /*
905          * Clear @task->pi_blocked_on. Requires protection by
906          * @task->pi_lock. Redundant operation for the @waiter == NULL
907          * case, but conditionals are more expensive than a redundant
908          * store.
909          */
910         raw_spin_lock(&task->pi_lock);
911         task->pi_blocked_on = NULL;
912         /*
913          * Finish the lock acquisition. @task is the new owner. If
914          * other waiters exist we have to insert the highest priority
915          * waiter into @task->pi_waiters tree.
916          */
917         if (rt_mutex_has_waiters(lock))
918                 rt_mutex_enqueue_pi(task, rt_mutex_top_waiter(lock));
919         raw_spin_unlock(&task->pi_lock);
920 
921 takeit:
922         /* We got the lock. */
923         debug_rt_mutex_lock(lock);
924 
925         /*
926          * This either preserves the RT_MUTEX_HAS_WAITERS bit if there
927          * are still waiters or clears it.
928          */
929         rt_mutex_set_owner(lock, task);
930 
931         return 1;
932 }
933 
934 /*
935  * Task blocks on lock.
936  *
937  * Prepare waiter and propagate pi chain
938  *
939  * This must be called with lock->wait_lock held and interrupts disabled
940  */
941 static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
942                                    struct rt_mutex_waiter *waiter,
943                                    struct task_struct *task,
944                                    enum rtmutex_chainwalk chwalk)
945 {
946         struct task_struct *owner = rt_mutex_owner(lock);
947         struct rt_mutex_waiter *top_waiter = waiter;
948         struct rt_mutex *next_lock;
949         int chain_walk = 0, res;
950 
951         lockdep_assert_held(&lock->wait_lock);
952 
953         /*
954          * Early deadlock detection. We really don't want the task to
955          * enqueue on itself just to untangle the mess later. It's not
956          * only an optimization. We drop the locks, so another waiter
957          * can come in before the chain walk detects the deadlock. So
958          * the other will detect the deadlock and return -EDEADLOCK,
959          * which is wrong, as the other waiter is not in a deadlock
960          * situation.
961          */
962         if (owner == task)
963                 return -EDEADLK;
964 
965         raw_spin_lock(&task->pi_lock);
966         rt_mutex_adjust_prio(task);
967         waiter->task = task;
968         waiter->lock = lock;
969         waiter->prio = task->prio;
970         waiter->deadline = task->dl.deadline;
971 
972         /* Get the top priority waiter on the lock */
973         if (rt_mutex_has_waiters(lock))
974                 top_waiter = rt_mutex_top_waiter(lock);
975         rt_mutex_enqueue(lock, waiter);
976 
977         task->pi_blocked_on = waiter;
978 
979         raw_spin_unlock(&task->pi_lock);
980 
981         if (!owner)
982                 return 0;
983 
984         raw_spin_lock(&owner->pi_lock);
985         if (waiter == rt_mutex_top_waiter(lock)) {
986                 rt_mutex_dequeue_pi(owner, top_waiter);
987                 rt_mutex_enqueue_pi(owner, waiter);
988 
989                 rt_mutex_adjust_prio(owner);
990                 if (owner->pi_blocked_on)
991                         chain_walk = 1;
992         } else if (rt_mutex_cond_detect_deadlock(waiter, chwalk)) {
993                 chain_walk = 1;
994         }
995 
996         /* Store the lock on which owner is blocked or NULL */
997         next_lock = task_blocked_on_lock(owner);
998 
999         raw_spin_unlock(&owner->pi_lock);
1000         /*
1001          * Even if full deadlock detection is on, if the owner is not
1002          * blocked itself, we can avoid finding this out in the chain
1003          * walk.
1004          */
1005         if (!chain_walk || !next_lock)
1006                 return 0;
1007 
1008         /*
1009          * The owner can't disappear while holding a lock,
1010          * so the owner struct is protected by wait_lock.
1011          * Gets dropped in rt_mutex_adjust_prio_chain()!
1012          */
1013         get_task_struct(owner);
1014 
1015         raw_spin_unlock_irq(&lock->wait_lock);
1016 
1017         res = rt_mutex_adjust_prio_chain(owner, chwalk, lock,
1018                                          next_lock, waiter, task);
1019 
1020         raw_spin_lock_irq(&lock->wait_lock);
1021 
1022         return res;
1023 }
1024 
1025 /*
1026  * Remove the top waiter from the current tasks pi waiter tree and
1027  * queue it up.
1028  *
1029  * Called with lock->wait_lock held and interrupts disabled.
1030  */
1031 static void mark_wakeup_next_waiter(struct wake_q_head *wake_q,
1032                                     struct rt_mutex *lock)
1033 {
1034         struct rt_mutex_waiter *waiter;
1035 
1036         raw_spin_lock(&current->pi_lock);
1037 
1038         waiter = rt_mutex_top_waiter(lock);
1039 
1040         /*
1041          * Remove it from current->pi_waiters and deboost.
1042          *
1043          * We must in fact deboost here in order to ensure we call
1044          * rt_mutex_setprio() to update p->pi_top_task before the
1045          * task unblocks.
1046          */
1047         rt_mutex_dequeue_pi(current, waiter);
1048         rt_mutex_adjust_prio(current);
1049 
1050         /*
1051          * As we are waking up the top waiter, and the waiter stays
1052          * queued on the lock until it gets the lock, this lock
1053          * obviously has waiters. Just set the bit here and this has
1054          * the added benefit of forcing all new tasks into the
1055          * slow path making sure no task of lower priority than
1056          * the top waiter can steal this lock.
1057          */
1058         lock->owner = (void *) RT_MUTEX_HAS_WAITERS;
1059 
1060         /*
1061          * We deboosted before waking the top waiter task such that we don't
1062          * run two tasks with the 'same' priority (and ensure the
1063          * p->pi_top_task pointer points to a blocked task). This however can
1064          * lead to priority inversion if we would get preempted after the
1065          * deboost but before waking our donor task, hence the preempt_disable()
1066          * before unlock.
1067          *
1068          * Pairs with preempt_enable() in rt_mutex_postunlock();
1069          */
1070         preempt_disable();
1071         wake_q_add(wake_q, waiter->task);
1072         raw_spin_unlock(&current->pi_lock);
1073 }
1074 
1075 /*
1076  * Remove a waiter from a lock and give up
1077  *
1078  * Must be called with lock->wait_lock held and interrupts disabled. I must
1079  * have just failed to try_to_take_rt_mutex().
1080  */
1081 static void remove_waiter(struct rt_mutex *lock,
1082                           struct rt_mutex_waiter *waiter)
1083 {
1084         bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
1085         struct task_struct *owner = rt_mutex_owner(lock);
1086         struct rt_mutex *next_lock;
1087 
1088         lockdep_assert_held(&lock->wait_lock);
1089 
1090         raw_spin_lock(&current->pi_lock);
1091         rt_mutex_dequeue(lock, waiter);
1092         current->pi_blocked_on = NULL;
1093         raw_spin_unlock(&current->pi_lock);
1094 
1095         /*
1096          * Only update priority if the waiter was the highest priority
1097          * waiter of the lock and there is an owner to update.
1098          */
1099         if (!owner || !is_top_waiter)
1100                 return;
1101 
1102         raw_spin_lock(&owner->pi_lock);
1103 
1104         rt_mutex_dequeue_pi(owner, waiter);
1105 
1106         if (rt_mutex_has_waiters(lock))
1107                 rt_mutex_enqueue_pi(owner, rt_mutex_top_waiter(lock));
1108 
1109         rt_mutex_adjust_prio(owner);
1110 
1111         /* Store the lock on which owner is blocked or NULL */
1112         next_lock = task_blocked_on_lock(owner);
1113 
1114         raw_spin_unlock(&owner->pi_lock);
1115 
1116         /*
1117          * Don't walk the chain, if the owner task is not blocked
1118          * itself.
1119          */
1120         if (!next_lock)
1121                 return;
1122 
1123         /* gets dropped in rt_mutex_adjust_prio_chain()! */
1124         get_task_struct(owner);
1125 
1126         raw_spin_unlock_irq(&lock->wait_lock);
1127 
1128         rt_mutex_adjust_prio_chain(owner, RT_MUTEX_MIN_CHAINWALK, lock,
1129                                    next_lock, NULL, current);
1130 
1131         raw_spin_lock_irq(&lock->wait_lock);
1132 }
1133 
1134 /*
1135  * Recheck the pi chain, in case we got a priority setting
1136  *
1137  * Called from sched_setscheduler
1138  */
1139 void rt_mutex_adjust_pi(struct task_struct *task)
1140 {
1141         struct rt_mutex_waiter *waiter;
1142         struct rt_mutex *next_lock;
1143         unsigned long flags;
1144 
1145         raw_spin_lock_irqsave(&task->pi_lock, flags);
1146 
1147         waiter = task->pi_blocked_on;
1148         if (!waiter || rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
1149                 raw_spin_unlock_irqrestore(&task->pi_lock, flags);
1150                 return;
1151         }
1152         next_lock = waiter->lock;
1153         raw_spin_unlock_irqrestore(&task->pi_lock, flags);
1154 
1155         /* gets dropped in rt_mutex_adjust_prio_chain()! */
1156         get_task_struct(task);
1157 
1158         rt_mutex_adjust_prio_chain(task, RT_MUTEX_MIN_CHAINWALK, NULL,
1159                                    next_lock, NULL, task);
1160 }
1161 
1162 void rt_mutex_init_waiter(struct rt_mutex_waiter *waiter)
1163 {
1164         debug_rt_mutex_init_waiter(waiter);
1165         RB_CLEAR_NODE(&waiter->pi_tree_entry);
1166         RB_CLEAR_NODE(&waiter->tree_entry);
1167         waiter->task = NULL;
1168 }
1169 
1170 /**
1171  * __rt_mutex_slowlock() - Perform the wait-wake-try-to-take loop
1172  * @lock:                the rt_mutex to take
1173  * @state:               the state the task should block in (TASK_INTERRUPTIBLE
1174  *                       or TASK_UNINTERRUPTIBLE)
1175  * @timeout:             the pre-initialized and started timer, or NULL for none
1176  * @waiter:              the pre-initialized rt_mutex_waiter
1177  *
1178  * Must be called with lock->wait_lock held and interrupts disabled
1179  */
1180 static int __sched
1181 __rt_mutex_slowlock(struct rt_mutex *lock, int state,
1182                     struct hrtimer_sleeper *timeout,
1183                     struct rt_mutex_waiter *waiter)
1184 {
1185         int ret = 0;
1186 
1187         for (;;) {
1188                 /* Try to acquire the lock: */
1189                 if (try_to_take_rt_mutex(lock, current, waiter))
1190                         break;
1191 
1192                 /*
1193                  * TASK_INTERRUPTIBLE checks for signals and
1194                  * timeout. Ignored otherwise.
1195                  */
1196                 if (likely(state == TASK_INTERRUPTIBLE)) {
1197                         /* Signal pending? */
1198                         if (signal_pending(current))
1199                                 ret = -EINTR;
1200                         if (timeout && !timeout->task)
1201                                 ret = -ETIMEDOUT;
1202                         if (ret)
1203                                 break;
1204                 }
1205 
1206                 raw_spin_unlock_irq(&lock->wait_lock);
1207 
1208                 debug_rt_mutex_print_deadlock(waiter);
1209 
1210                 schedule();
1211 
1212                 raw_spin_lock_irq(&lock->wait_lock);
1213                 set_current_state(state);
1214         }
1215 
1216         __set_current_state(TASK_RUNNING);
1217         return ret;
1218 }
1219 
1220 static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
1221                                      struct rt_mutex_waiter *w)
1222 {
1223         /*
1224          * If the result is not -EDEADLOCK or the caller requested
1225          * deadlock detection, nothing to do here.
1226          */
1227         if (res != -EDEADLOCK || detect_deadlock)
1228                 return;
1229 
1230         /*
1231          * Yell lowdly and stop the task right here.
1232          */
1233         rt_mutex_print_deadlock(w);
1234         while (1) {
1235                 set_current_state(TASK_INTERRUPTIBLE);
1236                 schedule();
1237         }
1238 }
1239 
1240 /*
1241  * Slow path lock function:
1242  */
1243 static int __sched
1244 rt_mutex_slowlock(struct rt_mutex *lock, int state,
1245                   struct hrtimer_sleeper *timeout,
1246                   enum rtmutex_chainwalk chwalk)
1247 {
1248         struct rt_mutex_waiter waiter;
1249         unsigned long flags;
1250         int ret = 0;
1251 
1252         rt_mutex_init_waiter(&waiter);
1253 
1254         /*
1255          * Technically we could use raw_spin_[un]lock_irq() here, but this can
1256          * be called in early boot if the cmpxchg() fast path is disabled
1257          * (debug, no architecture support). In this case we will acquire the
1258          * rtmutex with lock->wait_lock held. But we cannot unconditionally
1259          * enable interrupts in that early boot case. So we need to use the
1260          * irqsave/restore variants.
1261          */
1262         raw_spin_lock_irqsave(&lock->wait_lock, flags);
1263 
1264         /* Try to acquire the lock again: */
1265         if (try_to_take_rt_mutex(lock, current, NULL)) {
1266                 raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1267                 return 0;
1268         }
1269 
1270         set_current_state(state);
1271 
1272         /* Setup the timer, when timeout != NULL */
1273         if (unlikely(timeout))
1274                 hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
1275 
1276         ret = task_blocks_on_rt_mutex(lock, &waiter, current, chwalk);
1277 
1278         if (likely(!ret))
1279                 /* sleep on the mutex */
1280                 ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);
1281 
1282         if (unlikely(ret)) {
1283                 __set_current_state(TASK_RUNNING);
1284                 if (rt_mutex_has_waiters(lock))
1285                         remove_waiter(lock, &waiter);
1286                 rt_mutex_handle_deadlock(ret, chwalk, &waiter);
1287         }
1288 
1289         /*
1290          * try_to_take_rt_mutex() sets the waiter bit
1291          * unconditionally. We might have to fix that up.
1292          */
1293         fixup_rt_mutex_waiters(lock);
1294 
1295         raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1296 
1297         /* Remove pending timer: */
1298         if (unlikely(timeout))
1299                 hrtimer_cancel(&timeout->timer);
1300 
1301         debug_rt_mutex_free_waiter(&waiter);
1302 
1303         return ret;
1304 }
1305 
1306 /*
1307  * Slow path try-lock function:
1308  */
1309 static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
1310 {
1311         unsigned long flags;
1312         int ret;
1313 
1314         /*
1315          * If the lock already has an owner we fail to get the lock.
1316          * This can be done without taking the @lock->wait_lock as
1317          * it is only being read, and this is a trylock anyway.
1318          */
1319         if (rt_mutex_owner(lock))
1320                 return 0;
1321 
1322         /*
1323          * The mutex has currently no owner. Lock the wait lock and try to
1324          * acquire the lock. We use irqsave here to support early boot calls.
1325          */
1326         raw_spin_lock_irqsave(&lock->wait_lock, flags);
1327 
1328         ret = try_to_take_rt_mutex(lock, current, NULL);
1329 
1330         /*
1331          * try_to_take_rt_mutex() sets the lock waiters bit
1332          * unconditionally. Clean this up.
1333          */
1334         fixup_rt_mutex_waiters(lock);
1335 
1336         raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1337 
1338         return ret;
1339 }
1340 
1341 /*
1342  * Slow path to release a rt-mutex.
1343  *
1344  * Return whether the current task needs to call rt_mutex_postunlock().
1345  */
1346 static bool __sched rt_mutex_slowunlock(struct rt_mutex *lock,
1347                                         struct wake_q_head *wake_q)
1348 {
1349         unsigned long flags;
1350 
1351         /* irqsave required to support early boot calls */
1352         raw_spin_lock_irqsave(&lock->wait_lock, flags);
1353 
1354         debug_rt_mutex_unlock(lock);
1355 
1356         /*
1357          * We must be careful here if the fast path is enabled. If we
1358          * have no waiters queued we cannot set owner to NULL here
1359          * because of:
1360          *
1361          * foo->lock->owner = NULL;
1362          *                      rtmutex_lock(foo->lock);   <- fast path
1363          *                      free = atomic_dec_and_test(foo->refcnt);
1364          *                      rtmutex_unlock(foo->lock); <- fast path
1365          *                      if (free)
1366          *                              kfree(foo);
1367          * raw_spin_unlock(foo->lock->wait_lock);
1368          *
1369          * So for the fastpath enabled kernel:
1370          *
1371          * Nothing can set the waiters bit as long as we hold
1372          * lock->wait_lock. So we do the following sequence:
1373          *
1374          *      owner = rt_mutex_owner(lock);
1375          *      clear_rt_mutex_waiters(lock);
1376          *      raw_spin_unlock(&lock->wait_lock);
1377          *      if (cmpxchg(&lock->owner, owner, 0) == owner)
1378          *              return;
1379          *      goto retry;
1380          *
1381          * The fastpath disabled variant is simple as all access to
1382          * lock->owner is serialized by lock->wait_lock:
1383          *
1384          *      lock->owner = NULL;
1385          *      raw_spin_unlock(&lock->wait_lock);
1386          */
1387         while (!rt_mutex_has_waiters(lock)) {
1388                 /* Drops lock->wait_lock ! */
1389                 if (unlock_rt_mutex_safe(lock, flags) == true)
1390                         return false;
1391                 /* Relock the rtmutex and try again */
1392                 raw_spin_lock_irqsave(&lock->wait_lock, flags);
1393         }
1394 
1395         /*
1396          * The wakeup next waiter path does not suffer from the above
1397          * race. See the comments there.
1398          *
1399          * Queue the next waiter for wakeup once we release the wait_lock.
1400          */
1401         mark_wakeup_next_waiter(wake_q, lock);
1402         raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1403 
1404         return true; /* call rt_mutex_postunlock() */
1405 }
1406 
1407 /*
1408  * debug aware fast / slowpath lock,trylock,unlock
1409  *
1410  * The atomic acquire/release ops are compiled away, when either the
1411  * architecture does not support cmpxchg or when debugging is enabled.
1412  */
1413 static inline int
1414 rt_mutex_fastlock(struct rt_mutex *lock, int state,
1415                   int (*slowfn)(struct rt_mutex *lock, int state,
1416                                 struct hrtimer_sleeper *timeout,
1417                                 enum rtmutex_chainwalk chwalk))
1418 {
1419         if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
1420                 return 0;
1421 
1422         return slowfn(lock, state, NULL, RT_MUTEX_MIN_CHAINWALK);
1423 }
1424 
1425 static inline int
1426 rt_mutex_timed_fastlock(struct rt_mutex *lock, int state,
1427                         struct hrtimer_sleeper *timeout,
1428                         enum rtmutex_chainwalk chwalk,
1429                         int (*slowfn)(struct rt_mutex *lock, int state,
1430                                       struct hrtimer_sleeper *timeout,
1431                                       enum rtmutex_chainwalk chwalk))
1432 {
1433         if (chwalk == RT_MUTEX_MIN_CHAINWALK &&
1434             likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
1435                 return 0;
1436 
1437         return slowfn(lock, state, timeout, chwalk);
1438 }
1439 
1440 static inline int
1441 rt_mutex_fasttrylock(struct rt_mutex *lock,
1442                      int (*slowfn)(struct rt_mutex *lock))
1443 {
1444         if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
1445                 return 1;
1446 
1447         return slowfn(lock);
1448 }
1449 
1450 /*
1451  * Performs the wakeup of the the top-waiter and re-enables preemption.
1452  */
1453 void rt_mutex_postunlock(struct wake_q_head *wake_q)
1454 {
1455         wake_up_q(wake_q);
1456 
1457         /* Pairs with preempt_disable() in rt_mutex_slowunlock() */
1458         preempt_enable();
1459 }
1460 
1461 static inline void
1462 rt_mutex_fastunlock(struct rt_mutex *lock,
1463                     bool (*slowfn)(struct rt_mutex *lock,
1464                                    struct wake_q_head *wqh))
1465 {
1466         DEFINE_WAKE_Q(wake_q);
1467 
1468         if (likely(rt_mutex_cmpxchg_release(lock, current, NULL)))
1469                 return;
1470 
1471         if (slowfn(lock, &wake_q))
1472                 rt_mutex_postunlock(&wake_q);
1473 }
1474 
1475 /**
1476  * rt_mutex_lock - lock a rt_mutex
1477  *
1478  * @lock: the rt_mutex to be locked
1479  */
1480 void __sched rt_mutex_lock(struct rt_mutex *lock)
1481 {
1482         might_sleep();
1483 
1484         rt_mutex_fastlock(lock, TASK_UNINTERRUPTIBLE, rt_mutex_slowlock);
1485 }
1486 EXPORT_SYMBOL_GPL(rt_mutex_lock);
1487 
1488 /**
1489  * rt_mutex_lock_interruptible - lock a rt_mutex interruptible
1490  *
1491  * @lock:               the rt_mutex to be locked
1492  *
1493  * Returns:
1494  *  0           on success
1495  * -EINTR       when interrupted by a signal
1496  */
1497 int __sched rt_mutex_lock_interruptible(struct rt_mutex *lock)
1498 {
1499         might_sleep();
1500 
1501         return rt_mutex_fastlock(lock, TASK_INTERRUPTIBLE, rt_mutex_slowlock);
1502 }
1503 EXPORT_SYMBOL_GPL(rt_mutex_lock_interruptible);
1504 
1505 /*
1506  * Futex variant, must not use fastpath.
1507  */
1508 int __sched rt_mutex_futex_trylock(struct rt_mutex *lock)
1509 {
1510         return rt_mutex_slowtrylock(lock);
1511 }
1512 
1513 /**
1514  * rt_mutex_timed_lock - lock a rt_mutex interruptible
1515  *                      the timeout structure is provided
1516  *                      by the caller
1517  *
1518  * @lock:               the rt_mutex to be locked
1519  * @timeout:            timeout structure or NULL (no timeout)
1520  *
1521  * Returns:
1522  *  0           on success
1523  * -EINTR       when interrupted by a signal
1524  * -ETIMEDOUT   when the timeout expired
1525  */
1526 int
1527 rt_mutex_timed_lock(struct rt_mutex *lock, struct hrtimer_sleeper *timeout)
1528 {
1529         might_sleep();
1530 
1531         return rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout,
1532                                        RT_MUTEX_MIN_CHAINWALK,
1533                                        rt_mutex_slowlock);
1534 }
1535 EXPORT_SYMBOL_GPL(rt_mutex_timed_lock);
1536 
1537 /**
1538  * rt_mutex_trylock - try to lock a rt_mutex
1539  *
1540  * @lock:       the rt_mutex to be locked
1541  *
1542  * This function can only be called in thread context. It's safe to
1543  * call it from atomic regions, but not from hard interrupt or soft
1544  * interrupt context.
1545  *
1546  * Returns 1 on success and 0 on contention
1547  */
1548 int __sched rt_mutex_trylock(struct rt_mutex *lock)
1549 {
1550         if (WARN_ON_ONCE(in_irq() || in_nmi() || in_serving_softirq()))
1551                 return 0;
1552 
1553         return rt_mutex_fasttrylock(lock, rt_mutex_slowtrylock);
1554 }
1555 EXPORT_SYMBOL_GPL(rt_mutex_trylock);
1556 
1557 /**
1558  * rt_mutex_unlock - unlock a rt_mutex
1559  *
1560  * @lock: the rt_mutex to be unlocked
1561  */
1562 void __sched rt_mutex_unlock(struct rt_mutex *lock)
1563 {
1564         rt_mutex_fastunlock(lock, rt_mutex_slowunlock);
1565 }
1566 EXPORT_SYMBOL_GPL(rt_mutex_unlock);
1567 
1568 /**
1569  * Futex variant, that since futex variants do not use the fast-path, can be
1570  * simple and will not need to retry.
1571  */
1572 bool __sched __rt_mutex_futex_unlock(struct rt_mutex *lock,
1573                                     struct wake_q_head *wake_q)
1574 {
1575         lockdep_assert_held(&lock->wait_lock);
1576 
1577         debug_rt_mutex_unlock(lock);
1578 
1579         if (!rt_mutex_has_waiters(lock)) {
1580                 lock->owner = NULL;
1581                 return false; /* done */
1582         }
1583 
1584         /*
1585          * We've already deboosted, mark_wakeup_next_waiter() will
1586          * retain preempt_disabled when we drop the wait_lock, to
1587          * avoid inversion prior to the wakeup.  preempt_disable()
1588          * therein pairs with rt_mutex_postunlock().
1589          */
1590         mark_wakeup_next_waiter(wake_q, lock);
1591 
1592         return true; /* call postunlock() */
1593 }
1594 
1595 void __sched rt_mutex_futex_unlock(struct rt_mutex *lock)
1596 {
1597         DEFINE_WAKE_Q(wake_q);
1598         bool postunlock;
1599 
1600         raw_spin_lock_irq(&lock->wait_lock);
1601         postunlock = __rt_mutex_futex_unlock(lock, &wake_q);
1602         raw_spin_unlock_irq(&lock->wait_lock);
1603 
1604         if (postunlock)
1605                 rt_mutex_postunlock(&wake_q);
1606 }
1607 
1608 /**
1609  * rt_mutex_destroy - mark a mutex unusable
1610  * @lock: the mutex to be destroyed
1611  *
1612  * This function marks the mutex uninitialized, and any subsequent
1613  * use of the mutex is forbidden. The mutex must not be locked when
1614  * this function is called.
1615  */
1616 void rt_mutex_destroy(struct rt_mutex *lock)
1617 {
1618         WARN_ON(rt_mutex_is_locked(lock));
1619 #ifdef CONFIG_DEBUG_RT_MUTEXES
1620         lock->magic = NULL;
1621 #endif
1622 }
1623 
1624 EXPORT_SYMBOL_GPL(rt_mutex_destroy);
1625 
1626 /**
1627  * __rt_mutex_init - initialize the rt lock
1628  *
1629  * @lock: the rt lock to be initialized
1630  *
1631  * Initialize the rt lock to unlocked state.
1632  *
1633  * Initializing of a locked rt lock is not allowed
1634  */
1635 void __rt_mutex_init(struct rt_mutex *lock, const char *name)
1636 {
1637         lock->owner = NULL;
1638         raw_spin_lock_init(&lock->wait_lock);
1639         lock->waiters = RB_ROOT;
1640         lock->waiters_leftmost = NULL;
1641 
1642         debug_rt_mutex_init(lock, name);
1643 }
1644 EXPORT_SYMBOL_GPL(__rt_mutex_init);
1645 
1646 /**
1647  * rt_mutex_init_proxy_locked - initialize and lock a rt_mutex on behalf of a
1648  *                              proxy owner
1649  *
1650  * @lock:       the rt_mutex to be locked
1651  * @proxy_owner:the task to set as owner
1652  *
1653  * No locking. Caller has to do serializing itself
1654  *
1655  * Special API call for PI-futex support. This initializes the rtmutex and
1656  * assigns it to @proxy_owner. Concurrent operations on the rtmutex are not
1657  * possible at this point because the pi_state which contains the rtmutex
1658  * is not yet visible to other tasks.
1659  */
1660 void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
1661                                 struct task_struct *proxy_owner)
1662 {
1663         __rt_mutex_init(lock, NULL);
1664         debug_rt_mutex_proxy_lock(lock, proxy_owner);
1665         rt_mutex_set_owner(lock, proxy_owner);
1666 }
1667 
1668 /**
1669  * rt_mutex_proxy_unlock - release a lock on behalf of owner
1670  *
1671  * @lock:       the rt_mutex to be locked
1672  *
1673  * No locking. Caller has to do serializing itself
1674  *
1675  * Special API call for PI-futex support. This merrily cleans up the rtmutex
1676  * (debugging) state. Concurrent operations on this rt_mutex are not
1677  * possible because it belongs to the pi_state which is about to be freed
1678  * and it is not longer visible to other tasks.
1679  */
1680 void rt_mutex_proxy_unlock(struct rt_mutex *lock,
1681                            struct task_struct *proxy_owner)
1682 {
1683         debug_rt_mutex_proxy_unlock(lock);
1684         rt_mutex_set_owner(lock, NULL);
1685 }
1686 
1687 int __rt_mutex_start_proxy_lock(struct rt_mutex *lock,
1688                               struct rt_mutex_waiter *waiter,
1689                               struct task_struct *task)
1690 {
1691         int ret;
1692 
1693         if (try_to_take_rt_mutex(lock, task, NULL))
1694                 return 1;
1695 
1696         /* We enforce deadlock detection for futexes */
1697         ret = task_blocks_on_rt_mutex(lock, waiter, task,
1698                                       RT_MUTEX_FULL_CHAINWALK);
1699 
1700         if (ret && !rt_mutex_owner(lock)) {
1701                 /*
1702                  * Reset the return value. We might have
1703                  * returned with -EDEADLK and the owner
1704                  * released the lock while we were walking the
1705                  * pi chain.  Let the waiter sort it out.
1706                  */
1707                 ret = 0;
1708         }
1709 
1710         if (unlikely(ret))
1711                 remove_waiter(lock, waiter);
1712 
1713         debug_rt_mutex_print_deadlock(waiter);
1714 
1715         return ret;
1716 }
1717 
1718 /**
1719  * rt_mutex_start_proxy_lock() - Start lock acquisition for another task
1720  * @lock:               the rt_mutex to take
1721  * @waiter:             the pre-initialized rt_mutex_waiter
1722  * @task:               the task to prepare
1723  *
1724  * Returns:
1725  *  0 - task blocked on lock
1726  *  1 - acquired the lock for task, caller should wake it up
1727  * <0 - error
1728  *
1729  * Special API call for FUTEX_REQUEUE_PI support.
1730  */
1731 int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
1732                               struct rt_mutex_waiter *waiter,
1733                               struct task_struct *task)
1734 {
1735         int ret;
1736 
1737         raw_spin_lock_irq(&lock->wait_lock);
1738         ret = __rt_mutex_start_proxy_lock(lock, waiter, task);
1739         raw_spin_unlock_irq(&lock->wait_lock);
1740 
1741         return ret;
1742 }
1743 
1744 /**
1745  * rt_mutex_next_owner - return the next owner of the lock
1746  *
1747  * @lock: the rt lock query
1748  *
1749  * Returns the next owner of the lock or NULL
1750  *
1751  * Caller has to serialize against other accessors to the lock
1752  * itself.
1753  *
1754  * Special API call for PI-futex support
1755  */
1756 struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock)
1757 {
1758         if (!rt_mutex_has_waiters(lock))
1759                 return NULL;
1760 
1761         return rt_mutex_top_waiter(lock)->task;
1762 }
1763 
1764 /**
1765  * rt_mutex_wait_proxy_lock() - Wait for lock acquisition
1766  * @lock:               the rt_mutex we were woken on
1767  * @to:                 the timeout, null if none. hrtimer should already have
1768  *                      been started.
1769  * @waiter:             the pre-initialized rt_mutex_waiter
1770  *
1771  * Wait for the the lock acquisition started on our behalf by
1772  * rt_mutex_start_proxy_lock(). Upon failure, the caller must call
1773  * rt_mutex_cleanup_proxy_lock().
1774  *
1775  * Returns:
1776  *  0 - success
1777  * <0 - error, one of -EINTR, -ETIMEDOUT
1778  *
1779  * Special API call for PI-futex support
1780  */
1781 int rt_mutex_wait_proxy_lock(struct rt_mutex *lock,
1782                                struct hrtimer_sleeper *to,
1783                                struct rt_mutex_waiter *waiter)
1784 {
1785         int ret;
1786 
1787         raw_spin_lock_irq(&lock->wait_lock);
1788         /* sleep on the mutex */
1789         set_current_state(TASK_INTERRUPTIBLE);
1790         ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter);
1791         /*
1792          * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might
1793          * have to fix that up.
1794          */
1795         fixup_rt_mutex_waiters(lock);
1796         raw_spin_unlock_irq(&lock->wait_lock);
1797 
1798         return ret;
1799 }
1800 
1801 /**
1802  * rt_mutex_cleanup_proxy_lock() - Cleanup failed lock acquisition
1803  * @lock:               the rt_mutex we were woken on
1804  * @waiter:             the pre-initialized rt_mutex_waiter
1805  *
1806  * Attempt to clean up after a failed rt_mutex_wait_proxy_lock().
1807  *
1808  * Unless we acquired the lock; we're still enqueued on the wait-list and can
1809  * in fact still be granted ownership until we're removed. Therefore we can
1810  * find we are in fact the owner and must disregard the
1811  * rt_mutex_wait_proxy_lock() failure.
1812  *
1813  * Returns:
1814  *  true  - did the cleanup, we done.
1815  *  false - we acquired the lock after rt_mutex_wait_proxy_lock() returned,
1816  *          caller should disregards its return value.
1817  *
1818  * Special API call for PI-futex support
1819  */
1820 bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock,
1821                                  struct rt_mutex_waiter *waiter)
1822 {
1823         bool cleanup = false;
1824 
1825         raw_spin_lock_irq(&lock->wait_lock);
1826         /*
1827          * Do an unconditional try-lock, this deals with the lock stealing
1828          * state where __rt_mutex_futex_unlock() -> mark_wakeup_next_waiter()
1829          * sets a NULL owner.
1830          *
1831          * We're not interested in the return value, because the subsequent
1832          * test on rt_mutex_owner() will infer that. If the trylock succeeded,
1833          * we will own the lock and it will have removed the waiter. If we
1834          * failed the trylock, we're still not owner and we need to remove
1835          * ourselves.
1836          */
1837         try_to_take_rt_mutex(lock, current, waiter);
1838         /*
1839          * Unless we're the owner; we're still enqueued on the wait_list.
1840          * So check if we became owner, if not, take us off the wait_list.
1841          */
1842         if (rt_mutex_owner(lock) != current) {
1843                 remove_waiter(lock, waiter);
1844                 cleanup = true;
1845         }
1846         /*
1847          * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might
1848          * have to fix that up.
1849          */
1850         fixup_rt_mutex_waiters(lock);
1851 
1852         raw_spin_unlock_irq(&lock->wait_lock);
1853 
1854         return cleanup;
1855 }
1856 

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