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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_root.rb_node;
275         struct rb_node *parent = NULL;
276         struct rt_mutex_waiter *entry;
277         bool leftmost = true;
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 = false;
287                 }
288         }
289 
290         rb_link_node(&waiter->tree_entry, parent, link);
291         rb_insert_color_cached(&waiter->tree_entry, &lock->waiters, leftmost);
292 }
293 
294 static void
295 rt_mutex_dequeue(struct rt_mutex *lock, struct rt_mutex_waiter *waiter)
296 {
297         if (RB_EMPTY_NODE(&waiter->tree_entry))
298                 return;
299 
300         rb_erase_cached(&waiter->tree_entry, &lock->waiters);
301         RB_CLEAR_NODE(&waiter->tree_entry);
302 }
303 
304 static void
305 rt_mutex_enqueue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
306 {
307         struct rb_node **link = &task->pi_waiters.rb_root.rb_node;
308         struct rb_node *parent = NULL;
309         struct rt_mutex_waiter *entry;
310         bool leftmost = true;
311 
312         while (*link) {
313                 parent = *link;
314                 entry = rb_entry(parent, struct rt_mutex_waiter, pi_tree_entry);
315                 if (rt_mutex_waiter_less(waiter, entry)) {
316                         link = &parent->rb_left;
317                 } else {
318                         link = &parent->rb_right;
319                         leftmost = false;
320                 }
321         }
322 
323         rb_link_node(&waiter->pi_tree_entry, parent, link);
324         rb_insert_color_cached(&waiter->pi_tree_entry, &task->pi_waiters, leftmost);
325 }
326 
327 static void
328 rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
329 {
330         if (RB_EMPTY_NODE(&waiter->pi_tree_entry))
331                 return;
332 
333         rb_erase_cached(&waiter->pi_tree_entry, &task->pi_waiters);
334         RB_CLEAR_NODE(&waiter->pi_tree_entry);
335 }
336 
337 static void rt_mutex_adjust_prio(struct task_struct *p)
338 {
339         struct task_struct *pi_task = NULL;
340 
341         lockdep_assert_held(&p->pi_lock);
342 
343         if (task_has_pi_waiters(p))
344                 pi_task = task_top_pi_waiter(p)->task;
345 
346         rt_mutex_setprio(p, pi_task);
347 }
348 
349 /*
350  * Deadlock detection is conditional:
351  *
352  * If CONFIG_DEBUG_RT_MUTEXES=n, deadlock detection is only conducted
353  * if the detect argument is == RT_MUTEX_FULL_CHAINWALK.
354  *
355  * If CONFIG_DEBUG_RT_MUTEXES=y, deadlock detection is always
356  * conducted independent of the detect argument.
357  *
358  * If the waiter argument is NULL this indicates the deboost path and
359  * deadlock detection is disabled independent of the detect argument
360  * and the config settings.
361  */
362 static bool rt_mutex_cond_detect_deadlock(struct rt_mutex_waiter *waiter,
363                                           enum rtmutex_chainwalk chwalk)
364 {
365         /*
366          * This is just a wrapper function for the following call,
367          * because debug_rt_mutex_detect_deadlock() smells like a magic
368          * debug feature and I wanted to keep the cond function in the
369          * main source file along with the comments instead of having
370          * two of the same in the headers.
371          */
372         return debug_rt_mutex_detect_deadlock(waiter, chwalk);
373 }
374 
375 /*
376  * Max number of times we'll walk the boosting chain:
377  */
378 int max_lock_depth = 1024;
379 
380 static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
381 {
382         return p->pi_blocked_on ? p->pi_blocked_on->lock : NULL;
383 }
384 
385 /*
386  * Adjust the priority chain. Also used for deadlock detection.
387  * Decreases task's usage by one - may thus free the task.
388  *
389  * @task:       the task owning the mutex (owner) for which a chain walk is
390  *              probably needed
391  * @chwalk:     do we have to carry out deadlock detection?
392  * @orig_lock:  the mutex (can be NULL if we are walking the chain to recheck
393  *              things for a task that has just got its priority adjusted, and
394  *              is waiting on a mutex)
395  * @next_lock:  the mutex on which the owner of @orig_lock was blocked before
396  *              we dropped its pi_lock. Is never dereferenced, only used for
397  *              comparison to detect lock chain changes.
398  * @orig_waiter: rt_mutex_waiter struct for the task that has just donated
399  *              its priority to the mutex owner (can be NULL in the case
400  *              depicted above or if the top waiter is gone away and we are
401  *              actually deboosting the owner)
402  * @top_task:   the current top waiter
403  *
404  * Returns 0 or -EDEADLK.
405  *
406  * Chain walk basics and protection scope
407  *
408  * [R] refcount on task
409  * [P] task->pi_lock held
410  * [L] rtmutex->wait_lock held
411  *
412  * Step Description                             Protected by
413  *      function arguments:
414  *      @task                                   [R]
415  *      @orig_lock if != NULL                   @top_task is blocked on it
416  *      @next_lock                              Unprotected. Cannot be
417  *                                              dereferenced. Only used for
418  *                                              comparison.
419  *      @orig_waiter if != NULL                 @top_task is blocked on it
420  *      @top_task                               current, or in case of proxy
421  *                                              locking protected by calling
422  *                                              code
423  *      again:
424  *        loop_sanity_check();
425  *      retry:
426  * [1]    lock(task->pi_lock);                  [R] acquire [P]
427  * [2]    waiter = task->pi_blocked_on;         [P]
428  * [3]    check_exit_conditions_1();            [P]
429  * [4]    lock = waiter->lock;                  [P]
430  * [5]    if (!try_lock(lock->wait_lock)) {     [P] try to acquire [L]
431  *          unlock(task->pi_lock);              release [P]
432  *          goto retry;
433  *        }
434  * [6]    check_exit_conditions_2();            [P] + [L]
435  * [7]    requeue_lock_waiter(lock, waiter);    [P] + [L]
436  * [8]    unlock(task->pi_lock);                release [P]
437  *        put_task_struct(task);                release [R]
438  * [9]    check_exit_conditions_3();            [L]
439  * [10]   task = owner(lock);                   [L]
440  *        get_task_struct(task);                [L] acquire [R]
441  *        lock(task->pi_lock);                  [L] acquire [P]
442  * [11]   requeue_pi_waiter(tsk, waiters(lock));[P] + [L]
443  * [12]   check_exit_conditions_4();            [P] + [L]
444  * [13]   unlock(task->pi_lock);                release [P]
445  *        unlock(lock->wait_lock);              release [L]
446  *        goto again;
447  */
448 static int rt_mutex_adjust_prio_chain(struct task_struct *task,
449                                       enum rtmutex_chainwalk chwalk,
450                                       struct rt_mutex *orig_lock,
451                                       struct rt_mutex *next_lock,
452                                       struct rt_mutex_waiter *orig_waiter,
453                                       struct task_struct *top_task)
454 {
455         struct rt_mutex_waiter *waiter, *top_waiter = orig_waiter;
456         struct rt_mutex_waiter *prerequeue_top_waiter;
457         int ret = 0, depth = 0;
458         struct rt_mutex *lock;
459         bool detect_deadlock;
460         bool requeue = true;
461 
462         detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter, chwalk);
463 
464         /*
465          * The (de)boosting is a step by step approach with a lot of
466          * pitfalls. We want this to be preemptible and we want hold a
467          * maximum of two locks per step. So we have to check
468          * carefully whether things change under us.
469          */
470  again:
471         /*
472          * We limit the lock chain length for each invocation.
473          */
474         if (++depth > max_lock_depth) {
475                 static int prev_max;
476 
477                 /*
478                  * Print this only once. If the admin changes the limit,
479                  * print a new message when reaching the limit again.
480                  */
481                 if (prev_max != max_lock_depth) {
482                         prev_max = max_lock_depth;
483                         printk(KERN_WARNING "Maximum lock depth %d reached "
484                                "task: %s (%d)\n", max_lock_depth,
485                                top_task->comm, task_pid_nr(top_task));
486                 }
487                 put_task_struct(task);
488 
489                 return -EDEADLK;
490         }
491 
492         /*
493          * We are fully preemptible here and only hold the refcount on
494          * @task. So everything can have changed under us since the
495          * caller or our own code below (goto retry/again) dropped all
496          * locks.
497          */
498  retry:
499         /*
500          * [1] Task cannot go away as we did a get_task() before !
501          */
502         raw_spin_lock_irq(&task->pi_lock);
503 
504         /*
505          * [2] Get the waiter on which @task is blocked on.
506          */
507         waiter = task->pi_blocked_on;
508 
509         /*
510          * [3] check_exit_conditions_1() protected by task->pi_lock.
511          */
512 
513         /*
514          * Check whether the end of the boosting chain has been
515          * reached or the state of the chain has changed while we
516          * dropped the locks.
517          */
518         if (!waiter)
519                 goto out_unlock_pi;
520 
521         /*
522          * Check the orig_waiter state. After we dropped the locks,
523          * the previous owner of the lock might have released the lock.
524          */
525         if (orig_waiter && !rt_mutex_owner(orig_lock))
526                 goto out_unlock_pi;
527 
528         /*
529          * We dropped all locks after taking a refcount on @task, so
530          * the task might have moved on in the lock chain or even left
531          * the chain completely and blocks now on an unrelated lock or
532          * on @orig_lock.
533          *
534          * We stored the lock on which @task was blocked in @next_lock,
535          * so we can detect the chain change.
536          */
537         if (next_lock != waiter->lock)
538                 goto out_unlock_pi;
539 
540         /*
541          * Drop out, when the task has no waiters. Note,
542          * top_waiter can be NULL, when we are in the deboosting
543          * mode!
544          */
545         if (top_waiter) {
546                 if (!task_has_pi_waiters(task))
547                         goto out_unlock_pi;
548                 /*
549                  * If deadlock detection is off, we stop here if we
550                  * are not the top pi waiter of the task. If deadlock
551                  * detection is enabled we continue, but stop the
552                  * requeueing in the chain walk.
553                  */
554                 if (top_waiter != task_top_pi_waiter(task)) {
555                         if (!detect_deadlock)
556                                 goto out_unlock_pi;
557                         else
558                                 requeue = false;
559                 }
560         }
561 
562         /*
563          * If the waiter priority is the same as the task priority
564          * then there is no further priority adjustment necessary.  If
565          * deadlock detection is off, we stop the chain walk. If its
566          * enabled we continue, but stop the requeueing in the chain
567          * walk.
568          */
569         if (rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
570                 if (!detect_deadlock)
571                         goto out_unlock_pi;
572                 else
573                         requeue = false;
574         }
575 
576         /*
577          * [4] Get the next lock
578          */
579         lock = waiter->lock;
580         /*
581          * [5] We need to trylock here as we are holding task->pi_lock,
582          * which is the reverse lock order versus the other rtmutex
583          * operations.
584          */
585         if (!raw_spin_trylock(&lock->wait_lock)) {
586                 raw_spin_unlock_irq(&task->pi_lock);
587                 cpu_relax();
588                 goto retry;
589         }
590 
591         /*
592          * [6] check_exit_conditions_2() protected by task->pi_lock and
593          * lock->wait_lock.
594          *
595          * Deadlock detection. If the lock is the same as the original
596          * lock which caused us to walk the lock chain or if the
597          * current lock is owned by the task which initiated the chain
598          * walk, we detected a deadlock.
599          */
600         if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
601                 debug_rt_mutex_deadlock(chwalk, orig_waiter, lock);
602                 raw_spin_unlock(&lock->wait_lock);
603                 ret = -EDEADLK;
604                 goto out_unlock_pi;
605         }
606 
607         /*
608          * If we just follow the lock chain for deadlock detection, no
609          * need to do all the requeue operations. To avoid a truckload
610          * of conditionals around the various places below, just do the
611          * minimum chain walk checks.
612          */
613         if (!requeue) {
614                 /*
615                  * No requeue[7] here. Just release @task [8]
616                  */
617                 raw_spin_unlock(&task->pi_lock);
618                 put_task_struct(task);
619 
620                 /*
621                  * [9] check_exit_conditions_3 protected by lock->wait_lock.
622                  * If there is no owner of the lock, end of chain.
623                  */
624                 if (!rt_mutex_owner(lock)) {
625                         raw_spin_unlock_irq(&lock->wait_lock);
626                         return 0;
627                 }
628 
629                 /* [10] Grab the next task, i.e. owner of @lock */
630                 task = rt_mutex_owner(lock);
631                 get_task_struct(task);
632                 raw_spin_lock(&task->pi_lock);
633 
634                 /*
635                  * No requeue [11] here. We just do deadlock detection.
636                  *
637                  * [12] Store whether owner is blocked
638                  * itself. Decision is made after dropping the locks
639                  */
640                 next_lock = task_blocked_on_lock(task);
641                 /*
642                  * Get the top waiter for the next iteration
643                  */
644                 top_waiter = rt_mutex_top_waiter(lock);
645 
646                 /* [13] Drop locks */
647                 raw_spin_unlock(&task->pi_lock);
648                 raw_spin_unlock_irq(&lock->wait_lock);
649 
650                 /* If owner is not blocked, end of chain. */
651                 if (!next_lock)
652                         goto out_put_task;
653                 goto again;
654         }
655 
656         /*
657          * Store the current top waiter before doing the requeue
658          * operation on @lock. We need it for the boost/deboost
659          * decision below.
660          */
661         prerequeue_top_waiter = rt_mutex_top_waiter(lock);
662 
663         /* [7] Requeue the waiter in the lock waiter tree. */
664         rt_mutex_dequeue(lock, waiter);
665 
666         /*
667          * Update the waiter prio fields now that we're dequeued.
668          *
669          * These values can have changed through either:
670          *
671          *   sys_sched_set_scheduler() / sys_sched_setattr()
672          *
673          * or
674          *
675          *   DL CBS enforcement advancing the effective deadline.
676          *
677          * Even though pi_waiters also uses these fields, and that tree is only
678          * updated in [11], we can do this here, since we hold [L], which
679          * serializes all pi_waiters access and rb_erase() does not care about
680          * the values of the node being removed.
681          */
682         waiter->prio = task->prio;
683         waiter->deadline = task->dl.deadline;
684 
685         rt_mutex_enqueue(lock, waiter);
686 
687         /* [8] Release the task */
688         raw_spin_unlock(&task->pi_lock);
689         put_task_struct(task);
690 
691         /*
692          * [9] check_exit_conditions_3 protected by lock->wait_lock.
693          *
694          * We must abort the chain walk if there is no lock owner even
695          * in the dead lock detection case, as we have nothing to
696          * follow here. This is the end of the chain we are walking.
697          */
698         if (!rt_mutex_owner(lock)) {
699                 /*
700                  * If the requeue [7] above changed the top waiter,
701                  * then we need to wake the new top waiter up to try
702                  * to get the lock.
703                  */
704                 if (prerequeue_top_waiter != rt_mutex_top_waiter(lock))
705                         wake_up_process(rt_mutex_top_waiter(lock)->task);
706                 raw_spin_unlock_irq(&lock->wait_lock);
707                 return 0;
708         }
709 
710         /* [10] Grab the next task, i.e. the owner of @lock */
711         task = rt_mutex_owner(lock);
712         get_task_struct(task);
713         raw_spin_lock(&task->pi_lock);
714 
715         /* [11] requeue the pi waiters if necessary */
716         if (waiter == rt_mutex_top_waiter(lock)) {
717                 /*
718                  * The waiter became the new top (highest priority)
719                  * waiter on the lock. Replace the previous top waiter
720                  * in the owner tasks pi waiters tree with this waiter
721                  * and adjust the priority of the owner.
722                  */
723                 rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
724                 rt_mutex_enqueue_pi(task, waiter);
725                 rt_mutex_adjust_prio(task);
726 
727         } else if (prerequeue_top_waiter == waiter) {
728                 /*
729                  * The waiter was the top waiter on the lock, but is
730                  * no longer the top prority waiter. Replace waiter in
731                  * the owner tasks pi waiters tree with the new top
732                  * (highest priority) waiter and adjust the priority
733                  * of the owner.
734                  * The new top waiter is stored in @waiter so that
735                  * @waiter == @top_waiter evaluates to true below and
736                  * we continue to deboost the rest of the chain.
737                  */
738                 rt_mutex_dequeue_pi(task, waiter);
739                 waiter = rt_mutex_top_waiter(lock);
740                 rt_mutex_enqueue_pi(task, waiter);
741                 rt_mutex_adjust_prio(task);
742         } else {
743                 /*
744                  * Nothing changed. No need to do any priority
745                  * adjustment.
746                  */
747         }
748 
749         /*
750          * [12] check_exit_conditions_4() protected by task->pi_lock
751          * and lock->wait_lock. The actual decisions are made after we
752          * dropped the locks.
753          *
754          * Check whether the task which owns the current lock is pi
755          * blocked itself. If yes we store a pointer to the lock for
756          * the lock chain change detection above. After we dropped
757          * task->pi_lock next_lock cannot be dereferenced anymore.
758          */
759         next_lock = task_blocked_on_lock(task);
760         /*
761          * Store the top waiter of @lock for the end of chain walk
762          * decision below.
763          */
764         top_waiter = rt_mutex_top_waiter(lock);
765 
766         /* [13] Drop the locks */
767         raw_spin_unlock(&task->pi_lock);
768         raw_spin_unlock_irq(&lock->wait_lock);
769 
770         /*
771          * Make the actual exit decisions [12], based on the stored
772          * values.
773          *
774          * We reached the end of the lock chain. Stop right here. No
775          * point to go back just to figure that out.
776          */
777         if (!next_lock)
778                 goto out_put_task;
779 
780         /*
781          * If the current waiter is not the top waiter on the lock,
782          * then we can stop the chain walk here if we are not in full
783          * deadlock detection mode.
784          */
785         if (!detect_deadlock && waiter != top_waiter)
786                 goto out_put_task;
787 
788         goto again;
789 
790  out_unlock_pi:
791         raw_spin_unlock_irq(&task->pi_lock);
792  out_put_task:
793         put_task_struct(task);
794 
795         return ret;
796 }
797 
798 /*
799  * Try to take an rt-mutex
800  *
801  * Must be called with lock->wait_lock held and interrupts disabled
802  *
803  * @lock:   The lock to be acquired.
804  * @task:   The task which wants to acquire the lock
805  * @waiter: The waiter that is queued to the lock's wait tree if the
806  *          callsite called task_blocked_on_lock(), otherwise NULL
807  */
808 static int try_to_take_rt_mutex(struct rt_mutex *lock, struct task_struct *task,
809                                 struct rt_mutex_waiter *waiter)
810 {
811         lockdep_assert_held(&lock->wait_lock);
812 
813         /*
814          * Before testing whether we can acquire @lock, we set the
815          * RT_MUTEX_HAS_WAITERS bit in @lock->owner. This forces all
816          * other tasks which try to modify @lock into the slow path
817          * and they serialize on @lock->wait_lock.
818          *
819          * The RT_MUTEX_HAS_WAITERS bit can have a transitional state
820          * as explained at the top of this file if and only if:
821          *
822          * - There is a lock owner. The caller must fixup the
823          *   transient state if it does a trylock or leaves the lock
824          *   function due to a signal or timeout.
825          *
826          * - @task acquires the lock and there are no other
827          *   waiters. This is undone in rt_mutex_set_owner(@task) at
828          *   the end of this function.
829          */
830         mark_rt_mutex_waiters(lock);
831 
832         /*
833          * If @lock has an owner, give up.
834          */
835         if (rt_mutex_owner(lock))
836                 return 0;
837 
838         /*
839          * If @waiter != NULL, @task has already enqueued the waiter
840          * into @lock waiter tree. If @waiter == NULL then this is a
841          * trylock attempt.
842          */
843         if (waiter) {
844                 /*
845                  * If waiter is not the highest priority waiter of
846                  * @lock, give up.
847                  */
848                 if (waiter != rt_mutex_top_waiter(lock))
849                         return 0;
850 
851                 /*
852                  * We can acquire the lock. Remove the waiter from the
853                  * lock waiters tree.
854                  */
855                 rt_mutex_dequeue(lock, waiter);
856 
857         } else {
858                 /*
859                  * If the lock has waiters already we check whether @task is
860                  * eligible to take over the lock.
861                  *
862                  * If there are no other waiters, @task can acquire
863                  * the lock.  @task->pi_blocked_on is NULL, so it does
864                  * not need to be dequeued.
865                  */
866                 if (rt_mutex_has_waiters(lock)) {
867                         /*
868                          * If @task->prio is greater than or equal to
869                          * the top waiter priority (kernel view),
870                          * @task lost.
871                          */
872                         if (!rt_mutex_waiter_less(task_to_waiter(task),
873                                                   rt_mutex_top_waiter(lock)))
874                                 return 0;
875 
876                         /*
877                          * The current top waiter stays enqueued. We
878                          * don't have to change anything in the lock
879                          * waiters order.
880                          */
881                 } else {
882                         /*
883                          * No waiters. Take the lock without the
884                          * pi_lock dance.@task->pi_blocked_on is NULL
885                          * and we have no waiters to enqueue in @task
886                          * pi waiters tree.
887                          */
888                         goto takeit;
889                 }
890         }
891 
892         /*
893          * Clear @task->pi_blocked_on. Requires protection by
894          * @task->pi_lock. Redundant operation for the @waiter == NULL
895          * case, but conditionals are more expensive than a redundant
896          * store.
897          */
898         raw_spin_lock(&task->pi_lock);
899         task->pi_blocked_on = NULL;
900         /*
901          * Finish the lock acquisition. @task is the new owner. If
902          * other waiters exist we have to insert the highest priority
903          * waiter into @task->pi_waiters tree.
904          */
905         if (rt_mutex_has_waiters(lock))
906                 rt_mutex_enqueue_pi(task, rt_mutex_top_waiter(lock));
907         raw_spin_unlock(&task->pi_lock);
908 
909 takeit:
910         /* We got the lock. */
911         debug_rt_mutex_lock(lock);
912 
913         /*
914          * This either preserves the RT_MUTEX_HAS_WAITERS bit if there
915          * are still waiters or clears it.
916          */
917         rt_mutex_set_owner(lock, task);
918 
919         return 1;
920 }
921 
922 /*
923  * Task blocks on lock.
924  *
925  * Prepare waiter and propagate pi chain
926  *
927  * This must be called with lock->wait_lock held and interrupts disabled
928  */
929 static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
930                                    struct rt_mutex_waiter *waiter,
931                                    struct task_struct *task,
932                                    enum rtmutex_chainwalk chwalk)
933 {
934         struct task_struct *owner = rt_mutex_owner(lock);
935         struct rt_mutex_waiter *top_waiter = waiter;
936         struct rt_mutex *next_lock;
937         int chain_walk = 0, res;
938 
939         lockdep_assert_held(&lock->wait_lock);
940 
941         /*
942          * Early deadlock detection. We really don't want the task to
943          * enqueue on itself just to untangle the mess later. It's not
944          * only an optimization. We drop the locks, so another waiter
945          * can come in before the chain walk detects the deadlock. So
946          * the other will detect the deadlock and return -EDEADLOCK,
947          * which is wrong, as the other waiter is not in a deadlock
948          * situation.
949          */
950         if (owner == task)
951                 return -EDEADLK;
952 
953         raw_spin_lock(&task->pi_lock);
954         waiter->task = task;
955         waiter->lock = lock;
956         waiter->prio = task->prio;
957         waiter->deadline = task->dl.deadline;
958 
959         /* Get the top priority waiter on the lock */
960         if (rt_mutex_has_waiters(lock))
961                 top_waiter = rt_mutex_top_waiter(lock);
962         rt_mutex_enqueue(lock, waiter);
963 
964         task->pi_blocked_on = waiter;
965 
966         raw_spin_unlock(&task->pi_lock);
967 
968         if (!owner)
969                 return 0;
970 
971         raw_spin_lock(&owner->pi_lock);
972         if (waiter == rt_mutex_top_waiter(lock)) {
973                 rt_mutex_dequeue_pi(owner, top_waiter);
974                 rt_mutex_enqueue_pi(owner, waiter);
975 
976                 rt_mutex_adjust_prio(owner);
977                 if (owner->pi_blocked_on)
978                         chain_walk = 1;
979         } else if (rt_mutex_cond_detect_deadlock(waiter, chwalk)) {
980                 chain_walk = 1;
981         }
982 
983         /* Store the lock on which owner is blocked or NULL */
984         next_lock = task_blocked_on_lock(owner);
985 
986         raw_spin_unlock(&owner->pi_lock);
987         /*
988          * Even if full deadlock detection is on, if the owner is not
989          * blocked itself, we can avoid finding this out in the chain
990          * walk.
991          */
992         if (!chain_walk || !next_lock)
993                 return 0;
994 
995         /*
996          * The owner can't disappear while holding a lock,
997          * so the owner struct is protected by wait_lock.
998          * Gets dropped in rt_mutex_adjust_prio_chain()!
999          */
1000         get_task_struct(owner);
1001 
1002         raw_spin_unlock_irq(&lock->wait_lock);
1003 
1004         res = rt_mutex_adjust_prio_chain(owner, chwalk, lock,
1005                                          next_lock, waiter, task);
1006 
1007         raw_spin_lock_irq(&lock->wait_lock);
1008 
1009         return res;
1010 }
1011 
1012 /*
1013  * Remove the top waiter from the current tasks pi waiter tree and
1014  * queue it up.
1015  *
1016  * Called with lock->wait_lock held and interrupts disabled.
1017  */
1018 static void mark_wakeup_next_waiter(struct wake_q_head *wake_q,
1019                                     struct rt_mutex *lock)
1020 {
1021         struct rt_mutex_waiter *waiter;
1022 
1023         raw_spin_lock(&current->pi_lock);
1024 
1025         waiter = rt_mutex_top_waiter(lock);
1026 
1027         /*
1028          * Remove it from current->pi_waiters and deboost.
1029          *
1030          * We must in fact deboost here in order to ensure we call
1031          * rt_mutex_setprio() to update p->pi_top_task before the
1032          * task unblocks.
1033          */
1034         rt_mutex_dequeue_pi(current, waiter);
1035         rt_mutex_adjust_prio(current);
1036 
1037         /*
1038          * As we are waking up the top waiter, and the waiter stays
1039          * queued on the lock until it gets the lock, this lock
1040          * obviously has waiters. Just set the bit here and this has
1041          * the added benefit of forcing all new tasks into the
1042          * slow path making sure no task of lower priority than
1043          * the top waiter can steal this lock.
1044          */
1045         lock->owner = (void *) RT_MUTEX_HAS_WAITERS;
1046 
1047         /*
1048          * We deboosted before waking the top waiter task such that we don't
1049          * run two tasks with the 'same' priority (and ensure the
1050          * p->pi_top_task pointer points to a blocked task). This however can
1051          * lead to priority inversion if we would get preempted after the
1052          * deboost but before waking our donor task, hence the preempt_disable()
1053          * before unlock.
1054          *
1055          * Pairs with preempt_enable() in rt_mutex_postunlock();
1056          */
1057         preempt_disable();
1058         wake_q_add(wake_q, waiter->task);
1059         raw_spin_unlock(&current->pi_lock);
1060 }
1061 
1062 /*
1063  * Remove a waiter from a lock and give up
1064  *
1065  * Must be called with lock->wait_lock held and interrupts disabled. I must
1066  * have just failed to try_to_take_rt_mutex().
1067  */
1068 static void remove_waiter(struct rt_mutex *lock,
1069                           struct rt_mutex_waiter *waiter)
1070 {
1071         bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
1072         struct task_struct *owner = rt_mutex_owner(lock);
1073         struct rt_mutex *next_lock;
1074 
1075         lockdep_assert_held(&lock->wait_lock);
1076 
1077         raw_spin_lock(&current->pi_lock);
1078         rt_mutex_dequeue(lock, waiter);
1079         current->pi_blocked_on = NULL;
1080         raw_spin_unlock(&current->pi_lock);
1081 
1082         /*
1083          * Only update priority if the waiter was the highest priority
1084          * waiter of the lock and there is an owner to update.
1085          */
1086         if (!owner || !is_top_waiter)
1087                 return;
1088 
1089         raw_spin_lock(&owner->pi_lock);
1090 
1091         rt_mutex_dequeue_pi(owner, waiter);
1092 
1093         if (rt_mutex_has_waiters(lock))
1094                 rt_mutex_enqueue_pi(owner, rt_mutex_top_waiter(lock));
1095 
1096         rt_mutex_adjust_prio(owner);
1097 
1098         /* Store the lock on which owner is blocked or NULL */
1099         next_lock = task_blocked_on_lock(owner);
1100 
1101         raw_spin_unlock(&owner->pi_lock);
1102 
1103         /*
1104          * Don't walk the chain, if the owner task is not blocked
1105          * itself.
1106          */
1107         if (!next_lock)
1108                 return;
1109 
1110         /* gets dropped in rt_mutex_adjust_prio_chain()! */
1111         get_task_struct(owner);
1112 
1113         raw_spin_unlock_irq(&lock->wait_lock);
1114 
1115         rt_mutex_adjust_prio_chain(owner, RT_MUTEX_MIN_CHAINWALK, lock,
1116                                    next_lock, NULL, current);
1117 
1118         raw_spin_lock_irq(&lock->wait_lock);
1119 }
1120 
1121 /*
1122  * Recheck the pi chain, in case we got a priority setting
1123  *
1124  * Called from sched_setscheduler
1125  */
1126 void rt_mutex_adjust_pi(struct task_struct *task)
1127 {
1128         struct rt_mutex_waiter *waiter;
1129         struct rt_mutex *next_lock;
1130         unsigned long flags;
1131 
1132         raw_spin_lock_irqsave(&task->pi_lock, flags);
1133 
1134         waiter = task->pi_blocked_on;
1135         if (!waiter || rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
1136                 raw_spin_unlock_irqrestore(&task->pi_lock, flags);
1137                 return;
1138         }
1139         next_lock = waiter->lock;
1140         raw_spin_unlock_irqrestore(&task->pi_lock, flags);
1141 
1142         /* gets dropped in rt_mutex_adjust_prio_chain()! */
1143         get_task_struct(task);
1144 
1145         rt_mutex_adjust_prio_chain(task, RT_MUTEX_MIN_CHAINWALK, NULL,
1146                                    next_lock, NULL, task);
1147 }
1148 
1149 void rt_mutex_init_waiter(struct rt_mutex_waiter *waiter)
1150 {
1151         debug_rt_mutex_init_waiter(waiter);
1152         RB_CLEAR_NODE(&waiter->pi_tree_entry);
1153         RB_CLEAR_NODE(&waiter->tree_entry);
1154         waiter->task = NULL;
1155 }
1156 
1157 /**
1158  * __rt_mutex_slowlock() - Perform the wait-wake-try-to-take loop
1159  * @lock:                the rt_mutex to take
1160  * @state:               the state the task should block in (TASK_INTERRUPTIBLE
1161  *                       or TASK_UNINTERRUPTIBLE)
1162  * @timeout:             the pre-initialized and started timer, or NULL for none
1163  * @waiter:              the pre-initialized rt_mutex_waiter
1164  *
1165  * Must be called with lock->wait_lock held and interrupts disabled
1166  */
1167 static int __sched
1168 __rt_mutex_slowlock(struct rt_mutex *lock, int state,
1169                     struct hrtimer_sleeper *timeout,
1170                     struct rt_mutex_waiter *waiter)
1171 {
1172         int ret = 0;
1173 
1174         for (;;) {
1175                 /* Try to acquire the lock: */
1176                 if (try_to_take_rt_mutex(lock, current, waiter))
1177                         break;
1178 
1179                 /*
1180                  * TASK_INTERRUPTIBLE checks for signals and
1181                  * timeout. Ignored otherwise.
1182                  */
1183                 if (likely(state == TASK_INTERRUPTIBLE)) {
1184                         /* Signal pending? */
1185                         if (signal_pending(current))
1186                                 ret = -EINTR;
1187                         if (timeout && !timeout->task)
1188                                 ret = -ETIMEDOUT;
1189                         if (ret)
1190                                 break;
1191                 }
1192 
1193                 raw_spin_unlock_irq(&lock->wait_lock);
1194 
1195                 debug_rt_mutex_print_deadlock(waiter);
1196 
1197                 schedule();
1198 
1199                 raw_spin_lock_irq(&lock->wait_lock);
1200                 set_current_state(state);
1201         }
1202 
1203         __set_current_state(TASK_RUNNING);
1204         return ret;
1205 }
1206 
1207 static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
1208                                      struct rt_mutex_waiter *w)
1209 {
1210         /*
1211          * If the result is not -EDEADLOCK or the caller requested
1212          * deadlock detection, nothing to do here.
1213          */
1214         if (res != -EDEADLOCK || detect_deadlock)
1215                 return;
1216 
1217         /*
1218          * Yell lowdly and stop the task right here.
1219          */
1220         rt_mutex_print_deadlock(w);
1221         while (1) {
1222                 set_current_state(TASK_INTERRUPTIBLE);
1223                 schedule();
1224         }
1225 }
1226 
1227 /*
1228  * Slow path lock function:
1229  */
1230 static int __sched
1231 rt_mutex_slowlock(struct rt_mutex *lock, int state,
1232                   struct hrtimer_sleeper *timeout,
1233                   enum rtmutex_chainwalk chwalk)
1234 {
1235         struct rt_mutex_waiter waiter;
1236         unsigned long flags;
1237         int ret = 0;
1238 
1239         rt_mutex_init_waiter(&waiter);
1240 
1241         /*
1242          * Technically we could use raw_spin_[un]lock_irq() here, but this can
1243          * be called in early boot if the cmpxchg() fast path is disabled
1244          * (debug, no architecture support). In this case we will acquire the
1245          * rtmutex with lock->wait_lock held. But we cannot unconditionally
1246          * enable interrupts in that early boot case. So we need to use the
1247          * irqsave/restore variants.
1248          */
1249         raw_spin_lock_irqsave(&lock->wait_lock, flags);
1250 
1251         /* Try to acquire the lock again: */
1252         if (try_to_take_rt_mutex(lock, current, NULL)) {
1253                 raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1254                 return 0;
1255         }
1256 
1257         set_current_state(state);
1258 
1259         /* Setup the timer, when timeout != NULL */
1260         if (unlikely(timeout))
1261                 hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
1262 
1263         ret = task_blocks_on_rt_mutex(lock, &waiter, current, chwalk);
1264 
1265         if (likely(!ret))
1266                 /* sleep on the mutex */
1267                 ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);
1268 
1269         if (unlikely(ret)) {
1270                 __set_current_state(TASK_RUNNING);
1271                 remove_waiter(lock, &waiter);
1272                 rt_mutex_handle_deadlock(ret, chwalk, &waiter);
1273         }
1274 
1275         /*
1276          * try_to_take_rt_mutex() sets the waiter bit
1277          * unconditionally. We might have to fix that up.
1278          */
1279         fixup_rt_mutex_waiters(lock);
1280 
1281         raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1282 
1283         /* Remove pending timer: */
1284         if (unlikely(timeout))
1285                 hrtimer_cancel(&timeout->timer);
1286 
1287         debug_rt_mutex_free_waiter(&waiter);
1288 
1289         return ret;
1290 }
1291 
1292 static inline int __rt_mutex_slowtrylock(struct rt_mutex *lock)
1293 {
1294         int ret = try_to_take_rt_mutex(lock, current, NULL);
1295 
1296         /*
1297          * try_to_take_rt_mutex() sets the lock waiters bit
1298          * unconditionally. Clean this up.
1299          */
1300         fixup_rt_mutex_waiters(lock);
1301 
1302         return ret;
1303 }
1304 
1305 /*
1306  * Slow path try-lock function:
1307  */
1308 static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
1309 {
1310         unsigned long flags;
1311         int ret;
1312 
1313         /*
1314          * If the lock already has an owner we fail to get the lock.
1315          * This can be done without taking the @lock->wait_lock as
1316          * it is only being read, and this is a trylock anyway.
1317          */
1318         if (rt_mutex_owner(lock))
1319                 return 0;
1320 
1321         /*
1322          * The mutex has currently no owner. Lock the wait lock and try to
1323          * acquire the lock. We use irqsave here to support early boot calls.
1324          */
1325         raw_spin_lock_irqsave(&lock->wait_lock, flags);
1326 
1327         ret = __rt_mutex_slowtrylock(lock);
1328 
1329         raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1330 
1331         return ret;
1332 }
1333 
1334 /*
1335  * Slow path to release a rt-mutex.
1336  *
1337  * Return whether the current task needs to call rt_mutex_postunlock().
1338  */
1339 static bool __sched rt_mutex_slowunlock(struct rt_mutex *lock,
1340                                         struct wake_q_head *wake_q)
1341 {
1342         unsigned long flags;
1343 
1344         /* irqsave required to support early boot calls */
1345         raw_spin_lock_irqsave(&lock->wait_lock, flags);
1346 
1347         debug_rt_mutex_unlock(lock);
1348 
1349         /*
1350          * We must be careful here if the fast path is enabled. If we
1351          * have no waiters queued we cannot set owner to NULL here
1352          * because of:
1353          *
1354          * foo->lock->owner = NULL;
1355          *                      rtmutex_lock(foo->lock);   <- fast path
1356          *                      free = atomic_dec_and_test(foo->refcnt);
1357          *                      rtmutex_unlock(foo->lock); <- fast path
1358          *                      if (free)
1359          *                              kfree(foo);
1360          * raw_spin_unlock(foo->lock->wait_lock);
1361          *
1362          * So for the fastpath enabled kernel:
1363          *
1364          * Nothing can set the waiters bit as long as we hold
1365          * lock->wait_lock. So we do the following sequence:
1366          *
1367          *      owner = rt_mutex_owner(lock);
1368          *      clear_rt_mutex_waiters(lock);
1369          *      raw_spin_unlock(&lock->wait_lock);
1370          *      if (cmpxchg(&lock->owner, owner, 0) == owner)
1371          *              return;
1372          *      goto retry;
1373          *
1374          * The fastpath disabled variant is simple as all access to
1375          * lock->owner is serialized by lock->wait_lock:
1376          *
1377          *      lock->owner = NULL;
1378          *      raw_spin_unlock(&lock->wait_lock);
1379          */
1380         while (!rt_mutex_has_waiters(lock)) {
1381                 /* Drops lock->wait_lock ! */
1382                 if (unlock_rt_mutex_safe(lock, flags) == true)
1383                         return false;
1384                 /* Relock the rtmutex and try again */
1385                 raw_spin_lock_irqsave(&lock->wait_lock, flags);
1386         }
1387 
1388         /*
1389          * The wakeup next waiter path does not suffer from the above
1390          * race. See the comments there.
1391          *
1392          * Queue the next waiter for wakeup once we release the wait_lock.
1393          */
1394         mark_wakeup_next_waiter(wake_q, lock);
1395         raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1396 
1397         return true; /* call rt_mutex_postunlock() */
1398 }
1399 
1400 /*
1401  * debug aware fast / slowpath lock,trylock,unlock
1402  *
1403  * The atomic acquire/release ops are compiled away, when either the
1404  * architecture does not support cmpxchg or when debugging is enabled.
1405  */
1406 static inline int
1407 rt_mutex_fastlock(struct rt_mutex *lock, int state,
1408                   int (*slowfn)(struct rt_mutex *lock, int state,
1409                                 struct hrtimer_sleeper *timeout,
1410                                 enum rtmutex_chainwalk chwalk))
1411 {
1412         if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
1413                 return 0;
1414 
1415         return slowfn(lock, state, NULL, RT_MUTEX_MIN_CHAINWALK);
1416 }
1417 
1418 static inline int
1419 rt_mutex_timed_fastlock(struct rt_mutex *lock, int state,
1420                         struct hrtimer_sleeper *timeout,
1421                         enum rtmutex_chainwalk chwalk,
1422                         int (*slowfn)(struct rt_mutex *lock, int state,
1423                                       struct hrtimer_sleeper *timeout,
1424                                       enum rtmutex_chainwalk chwalk))
1425 {
1426         if (chwalk == RT_MUTEX_MIN_CHAINWALK &&
1427             likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
1428                 return 0;
1429 
1430         return slowfn(lock, state, timeout, chwalk);
1431 }
1432 
1433 static inline int
1434 rt_mutex_fasttrylock(struct rt_mutex *lock,
1435                      int (*slowfn)(struct rt_mutex *lock))
1436 {
1437         if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
1438                 return 1;
1439 
1440         return slowfn(lock);
1441 }
1442 
1443 /*
1444  * Performs the wakeup of the the top-waiter and re-enables preemption.
1445  */
1446 void rt_mutex_postunlock(struct wake_q_head *wake_q)
1447 {
1448         wake_up_q(wake_q);
1449 
1450         /* Pairs with preempt_disable() in rt_mutex_slowunlock() */
1451         preempt_enable();
1452 }
1453 
1454 static inline void
1455 rt_mutex_fastunlock(struct rt_mutex *lock,
1456                     bool (*slowfn)(struct rt_mutex *lock,
1457                                    struct wake_q_head *wqh))
1458 {
1459         DEFINE_WAKE_Q(wake_q);
1460 
1461         if (likely(rt_mutex_cmpxchg_release(lock, current, NULL)))
1462                 return;
1463 
1464         if (slowfn(lock, &wake_q))
1465                 rt_mutex_postunlock(&wake_q);
1466 }
1467 
1468 static inline void __rt_mutex_lock(struct rt_mutex *lock, unsigned int subclass)
1469 {
1470         might_sleep();
1471 
1472         mutex_acquire(&lock->dep_map, subclass, 0, _RET_IP_);
1473         rt_mutex_fastlock(lock, TASK_UNINTERRUPTIBLE, rt_mutex_slowlock);
1474 }
1475 
1476 #ifdef CONFIG_DEBUG_LOCK_ALLOC
1477 /**
1478  * rt_mutex_lock_nested - lock a rt_mutex
1479  *
1480  * @lock: the rt_mutex to be locked
1481  * @subclass: the lockdep subclass
1482  */
1483 void __sched rt_mutex_lock_nested(struct rt_mutex *lock, unsigned int subclass)
1484 {
1485         __rt_mutex_lock(lock, subclass);
1486 }
1487 EXPORT_SYMBOL_GPL(rt_mutex_lock_nested);
1488 
1489 #else /* !CONFIG_DEBUG_LOCK_ALLOC */
1490 
1491 /**
1492  * rt_mutex_lock - lock a rt_mutex
1493  *
1494  * @lock: the rt_mutex to be locked
1495  */
1496 void __sched rt_mutex_lock(struct rt_mutex *lock)
1497 {
1498         __rt_mutex_lock(lock, 0);
1499 }
1500 EXPORT_SYMBOL_GPL(rt_mutex_lock);
1501 #endif
1502 
1503 /**
1504  * rt_mutex_lock_interruptible - lock a rt_mutex interruptible
1505  *
1506  * @lock:               the rt_mutex to be locked
1507  *
1508  * Returns:
1509  *  0           on success
1510  * -EINTR       when interrupted by a signal
1511  */
1512 int __sched rt_mutex_lock_interruptible(struct rt_mutex *lock)
1513 {
1514         int ret;
1515 
1516         might_sleep();
1517 
1518         mutex_acquire(&lock->dep_map, 0, 0, _RET_IP_);
1519         ret = rt_mutex_fastlock(lock, TASK_INTERRUPTIBLE, rt_mutex_slowlock);
1520         if (ret)
1521                 mutex_release(&lock->dep_map, 1, _RET_IP_);
1522 
1523         return ret;
1524 }
1525 EXPORT_SYMBOL_GPL(rt_mutex_lock_interruptible);
1526 
1527 /*
1528  * Futex variant, must not use fastpath.
1529  */
1530 int __sched rt_mutex_futex_trylock(struct rt_mutex *lock)
1531 {
1532         return rt_mutex_slowtrylock(lock);
1533 }
1534 
1535 int __sched __rt_mutex_futex_trylock(struct rt_mutex *lock)
1536 {
1537         return __rt_mutex_slowtrylock(lock);
1538 }
1539 
1540 /**
1541  * rt_mutex_timed_lock - lock a rt_mutex interruptible
1542  *                      the timeout structure is provided
1543  *                      by the caller
1544  *
1545  * @lock:               the rt_mutex to be locked
1546  * @timeout:            timeout structure or NULL (no timeout)
1547  *
1548  * Returns:
1549  *  0           on success
1550  * -EINTR       when interrupted by a signal
1551  * -ETIMEDOUT   when the timeout expired
1552  */
1553 int
1554 rt_mutex_timed_lock(struct rt_mutex *lock, struct hrtimer_sleeper *timeout)
1555 {
1556         int ret;
1557 
1558         might_sleep();
1559 
1560         mutex_acquire(&lock->dep_map, 0, 0, _RET_IP_);
1561         ret = rt_mutex_timed_fastlock(lock, TASK_INTERRUPTIBLE, timeout,
1562                                        RT_MUTEX_MIN_CHAINWALK,
1563                                        rt_mutex_slowlock);
1564         if (ret)
1565                 mutex_release(&lock->dep_map, 1, _RET_IP_);
1566 
1567         return ret;
1568 }
1569 EXPORT_SYMBOL_GPL(rt_mutex_timed_lock);
1570 
1571 /**
1572  * rt_mutex_trylock - try to lock a rt_mutex
1573  *
1574  * @lock:       the rt_mutex to be locked
1575  *
1576  * This function can only be called in thread context. It's safe to
1577  * call it from atomic regions, but not from hard interrupt or soft
1578  * interrupt context.
1579  *
1580  * Returns 1 on success and 0 on contention
1581  */
1582 int __sched rt_mutex_trylock(struct rt_mutex *lock)
1583 {
1584         int ret;
1585 
1586         if (WARN_ON_ONCE(in_irq() || in_nmi() || in_serving_softirq()))
1587                 return 0;
1588 
1589         ret = rt_mutex_fasttrylock(lock, rt_mutex_slowtrylock);
1590         if (ret)
1591                 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
1592 
1593         return ret;
1594 }
1595 EXPORT_SYMBOL_GPL(rt_mutex_trylock);
1596 
1597 /**
1598  * rt_mutex_unlock - unlock a rt_mutex
1599  *
1600  * @lock: the rt_mutex to be unlocked
1601  */
1602 void __sched rt_mutex_unlock(struct rt_mutex *lock)
1603 {
1604         mutex_release(&lock->dep_map, 1, _RET_IP_);
1605         rt_mutex_fastunlock(lock, rt_mutex_slowunlock);
1606 }
1607 EXPORT_SYMBOL_GPL(rt_mutex_unlock);
1608 
1609 /**
1610  * Futex variant, that since futex variants do not use the fast-path, can be
1611  * simple and will not need to retry.
1612  */
1613 bool __sched __rt_mutex_futex_unlock(struct rt_mutex *lock,
1614                                     struct wake_q_head *wake_q)
1615 {
1616         lockdep_assert_held(&lock->wait_lock);
1617 
1618         debug_rt_mutex_unlock(lock);
1619 
1620         if (!rt_mutex_has_waiters(lock)) {
1621                 lock->owner = NULL;
1622                 return false; /* done */
1623         }
1624 
1625         /*
1626          * We've already deboosted, mark_wakeup_next_waiter() will
1627          * retain preempt_disabled when we drop the wait_lock, to
1628          * avoid inversion prior to the wakeup.  preempt_disable()
1629          * therein pairs with rt_mutex_postunlock().
1630          */
1631         mark_wakeup_next_waiter(wake_q, lock);
1632 
1633         return true; /* call postunlock() */
1634 }
1635 
1636 void __sched rt_mutex_futex_unlock(struct rt_mutex *lock)
1637 {
1638         DEFINE_WAKE_Q(wake_q);
1639         unsigned long flags;
1640         bool postunlock;
1641 
1642         raw_spin_lock_irqsave(&lock->wait_lock, flags);
1643         postunlock = __rt_mutex_futex_unlock(lock, &wake_q);
1644         raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1645 
1646         if (postunlock)
1647                 rt_mutex_postunlock(&wake_q);
1648 }
1649 
1650 /**
1651  * rt_mutex_destroy - mark a mutex unusable
1652  * @lock: the mutex to be destroyed
1653  *
1654  * This function marks the mutex uninitialized, and any subsequent
1655  * use of the mutex is forbidden. The mutex must not be locked when
1656  * this function is called.
1657  */
1658 void rt_mutex_destroy(struct rt_mutex *lock)
1659 {
1660         WARN_ON(rt_mutex_is_locked(lock));
1661 #ifdef CONFIG_DEBUG_RT_MUTEXES
1662         lock->magic = NULL;
1663 #endif
1664 }
1665 EXPORT_SYMBOL_GPL(rt_mutex_destroy);
1666 
1667 /**
1668  * __rt_mutex_init - initialize the rt lock
1669  *
1670  * @lock: the rt lock to be initialized
1671  *
1672  * Initialize the rt lock to unlocked state.
1673  *
1674  * Initializing of a locked rt lock is not allowed
1675  */
1676 void __rt_mutex_init(struct rt_mutex *lock, const char *name,
1677                      struct lock_class_key *key)
1678 {
1679         lock->owner = NULL;
1680         raw_spin_lock_init(&lock->wait_lock);
1681         lock->waiters = RB_ROOT_CACHED;
1682 
1683         if (name && key)
1684                 debug_rt_mutex_init(lock, name, key);
1685 }
1686 EXPORT_SYMBOL_GPL(__rt_mutex_init);
1687 
1688 /**
1689  * rt_mutex_init_proxy_locked - initialize and lock a rt_mutex on behalf of a
1690  *                              proxy owner
1691  *
1692  * @lock:       the rt_mutex to be locked
1693  * @proxy_owner:the task to set as owner
1694  *
1695  * No locking. Caller has to do serializing itself
1696  *
1697  * Special API call for PI-futex support. This initializes the rtmutex and
1698  * assigns it to @proxy_owner. Concurrent operations on the rtmutex are not
1699  * possible at this point because the pi_state which contains the rtmutex
1700  * is not yet visible to other tasks.
1701  */
1702 void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
1703                                 struct task_struct *proxy_owner)
1704 {
1705         __rt_mutex_init(lock, NULL, NULL);
1706         debug_rt_mutex_proxy_lock(lock, proxy_owner);
1707         rt_mutex_set_owner(lock, proxy_owner);
1708 }
1709 
1710 /**
1711  * rt_mutex_proxy_unlock - release a lock on behalf of owner
1712  *
1713  * @lock:       the rt_mutex to be locked
1714  *
1715  * No locking. Caller has to do serializing itself
1716  *
1717  * Special API call for PI-futex support. This merrily cleans up the rtmutex
1718  * (debugging) state. Concurrent operations on this rt_mutex are not
1719  * possible because it belongs to the pi_state which is about to be freed
1720  * and it is not longer visible to other tasks.
1721  */
1722 void rt_mutex_proxy_unlock(struct rt_mutex *lock,
1723                            struct task_struct *proxy_owner)
1724 {
1725         debug_rt_mutex_proxy_unlock(lock);
1726         rt_mutex_set_owner(lock, NULL);
1727 }
1728 
1729 /**
1730  * __rt_mutex_start_proxy_lock() - Start lock acquisition for another task
1731  * @lock:               the rt_mutex to take
1732  * @waiter:             the pre-initialized rt_mutex_waiter
1733  * @task:               the task to prepare
1734  *
1735  * Starts the rt_mutex acquire; it enqueues the @waiter and does deadlock
1736  * detection. It does not wait, see rt_mutex_wait_proxy_lock() for that.
1737  *
1738  * NOTE: does _NOT_ remove the @waiter on failure; must either call
1739  * rt_mutex_wait_proxy_lock() or rt_mutex_cleanup_proxy_lock() after this.
1740  *
1741  * Returns:
1742  *  0 - task blocked on lock
1743  *  1 - acquired the lock for task, caller should wake it up
1744  * <0 - error
1745  *
1746  * Special API call for PI-futex support.
1747  */
1748 int __rt_mutex_start_proxy_lock(struct rt_mutex *lock,
1749                               struct rt_mutex_waiter *waiter,
1750                               struct task_struct *task)
1751 {
1752         int ret;
1753 
1754         lockdep_assert_held(&lock->wait_lock);
1755 
1756         if (try_to_take_rt_mutex(lock, task, NULL))
1757                 return 1;
1758 
1759         /* We enforce deadlock detection for futexes */
1760         ret = task_blocks_on_rt_mutex(lock, waiter, task,
1761                                       RT_MUTEX_FULL_CHAINWALK);
1762 
1763         if (ret && !rt_mutex_owner(lock)) {
1764                 /*
1765                  * Reset the return value. We might have
1766                  * returned with -EDEADLK and the owner
1767                  * released the lock while we were walking the
1768                  * pi chain.  Let the waiter sort it out.
1769                  */
1770                 ret = 0;
1771         }
1772 
1773         debug_rt_mutex_print_deadlock(waiter);
1774 
1775         return ret;
1776 }
1777 
1778 /**
1779  * rt_mutex_start_proxy_lock() - Start lock acquisition for another task
1780  * @lock:               the rt_mutex to take
1781  * @waiter:             the pre-initialized rt_mutex_waiter
1782  * @task:               the task to prepare
1783  *
1784  * Starts the rt_mutex acquire; it enqueues the @waiter and does deadlock
1785  * detection. It does not wait, see rt_mutex_wait_proxy_lock() for that.
1786  *
1787  * NOTE: unlike __rt_mutex_start_proxy_lock this _DOES_ remove the @waiter
1788  * on failure.
1789  *
1790  * Returns:
1791  *  0 - task blocked on lock
1792  *  1 - acquired the lock for task, caller should wake it up
1793  * <0 - error
1794  *
1795  * Special API call for PI-futex support.
1796  */
1797 int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
1798                               struct rt_mutex_waiter *waiter,
1799                               struct task_struct *task)
1800 {
1801         int ret;
1802 
1803         raw_spin_lock_irq(&lock->wait_lock);
1804         ret = __rt_mutex_start_proxy_lock(lock, waiter, task);
1805         if (unlikely(ret))
1806                 remove_waiter(lock, waiter);
1807         raw_spin_unlock_irq(&lock->wait_lock);
1808 
1809         return ret;
1810 }
1811 
1812 /**
1813  * rt_mutex_next_owner - return the next owner of the lock
1814  *
1815  * @lock: the rt lock query
1816  *
1817  * Returns the next owner of the lock or NULL
1818  *
1819  * Caller has to serialize against other accessors to the lock
1820  * itself.
1821  *
1822  * Special API call for PI-futex support
1823  */
1824 struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock)
1825 {
1826         if (!rt_mutex_has_waiters(lock))
1827                 return NULL;
1828 
1829         return rt_mutex_top_waiter(lock)->task;
1830 }
1831 
1832 /**
1833  * rt_mutex_wait_proxy_lock() - Wait for lock acquisition
1834  * @lock:               the rt_mutex we were woken on
1835  * @to:                 the timeout, null if none. hrtimer should already have
1836  *                      been started.
1837  * @waiter:             the pre-initialized rt_mutex_waiter
1838  *
1839  * Wait for the the lock acquisition started on our behalf by
1840  * rt_mutex_start_proxy_lock(). Upon failure, the caller must call
1841  * rt_mutex_cleanup_proxy_lock().
1842  *
1843  * Returns:
1844  *  0 - success
1845  * <0 - error, one of -EINTR, -ETIMEDOUT
1846  *
1847  * Special API call for PI-futex support
1848  */
1849 int rt_mutex_wait_proxy_lock(struct rt_mutex *lock,
1850                                struct hrtimer_sleeper *to,
1851                                struct rt_mutex_waiter *waiter)
1852 {
1853         int ret;
1854 
1855         raw_spin_lock_irq(&lock->wait_lock);
1856         /* sleep on the mutex */
1857         set_current_state(TASK_INTERRUPTIBLE);
1858         ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter);
1859         /*
1860          * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might
1861          * have to fix that up.
1862          */
1863         fixup_rt_mutex_waiters(lock);
1864         raw_spin_unlock_irq(&lock->wait_lock);
1865 
1866         return ret;
1867 }
1868 
1869 /**
1870  * rt_mutex_cleanup_proxy_lock() - Cleanup failed lock acquisition
1871  * @lock:               the rt_mutex we were woken on
1872  * @waiter:             the pre-initialized rt_mutex_waiter
1873  *
1874  * Attempt to clean up after a failed __rt_mutex_start_proxy_lock() or
1875  * rt_mutex_wait_proxy_lock().
1876  *
1877  * Unless we acquired the lock; we're still enqueued on the wait-list and can
1878  * in fact still be granted ownership until we're removed. Therefore we can
1879  * find we are in fact the owner and must disregard the
1880  * rt_mutex_wait_proxy_lock() failure.
1881  *
1882  * Returns:
1883  *  true  - did the cleanup, we done.
1884  *  false - we acquired the lock after rt_mutex_wait_proxy_lock() returned,
1885  *          caller should disregards its return value.
1886  *
1887  * Special API call for PI-futex support
1888  */
1889 bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock,
1890                                  struct rt_mutex_waiter *waiter)
1891 {
1892         bool cleanup = false;
1893 
1894         raw_spin_lock_irq(&lock->wait_lock);
1895         /*
1896          * Do an unconditional try-lock, this deals with the lock stealing
1897          * state where __rt_mutex_futex_unlock() -> mark_wakeup_next_waiter()
1898          * sets a NULL owner.
1899          *
1900          * We're not interested in the return value, because the subsequent
1901          * test on rt_mutex_owner() will infer that. If the trylock succeeded,
1902          * we will own the lock and it will have removed the waiter. If we
1903          * failed the trylock, we're still not owner and we need to remove
1904          * ourselves.
1905          */
1906         try_to_take_rt_mutex(lock, current, waiter);
1907         /*
1908          * Unless we're the owner; we're still enqueued on the wait_list.
1909          * So check if we became owner, if not, take us off the wait_list.
1910          */
1911         if (rt_mutex_owner(lock) != current) {
1912                 remove_waiter(lock, waiter);
1913                 cleanup = true;
1914         }
1915         /*
1916          * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might
1917          * have to fix that up.
1918          */
1919         fixup_rt_mutex_waiters(lock);
1920 
1921         raw_spin_unlock_irq(&lock->wait_lock);
1922 
1923         return cleanup;
1924 }
1925 

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