summaryrefslogtreecommitdiff
path: root/absl/synchronization/mutex.h
blob: 368684bf2e27a2bebc19b8e7018db9f71a92159c (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// mutex.h
// -----------------------------------------------------------------------------
//
// This header file defines a `Mutex` -- a mutually exclusive lock -- and the
// most common type of synchronization primitive for facilitating locks on
// shared resources. A mutex is used to prevent multiple threads from accessing
// and/or writing to a shared resource concurrently.
//
// Unlike a `std::mutex`, the Abseil `Mutex` provides the following additional
// features:
//   * Conditional predicates intrinsic to the `Mutex` object
//   * Reader/writer locks, in addition to standard exclusive/writer locks
//   * Deadlock detection and debug support.
//
// The following helper classes are also defined within this file:
//
//  MutexLock - An RAII wrapper to acquire and release a `Mutex` for exclusive/
//              write access within the current scope.
//  ReaderMutexLock
//            - An RAII wrapper to acquire and release a `Mutex` for shared/read
//              access within the current scope.
//
//  WriterMutexLock
//            - Alias for `MutexLock` above, designed for use in distinguishing
//              reader and writer locks within code.
//
// In addition to simple mutex locks, this file also defines ways to perform
// locking under certain conditions.
//
//  Condition   - (Preferred) Used to wait for a particular predicate that
//                depends on state protected by the `Mutex` to become true.
//  CondVar     - A lower-level variant of `Condition` that relies on
//                application code to explicitly signal the `CondVar` when
//                a condition has been met.
//
// See below for more information on using `Condition` or `CondVar`.
//
// Mutexes and mutex behavior can be quite complicated. The information within
// this header file is limited, as a result. Please consult the Mutex guide for
// more complete information and examples.

#ifndef ABSL_SYNCHRONIZATION_MUTEX_H_
#define ABSL_SYNCHRONIZATION_MUTEX_H_

#include <atomic>
#include <cstdint>
#include <string>

#include "absl/base/internal/identity.h"
#include "absl/base/internal/low_level_alloc.h"
#include "absl/base/internal/thread_identity.h"
#include "absl/base/internal/tsan_mutex_interface.h"
#include "absl/base/port.h"
#include "absl/base/thread_annotations.h"
#include "absl/synchronization/internal/kernel_timeout.h"
#include "absl/synchronization/internal/per_thread_sem.h"
#include "absl/time/time.h"

// Decide if we should use the non-production implementation because
// the production implementation hasn't been fully ported yet.
#ifdef ABSL_INTERNAL_USE_NONPROD_MUTEX
#error ABSL_INTERNAL_USE_NONPROD_MUTEX cannot be directly set
#elif defined(ABSL_LOW_LEVEL_ALLOC_MISSING)
#define ABSL_INTERNAL_USE_NONPROD_MUTEX 1
#include "absl/synchronization/internal/mutex_nonprod.inc"
#endif

namespace absl {

class Condition;
struct SynchWaitParams;

// -----------------------------------------------------------------------------
// Mutex
// -----------------------------------------------------------------------------
//
// A `Mutex` is a non-reentrant (aka non-recursive) Mutually Exclusive lock
// on some resource, typically a variable or data structure with associated
// invariants. Proper usage of mutexes prevents concurrent access by different
// threads to the same resource.
//
// A `Mutex` has two basic operations: `Mutex::Lock()` and `Mutex::Unlock()`.
// The `Lock()` operation *acquires* a `Mutex` (in a state known as an
// *exclusive* -- or write -- lock), while the `Unlock()` operation *releases* a
// Mutex. During the span of time between the Lock() and Unlock() operations,
// a mutex is said to be *held*. By design all mutexes support exclusive/write
// locks, as this is the most common way to use a mutex.
//
// The `Mutex` state machine for basic lock/unlock operations is quite simple:
//
// |                | Lock()     | Unlock() |
// |----------------+------------+----------|
// | Free           | Exclusive  | invalid  |
// | Exclusive      | blocks     | Free     |
//
// Attempts to `Unlock()` must originate from the thread that performed the
// corresponding `Lock()` operation.
//
// An "invalid" operation is disallowed by the API. The `Mutex` implementation
// is allowed to do anything on an invalid call, including but not limited to
// crashing with a useful error message, silently succeeding, or corrupting
// data structures. In debug mode, the implementation attempts to crash with a
// useful error message.
//
// `Mutex` is not guaranteed to be "fair" in prioritizing waiting threads; it
// is, however, approximately fair over long periods, and starvation-free for
// threads at the same priority.
//
// The lock/unlock primitives are now annotated with lock annotations
// defined in (base/thread_annotations.h). When writing multi-threaded code,
// you should use lock annotations whenever possible to document your lock
// synchronization policy. Besides acting as documentation, these annotations
// also help compilers or static analysis tools to identify and warn about
// issues that could potentially result in race conditions and deadlocks.
//
// For more information about the lock annotations, please see
// [Thread Safety Analysis](http://clang.llvm.org/docs/ThreadSafetyAnalysis.html)
// in the Clang documentation.
//
// See also `MutexLock`, below, for scoped `Mutex` acquisition.

class LOCKABLE Mutex {
 public:
  Mutex();
  ~Mutex();

  // Mutex::Lock()
  //
  // Blocks the calling thread, if necessary, until this `Mutex` is free, and
  // then acquires it exclusively. (This lock is also known as a "write lock.")
  void Lock() EXCLUSIVE_LOCK_FUNCTION();

  // Mutex::Unlock()
  //
  // Releases this `Mutex` and returns it from the exclusive/write state to the
  // free state. Caller must hold the `Mutex` exclusively.
  void Unlock() UNLOCK_FUNCTION();

  // Mutex::TryLock()
  //
  // If the mutex can be acquired without blocking, does so exclusively and
  // returns `true`. Otherwise, returns `false`. Returns `true` with high
  // probability if the `Mutex` was free.
  bool TryLock() EXCLUSIVE_TRYLOCK_FUNCTION(true);

  // Mutex::AssertHeld()
  //
  // Return immediately if this thread holds the `Mutex` exclusively (in write
  // mode). Otherwise, may report an error (typically by crashing with a
  // diagnostic), or may return immediately.
  void AssertHeld() const ASSERT_EXCLUSIVE_LOCK();

  // ---------------------------------------------------------------------------
  // Reader-Writer Locking
  // ---------------------------------------------------------------------------

  // A Mutex can also be used as a starvation-free reader-writer lock.
  // Neither read-locks nor write-locks are reentrant/recursive to avoid
  // potential client programming errors.
  //
  // The Mutex API provides `Writer*()` aliases for the existing `Lock()`,
  // `Unlock()` and `TryLock()` methods for use within applications mixing
  // reader/writer locks. Using `Reader*()` and `Writer*()` operations in this
  // manner can make locking behavior clearer when mixing read and write modes.
  //
  // Introducing reader locks necessarily complicates the `Mutex` state
  // machine somewhat. The table below illustrates the allowed state transitions
  // of a mutex in such cases. Note that ReaderLock() may block even if the lock
  // is held in shared mode; this occurs when another thread is blocked on a
  // call to WriterLock().
  //
  // ---------------------------------------------------------------------------
  //     Operation: WriterLock() Unlock()  ReaderLock()           ReaderUnlock()
  // ---------------------------------------------------------------------------
  // State
  // ---------------------------------------------------------------------------
  // Free           Exclusive    invalid   Shared(1)              invalid
  // Shared(1)      blocks       invalid   Shared(2) or blocks    Free
  // Shared(n) n>1  blocks       invalid   Shared(n+1) or blocks  Shared(n-1)
  // Exclusive      blocks       Free      blocks                 invalid
  // ---------------------------------------------------------------------------
  //
  // In comments below, "shared" refers to a state of Shared(n) for any n > 0.

  // Mutex::ReaderLock()
  //
  // Blocks the calling thread, if necessary, until this `Mutex` is either free,
  // or in shared mode, and then acquires a share of it. Note that
  // `ReaderLock()` will block if some other thread has an exclusive/writer lock
  // on the mutex.

  void ReaderLock() SHARED_LOCK_FUNCTION();

  // Mutex::ReaderUnlock()
  //
  // Releases a read share of this `Mutex`. `ReaderUnlock` may return a mutex to
  // the free state if this thread holds the last reader lock on the mutex. Note
  // that you cannot call `ReaderUnlock()` on a mutex held in write mode.
  void ReaderUnlock() UNLOCK_FUNCTION();

  // Mutex::ReaderTryLock()
  //
  // If the mutex can be acquired without blocking, acquires this mutex for
  // shared access and returns `true`. Otherwise, returns `false`. Returns
  // `true` with high probability if the `Mutex` was free or shared.
  bool ReaderTryLock() SHARED_TRYLOCK_FUNCTION(true);

  // Mutex::AssertReaderHeld()
  //
  // Returns immediately if this thread holds the `Mutex` in at least shared
  // mode (read mode). Otherwise, may report an error (typically by
  // crashing with a diagnostic), or may return immediately.
  void AssertReaderHeld() const ASSERT_SHARED_LOCK();

  // Mutex::WriterLock()
  // Mutex::WriterUnlock()
  // Mutex::WriterTryLock()
  //
  // Aliases for `Mutex::Lock()`, `Mutex::Unlock()`, and `Mutex::TryLock()`.
  //
  // These methods may be used (along with the complementary `Reader*()`
  // methods) to distingish simple exclusive `Mutex` usage (`Lock()`,
  // etc.) from reader/writer lock usage.
  void WriterLock() EXCLUSIVE_LOCK_FUNCTION() { this->Lock(); }

  void WriterUnlock() UNLOCK_FUNCTION() { this->Unlock(); }

  bool WriterTryLock() EXCLUSIVE_TRYLOCK_FUNCTION(true) {
    return this->TryLock();
  }

  // ---------------------------------------------------------------------------
  // Conditional Critical Regions
  // ---------------------------------------------------------------------------

  // Conditional usage of a `Mutex` can occur using two distinct paradigms:
  //
  //   * Use of `Mutex` member functions with `Condition` objects.
  //   * Use of the separate `CondVar` abstraction.
  //
  // In general, prefer use of `Condition` and the `Mutex` member functions
  // listed below over `CondVar`. When there are multiple threads waiting on
  // distinctly different conditions, however, a battery of `CondVar`s may be
  // more efficient. This section discusses use of `Condition` objects.
  //
  // `Mutex` contains member functions for performing lock operations only under
  // certain conditions, of class `Condition`. For correctness, the `Condition`
  // must return a boolean that is a pure function, only of state protected by
  // the `Mutex`. The condition must be invariant w.r.t. environmental state
  // such as thread, cpu id, or time, and must be `noexcept`. The condition will
  // always be invoked with the mutex held in at least read mode, so you should
  // not block it for long periods or sleep it on a timer.
  //
  // Since a condition must not depend directly on the current time, use
  // `*WithTimeout()` member function variants to make your condition
  // effectively true after a given duration, or `*WithDeadline()` variants to
  // make your condition effectively true after a given time.
  //
  // The condition function should have no side-effects aside from debug
  // logging; as a special exception, the function may acquire other mutexes
  // provided it releases all those that it acquires.  (This exception was
  // required to allow logging.)

  // Mutex::Await()
  //
  // Unlocks this `Mutex` and blocks until simultaneously both `cond` is `true`
  // and this `Mutex` can be reacquired, then reacquires this `Mutex` in the
  // same mode in which it was previously held. If the condition is initially
  // `true`, `Await()` *may* skip the release/re-acquire step.
  //
  // `Await()` requires that this thread holds this `Mutex` in some mode.
  void Await(const Condition &cond);

  // Mutex::LockWhen()
  // Mutex::ReaderLockWhen()
  // Mutex::WriterLockWhen()
  //
  // Blocks until simultaneously both `cond` is `true` and this` Mutex` can
  // be acquired, then atomically acquires this `Mutex`. `LockWhen()` is
  // logically equivalent to `*Lock(); Await();` though they may have different
  // performance characteristics.
  void LockWhen(const Condition &cond) EXCLUSIVE_LOCK_FUNCTION();

  void ReaderLockWhen(const Condition &cond) SHARED_LOCK_FUNCTION();

  void WriterLockWhen(const Condition &cond) EXCLUSIVE_LOCK_FUNCTION() {
    this->LockWhen(cond);
  }

  // ---------------------------------------------------------------------------
  // Mutex Variants with Timeouts/Deadlines
  // ---------------------------------------------------------------------------

  // Mutex::AwaitWithTimeout()
  // Mutex::AwaitWithDeadline()
  //
  // If `cond` is initially true, do nothing, or act as though `cond` is
  // initially false.
  //
  // If `cond` is initially false, unlock this `Mutex` and block until
  // simultaneously:
  //   - either `cond` is true or the {timeout has expired, deadline has passed}
  //     and
  //   - this `Mutex` can be reacquired,
  // then reacquire this `Mutex` in the same mode in which it was previously
  // held, returning `true` iff `cond` is `true` on return.
  //
  // Deadlines in the past are equivalent to an immediate deadline.
  // Negative timeouts are equivalent to a zero timeout.
  //
  // This method requires that this thread holds this `Mutex` in some mode.
  bool AwaitWithTimeout(const Condition &cond, absl::Duration timeout);

  bool AwaitWithDeadline(const Condition &cond, absl::Time deadline);

  // Mutex::LockWhenWithTimeout()
  // Mutex::ReaderLockWhenWithTimeout()
  // Mutex::WriterLockWhenWithTimeout()
  //
  // Blocks until simultaneously both:
  //   - either `cond` is `true` or the timeout has expired, and
  //   - this `Mutex` can be acquired,
  // then atomically acquires this `Mutex`, returning `true` iff `cond` is
  // `true` on return.
  //
  // Negative timeouts are equivalent to a zero timeout.
  bool LockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
      EXCLUSIVE_LOCK_FUNCTION();
  bool ReaderLockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
      SHARED_LOCK_FUNCTION();
  bool WriterLockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
      EXCLUSIVE_LOCK_FUNCTION() {
    return this->LockWhenWithTimeout(cond, timeout);
  }

  // Mutex::LockWhenWithDeadline()
  // Mutex::ReaderLockWhenWithDeadline()
  // Mutex::WriterLockWhenWithDeadline()
  //
  // Blocks until simultaneously both:
  //   - either `cond` is `true` or the deadline has been passed, and
  //   - this `Mutex` can be acquired,
  // then atomically acquires this Mutex, returning `true` iff `cond` is `true`
  // on return.
  //
  // Deadlines in the past are equivalent to an immediate deadline.
  bool LockWhenWithDeadline(const Condition &cond, absl::Time deadline)
      EXCLUSIVE_LOCK_FUNCTION();
  bool ReaderLockWhenWithDeadline(const Condition &cond, absl::Time deadline)
      SHARED_LOCK_FUNCTION();
  bool WriterLockWhenWithDeadline(const Condition &cond, absl::Time deadline)
      EXCLUSIVE_LOCK_FUNCTION() {
    return this->LockWhenWithDeadline(cond, deadline);
  }

  // ---------------------------------------------------------------------------
  // Debug Support: Invariant Checking, Deadlock Detection, Logging.
  // ---------------------------------------------------------------------------

  // Mutex::EnableInvariantDebugging()
  //
  // If `invariant`!=null and if invariant debugging has been enabled globally,
  // cause `(*invariant)(arg)` to be called at moments when the invariant for
  // this `Mutex` should hold (for example: just after acquire, just before
  // release).
  //
  // The routine `invariant` should have no side-effects since it is not
  // guaranteed how many times it will be called; it should check the invariant
  // and crash if it does not hold. Enabling global invariant debugging may
  // substantially reduce `Mutex` performance; it should be set only for
  // non-production runs.  Optimization options may also disable invariant
  // checks.
  void EnableInvariantDebugging(void (*invariant)(void *), void *arg);

  // Mutex::EnableDebugLog()
  //
  // Cause all subsequent uses of this `Mutex` to be logged via
  // `ABSL_RAW_LOG(INFO)`. Log entries are tagged with `name` if no previous
  // call to `EnableInvariantDebugging()` or `EnableDebugLog()` has been made.
  //
  // Note: This method substantially reduces `Mutex` performance.
  void EnableDebugLog(const char *name);

  // Deadlock detection

  // Mutex::ForgetDeadlockInfo()
  //
  // Forget any deadlock-detection information previously gathered
  // about this `Mutex`. Call this method in debug mode when the lock ordering
  // of a `Mutex` changes.
  void ForgetDeadlockInfo();

  // Mutex::AssertNotHeld()
  //
  // Return immediately if this thread does not hold this `Mutex` in any
  // mode; otherwise, may report an error (typically by crashing with a
  // diagnostic), or may return immediately.
  //
  // Currently this check is performed only if all of:
  //    - in debug mode
  //    - SetMutexDeadlockDetectionMode() has been set to kReport or kAbort
  //    - number of locks concurrently held by this thread is not large.
  // are true.
  void AssertNotHeld() const;

  // Special cases.

  // A `MuHow` is a constant that indicates how a lock should be acquired.
  // Internal implementation detail.  Clients should ignore.
  typedef const struct MuHowS *MuHow;

  // Mutex::InternalAttemptToUseMutexInFatalSignalHandler()
  //
  // Causes the `Mutex` implementation to prepare itself for re-entry caused by
  // future use of `Mutex` within a fatal signal handler. This method is
  // intended for use only for last-ditch attempts to log crash information.
  // It does not guarantee that attempts to use Mutexes within the handler will
  // not deadlock; it merely makes other faults less likely.
  //
  // WARNING:  This routine must be invoked from a signal handler, and the
  // signal handler must either loop forever or terminate the process.
  // Attempts to return from (or `longjmp` out of) the signal handler once this
  // call has been made may cause arbitrary program behaviour including
  // crashes and deadlocks.
  static void InternalAttemptToUseMutexInFatalSignalHandler();

 private:
#ifdef ABSL_INTERNAL_USE_NONPROD_MUTEX
  friend class CondVar;

  synchronization_internal::MutexImpl *impl() { return impl_.get(); }

  synchronization_internal::SynchronizationStorage<
      synchronization_internal::MutexImpl>
      impl_;
#else
  std::atomic<intptr_t> mu_;  // The Mutex state.

  // Post()/Wait() versus associated PerThreadSem; in class for required
  // friendship with PerThreadSem.
  static inline void IncrementSynchSem(Mutex *mu,
                                       base_internal::PerThreadSynch *w);
  static inline bool DecrementSynchSem(
      Mutex *mu, base_internal::PerThreadSynch *w,
      synchronization_internal::KernelTimeout t);

  // slow path acquire
  void LockSlowLoop(SynchWaitParams *waitp, int flags);
  // wrappers around LockSlowLoop()
  bool LockSlowWithDeadline(MuHow how, const Condition *cond,
                            synchronization_internal::KernelTimeout t,
                            int flags);
  void LockSlow(MuHow how, const Condition *cond,
                int flags) ABSL_ATTRIBUTE_COLD;
  // slow path release
  void UnlockSlow(SynchWaitParams *waitp) ABSL_ATTRIBUTE_COLD;
  // Common code between Await() and AwaitWithTimeout/Deadline()
  bool AwaitCommon(const Condition &cond,
                   synchronization_internal::KernelTimeout t);
  // Attempt to remove thread s from queue.
  void TryRemove(base_internal::PerThreadSynch *s);
  // Block a thread on mutex.
  void Block(base_internal::PerThreadSynch *s);
  // Wake a thread; return successor.
  base_internal::PerThreadSynch *Wakeup(base_internal::PerThreadSynch *w);

  friend class CondVar;   // for access to Trans()/Fer().
  void Trans(MuHow how);  // used for CondVar->Mutex transfer
  void Fer(
      base_internal::PerThreadSynch *w);  // used for CondVar->Mutex transfer
#endif

  // Catch the error of writing Mutex when intending MutexLock.
  Mutex(const volatile Mutex * /*ignored*/) {}  // NOLINT(runtime/explicit)

  Mutex(const Mutex&) = delete;
  Mutex& operator=(const Mutex&) = delete;
};

// -----------------------------------------------------------------------------
// Mutex RAII Wrappers
// -----------------------------------------------------------------------------

// MutexLock
//
// `MutexLock` is a helper class, which acquires and releases a `Mutex` via
// RAII.
//
// Example:
//
// Class Foo {
//
//   Foo::Bar* Baz() {
//     MutexLock l(&lock_);
//     ...
//     return bar;
//   }
//
// private:
//   Mutex lock_;
// };
class SCOPED_LOCKABLE MutexLock {
 public:
  explicit MutexLock(Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu) : mu_(mu) {
    this->mu_->Lock();
  }

  MutexLock(const MutexLock &) = delete;  // NOLINT(runtime/mutex)
  MutexLock(MutexLock&&) = delete;  // NOLINT(runtime/mutex)
  MutexLock& operator=(const MutexLock&) = delete;
  MutexLock& operator=(MutexLock&&) = delete;

  ~MutexLock() UNLOCK_FUNCTION() { this->mu_->Unlock(); }

 private:
  Mutex *const mu_;
};

// ReaderMutexLock
//
// The `ReaderMutexLock` is a helper class, like `MutexLock`, which acquires and
// releases a shared lock on a `Mutex` via RAII.
class SCOPED_LOCKABLE ReaderMutexLock {
 public:
  explicit ReaderMutexLock(Mutex *mu) SHARED_LOCK_FUNCTION(mu)
      :  mu_(mu) {
    mu->ReaderLock();
  }

  ReaderMutexLock(const ReaderMutexLock&) = delete;
  ReaderMutexLock(ReaderMutexLock&&) = delete;
  ReaderMutexLock& operator=(const ReaderMutexLock&) = delete;
  ReaderMutexLock& operator=(ReaderMutexLock&&) = delete;

  ~ReaderMutexLock() UNLOCK_FUNCTION() {
    this->mu_->ReaderUnlock();
  }

 private:
  Mutex *const mu_;
};

// WriterMutexLock
//
// The `WriterMutexLock` is a helper class, like `MutexLock`, which acquires and
// releases a write (exclusive) lock on a `Mutex` va RAII.
class SCOPED_LOCKABLE WriterMutexLock {
 public:
  explicit WriterMutexLock(Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu)
      : mu_(mu) {
    mu->WriterLock();
  }

  WriterMutexLock(const WriterMutexLock&) = delete;
  WriterMutexLock(WriterMutexLock&&) = delete;
  WriterMutexLock& operator=(const WriterMutexLock&) = delete;
  WriterMutexLock& operator=(WriterMutexLock&&) = delete;

  ~WriterMutexLock() UNLOCK_FUNCTION() {
    this->mu_->WriterUnlock();
  }

 private:
  Mutex *const mu_;
};

// -----------------------------------------------------------------------------
// Condition
// -----------------------------------------------------------------------------
//
// As noted above, `Mutex` contains a number of member functions which take a
// `Condition` as a argument; clients can wait for conditions to become `true`
// before attempting to acquire the mutex. These sections are known as
// "condition critical" sections. To use a `Condition`, you simply need to
// construct it, and use within an appropriate `Mutex` member function;
// everything else in the `Condition` class is an implementation detail.
//
// A `Condition` is specified as a function pointer which returns a boolean.
// `Condition` functions should be pure functions -- their results should depend
// only on passed arguments, should not consult any external state (such as
// clocks), and should have no side-effects, aside from debug logging. Any
// objects that the function may access should be limited to those which are
// constant while the mutex is blocked on the condition (e.g. a stack variable),
// or objects of state protected explicitly by the mutex.
//
// No matter which construction is used for `Condition`, the underlying
// function pointer / functor / callable must not throw any
// exceptions. Correctness of `Mutex` / `Condition` is not guaranteed in
// the face of a throwing `Condition`. (When Abseil is allowed to depend
// on C++17, these function pointers will be explicitly marked
// `noexcept`; until then this requirement cannot be enforced in the
// type system.)
//
// Note: to use a `Condition`, you need only construct it and pass it within the
// appropriate `Mutex' member function, such as `Mutex::Await()`.
//
// Example:
//
//   // assume count_ is not internal reference count
//   int count_ GUARDED_BY(mu_);
//
//   mu_.LockWhen(Condition(+[](int* count) { return *count == 0; },
//         &count_));
//
// When multiple threads are waiting on exactly the same condition, make sure
// that they are constructed with the same parameters (same pointer to function
// + arg, or same pointer to object + method), so that the mutex implementation
// can avoid redundantly evaluating the same condition for each thread.
class Condition {
 public:
  // A Condition that returns the result of "(*func)(arg)"
  Condition(bool (*func)(void *), void *arg);

  // Templated version for people who are averse to casts.
  //
  // To use a lambda, prepend it with unary plus, which converts the lambda
  // into a function pointer:
  //     Condition(+[](T* t) { return ...; }, arg).
  //
  // Note: lambdas in this case must contain no bound variables.
  //
  // See class comment for performance advice.
  template<typename T>
  Condition(bool (*func)(T *), T *arg);

  // Templated version for invoking a method that returns a `bool`.
  //
  // `Condition(object, &Class::Method)` constructs a `Condition` that evaluates
  // `object->Method()`.
  //
  // Implementation Note: `absl::internal::identity` is used to allow methods to
  // come from base classes. A simpler signature like
  // `Condition(T*, bool (T::*)())` does not suffice.
  template<typename T>
  Condition(T *object, bool (absl::internal::identity<T>::type::* method)());

  // Same as above, for const members
  template<typename T>
  Condition(const T *object,
            bool (absl::internal::identity<T>::type::* method)() const);

  // A Condition that returns the value of `*cond`
  explicit Condition(const bool *cond);

  // Templated version for invoking a functor that returns a `bool`.
  // This approach accepts pointers to non-mutable lambdas, `std::function`,
  // the result of` std::bind` and user-defined functors that define
  // `bool F::operator()() const`.
  //
  // Example:
  //
  //   auto reached = [this, current]() {
  //     mu_.AssertReaderHeld();                // For annotalysis.
  //     return processed_ >= current;
  //   };
  //   mu_.Await(Condition(&reached));

  // See class comment for performance advice. In particular, if there
  // might be more than one waiter for the same condition, make sure
  // that all waiters construct the condition with the same pointers.

  // Implementation note: The second template parameter ensures that this
  // constructor doesn't participate in overload resolution if T doesn't have
  // `bool operator() const`.
  template <typename T, typename E = decltype(
      static_cast<bool (T::*)() const>(&T::operator()))>
  explicit Condition(const T *obj)
      : Condition(obj, static_cast<bool (T::*)() const>(&T::operator())) {}

  // A Condition that always returns `true`.
  static const Condition kTrue;

  // Evaluates the condition.
  bool Eval() const;

  // Returns `true` if the two conditions are guaranteed to return the same
  // value if evaluated at the same time, `false` if the evaluation *may* return
  // different results.
  //
  // Two `Condition` values are guaranteed equal if both their `func` and `arg`
  // components are the same. A null pointer is equivalent to a `true`
  // condition.
  static bool GuaranteedEqual(const Condition *a, const Condition *b);

 private:
  typedef bool (*InternalFunctionType)(void * arg);
  typedef bool (Condition::*InternalMethodType)();
  typedef bool (*InternalMethodCallerType)(void * arg,
                                           InternalMethodType internal_method);

  bool (*eval_)(const Condition*);  // Actual evaluator
  InternalFunctionType function_;   // function taking pointer returning bool
  InternalMethodType method_;       // method returning bool
  void *arg_;                       // arg of function_ or object of method_

  Condition();        // null constructor used only to create kTrue

  // Various functions eval_ can point to:
  static bool CallVoidPtrFunction(const Condition*);
  template <typename T> static bool CastAndCallFunction(const Condition* c);
  template <typename T> static bool CastAndCallMethod(const Condition* c);
};

// -----------------------------------------------------------------------------
// CondVar
// -----------------------------------------------------------------------------
//
// A condition variable, reflecting state evaluated separately outside of the
// `Mutex` object, which can be signaled to wake callers.
// This class is not normally needed; use `Mutex` member functions such as
// `Mutex::Await()` and intrinsic `Condition` abstractions. In rare cases
// with many threads and many conditions, `CondVar` may be faster.
//
// The implementation may deliver signals to any condition variable at
// any time, even when no call to `Signal()` or `SignalAll()` is made; as a
// result, upon being awoken, you must check the logical condition you have
// been waiting upon.
//
// Examples:
//
// Usage for a thread waiting for some condition C protected by mutex mu:
//       mu.Lock();
//       while (!C) { cv->Wait(&mu); }        // releases and reacquires mu
//       //  C holds; process data
//       mu.Unlock();
//
// Usage to wake T is:
//       mu.Lock();
//      // process data, possibly establishing C
//      if (C) { cv->Signal(); }
//      mu.Unlock();
//
// If C may be useful to more than one waiter, use `SignalAll()` instead of
// `Signal()`.
//
// With this implementation it is efficient to use `Signal()/SignalAll()` inside
// the locked region; this usage can make reasoning about your program easier.
//
class CondVar {
 public:
  CondVar();
  ~CondVar();

  // CondVar::Wait()
  //
  // Atomically releases a `Mutex` and blocks on this condition variable.
  // Waits until awakened by a call to `Signal()` or `SignalAll()` (or a
  // spurious wakeup), then reacquires the `Mutex` and returns.
  //
  // Requires and ensures that the current thread holds the `Mutex`.
  void Wait(Mutex *mu);

  // CondVar::WaitWithTimeout()
  //
  // Atomically releases a `Mutex` and blocks on this condition variable.
  // Waits until awakened by a call to `Signal()` or `SignalAll()` (or a
  // spurious wakeup), or until the timeout has expired, then reacquires
  // the `Mutex` and returns.
  //
  // Returns true if the timeout has expired without this `CondVar`
  // being signalled in any manner. If both the timeout has expired
  // and this `CondVar` has been signalled, the implementation is free
  // to return `true` or `false`.
  //
  // Requires and ensures that the current thread holds the `Mutex`.
  bool WaitWithTimeout(Mutex *mu, absl::Duration timeout);

  // CondVar::WaitWithDeadline()
  //
  // Atomically releases a `Mutex` and blocks on this condition variable.
  // Waits until awakened by a call to `Signal()` or `SignalAll()` (or a
  // spurious wakeup), or until the deadline has passed, then reacquires
  // the `Mutex` and returns.
  //
  // Deadlines in the past are equivalent to an immediate deadline.
  //
  // Returns true if the deadline has passed without this `CondVar`
  // being signalled in any manner. If both the deadline has passed
  // and this `CondVar` has been signalled, the implementation is free
  // to return `true` or `false`.
  //
  // Requires and ensures that the current thread holds the `Mutex`.
  bool WaitWithDeadline(Mutex *mu, absl::Time deadline);

  // CondVar::Signal()
  //
  // Signal this `CondVar`; wake at least one waiter if one exists.
  void Signal();

  // CondVar::SignalAll()
  //
  // Signal this `CondVar`; wake all waiters.
  void SignalAll();

  // CondVar::EnableDebugLog()
  //
  // Causes all subsequent uses of this `CondVar` to be logged via
  // `ABSL_RAW_LOG(INFO)`. Log entries are tagged with `name` if `name != 0`.
  // Note: this method substantially reduces `CondVar` performance.
  void EnableDebugLog(const char *name);

 private:
#ifdef ABSL_INTERNAL_USE_NONPROD_MUTEX
  synchronization_internal::CondVarImpl *impl() { return impl_.get(); }
  synchronization_internal::SynchronizationStorage<
      synchronization_internal::CondVarImpl>
      impl_;
#else
  bool WaitCommon(Mutex *mutex, synchronization_internal::KernelTimeout t);
  void Remove(base_internal::PerThreadSynch *s);
  void Wakeup(base_internal::PerThreadSynch *w);
  std::atomic<intptr_t> cv_;  // Condition variable state.
#endif
  CondVar(const CondVar&) = delete;
  CondVar& operator=(const CondVar&) = delete;
};


// Variants of MutexLock.
//
// If you find yourself using one of these, consider instead using
// Mutex::Unlock() and/or if-statements for clarity.

// MutexLockMaybe
//
// MutexLockMaybe is like MutexLock, but is a no-op when mu is null.
class SCOPED_LOCKABLE MutexLockMaybe {
 public:
  explicit MutexLockMaybe(Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu)
      : mu_(mu) { if (this->mu_ != nullptr) { this->mu_->Lock(); } }
  ~MutexLockMaybe() UNLOCK_FUNCTION() {
    if (this->mu_ != nullptr) { this->mu_->Unlock(); }
  }
 private:
  Mutex *const mu_;
  MutexLockMaybe(const MutexLockMaybe&) = delete;
  MutexLockMaybe(MutexLockMaybe&&) = delete;
  MutexLockMaybe& operator=(const MutexLockMaybe&) = delete;
  MutexLockMaybe& operator=(MutexLockMaybe&&) = delete;
};

// ReleaseableMutexLock
//
// ReleasableMutexLock is like MutexLock, but permits `Release()` of its
// mutex before destruction. `Release()` may be called at most once.
class SCOPED_LOCKABLE ReleasableMutexLock {
 public:
  explicit ReleasableMutexLock(Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu)
      : mu_(mu) {
    this->mu_->Lock();
  }
  ~ReleasableMutexLock() UNLOCK_FUNCTION() {
    if (this->mu_ != nullptr) { this->mu_->Unlock(); }
  }

  void Release() UNLOCK_FUNCTION();

 private:
  Mutex *mu_;
  ReleasableMutexLock(const ReleasableMutexLock&) = delete;
  ReleasableMutexLock(ReleasableMutexLock&&) = delete;
  ReleasableMutexLock& operator=(const ReleasableMutexLock&) = delete;
  ReleasableMutexLock& operator=(ReleasableMutexLock&&) = delete;
};

#ifdef ABSL_INTERNAL_USE_NONPROD_MUTEX
#else
inline Mutex::Mutex() : mu_(0) {
  ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);
}

inline CondVar::CondVar() : cv_(0) {}
#endif

// static
template <typename T>
bool Condition::CastAndCallMethod(const Condition *c) {
  typedef bool (T::*MemberType)();
  MemberType rm = reinterpret_cast<MemberType>(c->method_);
  T *x = static_cast<T *>(c->arg_);
  return (x->*rm)();
}

// static
template <typename T>
bool Condition::CastAndCallFunction(const Condition *c) {
  typedef bool (*FuncType)(T *);
  FuncType fn = reinterpret_cast<FuncType>(c->function_);
  T *x = static_cast<T *>(c->arg_);
  return (*fn)(x);
}

template <typename T>
inline Condition::Condition(bool (*func)(T *), T *arg)
    : eval_(&CastAndCallFunction<T>),
      function_(reinterpret_cast<InternalFunctionType>(func)),
      method_(nullptr),
      arg_(const_cast<void *>(static_cast<const void *>(arg))) {}

template <typename T>
inline Condition::Condition(T *object,
                            bool (absl::internal::identity<T>::type::*method)())
    : eval_(&CastAndCallMethod<T>),
      function_(nullptr),
      method_(reinterpret_cast<InternalMethodType>(method)),
      arg_(object) {}

template <typename T>
inline Condition::Condition(const T *object,
                            bool (absl::internal::identity<T>::type::*method)()
                                const)
    : eval_(&CastAndCallMethod<T>),
      function_(nullptr),
      method_(reinterpret_cast<InternalMethodType>(method)),
      arg_(reinterpret_cast<void *>(const_cast<T *>(object))) {}

// Register a hook for profiling support.
//
// The function pointer registered here will be called whenever a mutex is
// contended.  The callback is given the absl/base/cycleclock.h timestamp when
// waiting began.
//
// Calls to this function do not race or block, but there is no ordering
// guaranteed between calls to this function and call to the provided hook.
// In particular, the previously registered hook may still be called for some
// time after this function returns.
void RegisterMutexProfiler(void (*fn)(int64_t wait_timestamp));

// Register a hook for Mutex tracing.
//
// The function pointer registered here will be called whenever a mutex is
// contended.  The callback is given an opaque handle to the contended mutex,
// an event name, and the number of wait cycles (as measured by
// //absl/base/internal/cycleclock.h, and which may not be real
// "cycle" counts.)
//
// The only event name currently sent is "slow release".
//
// This has the same memory ordering concerns as RegisterMutexProfiler() above.
void RegisterMutexTracer(void (*fn)(const char *msg, const void *obj,
                              int64_t wait_cycles));

// TODO(gfalcon): Combine RegisterMutexProfiler() and RegisterMutexTracer()
// into a single interface, since they are only ever called in pairs.

// Register a hook for CondVar tracing.
//
// The function pointer registered here will be called here on various CondVar
// events.  The callback is given an opaque handle to the CondVar object and
// a std::string identifying the event.  This is thread-safe, but only a single
// tracer can be registered.
//
// Events that can be sent are "Wait", "Unwait", "Signal wakeup", and
// "SignalAll wakeup".
//
// This has the same memory ordering concerns as RegisterMutexProfiler() above.
void RegisterCondVarTracer(void (*fn)(const char *msg, const void *cv));

// Register a hook for symbolizing stack traces in deadlock detector reports.
//
// 'pc' is the program counter being symbolized, 'out' is the buffer to write
// into, and 'out_size' is the size of the buffer.  This function can return
// false if symbolizing failed, or true if a null-terminated symbol was written
// to 'out.'
//
// This has the same memory ordering concerns as RegisterMutexProfiler() above.
//
// DEPRECATED: The default symbolizer function is absl::Symbolize() and the
// ability to register a different hook for symbolizing stack traces will be
// removed on or after 2023-05-01.
ABSL_DEPRECATED("absl::RegisterSymbolizer() is deprecated and will be removed "
                "on or after 2023-05-01")
void RegisterSymbolizer(bool (*fn)(const void *pc, char *out, int out_size));

// EnableMutexInvariantDebugging()
//
// Enable or disable global support for Mutex invariant debugging.  If enabled,
// then invariant predicates can be registered per-Mutex for debug checking.
// See Mutex::EnableInvariantDebugging().
void EnableMutexInvariantDebugging(bool enabled);

// When in debug mode, and when the feature has been enabled globally, the
// implementation will keep track of lock ordering and complain (or optionally
// crash) if a cycle is detected in the acquired-before graph.

// Possible modes of operation for the deadlock detector in debug mode.
enum class OnDeadlockCycle {
  kIgnore,  // Neither report on nor attempt to track cycles in lock ordering
  kReport,  // Report lock cycles to stderr when detected
  kAbort,  // Report lock cycles to stderr when detected, then abort
};

// SetMutexDeadlockDetectionMode()
//
// Enable or disable global support for detection of potential deadlocks
// due to Mutex lock ordering inversions.  When set to 'kIgnore', tracking of
// lock ordering is disabled.  Otherwise, in debug builds, a lock ordering graph
// will be maintained internally, and detected cycles will be reported in
// the manner chosen here.
void SetMutexDeadlockDetectionMode(OnDeadlockCycle mode);

}  // namespace absl

// In some build configurations we pass --detect-odr-violations to the
// gold linker.  This causes it to flag weak symbol overrides as ODR
// violations.  Because ODR only applies to C++ and not C,
// --detect-odr-violations ignores symbols not mangled with C++ names.
// By changing our extension points to be extern "C", we dodge this
// check.
extern "C" {
void AbslInternalMutexYield();
}  // extern "C"
#endif  // ABSL_SYNCHRONIZATION_MUTEX_H_