16 // mutex.h
19 // This header file defines a `Mutex` -- a mutually exclusive lock -- and the
21 // shared resources. A mutex is used to prevent multiple threads from accessing
24 // Unlike a `std::mutex`, the Abseil `Mutex` provides the following additional
26 //   * Conditional predicates intrinsic to the `Mutex` object
32 //  MutexLock - An RAII wrapper to acquire and release a `Mutex` for exclusive/
35 //            - An RAII wrapper to acquire and release a `Mutex` for shared/read
42 // In addition to simple mutex locks, this file also defines ways to perform
46 //                depends on state protected by the `Mutex` to become true.
53 // Mutexes and mutex behavior can be quite complicated. The information within
54 // this header file is limited, as a result. Please consult the Mutex guide for
91 // Mutex
94 // A `Mutex` is a non-reentrant (aka non-recursive) Mutually Exclusive lock
99 // A `Mutex` has two basic operations: `Mutex::Lock()` and `Mutex::Unlock()`.
100 // The `Lock()` operation *acquires* a `Mutex` (in a state known as an
102 // Mutex. During the span of time between the Lock() and Unlock() operations,
103 // a mutex is said to be *held*. By design all mutexes support exclusive/write
104 // locks, as this is the most common way to use a mutex.
106 // The `Mutex` state machine for basic lock/unlock operations is quite simple:
116 // An "invalid" operation is disallowed by the API. The `Mutex` implementation
122 // `Mutex` is not guaranteed to be "fair" in prioritizing waiting threads; it
137 // See also `MutexLock`, below, for scoped `Mutex` acquisition.
139 class ABSL_LOCKABLE Mutex {
141   // Creates a `Mutex` that is not held by anyone. This constructor is
144   // To create `Mutex` instances with static storage duration
146   // `Mutex::Mutex(absl::kConstInit)` below instead.
147   Mutex();
149   // Creates a mutex with static storage duration.  A global variable
158   //   ABSL_CONST_INIT Mutex mu(absl::kConstInit);
160   explicit constexpr Mutex(absl::ConstInitType);
162   ~Mutex();
164   // Mutex::Lock()
166   // Blocks the calling thread, if necessary, until this `Mutex` is free, and
170   // Mutex::Unlock()
172   // Releases this `Mutex` and returns it from the exclusive/write state to the
173   // free state. Caller must hold the `Mutex` exclusively.
176   // Mutex::TryLock()
178   // If the mutex can be acquired without blocking, does so exclusively and
180   // probability if the `Mutex` was free.
183   // Mutex::AssertHeld()
185   // Return immediately if this thread holds the `Mutex` exclusively (in write
194   // A Mutex can also be used as a starvation-free reader-writer lock.
198   // The Mutex API provides `Writer*()` aliases for the existing `Lock()`,
203   // Introducing reader locks necessarily complicates the `Mutex` state
205   // of a mutex in such cases. Note that ReaderLock() may block even if the lock
222   // Mutex::ReaderLock()
224   // Blocks the calling thread, if necessary, until this `Mutex` is either free,
227   // on the mutex.
231   // Mutex::ReaderUnlock()
233   // Releases a read share of this `Mutex`. `ReaderUnlock` may return a mutex to
234   // the free state if this thread holds the last reader lock on the mutex. Note
235   // that you cannot call `ReaderUnlock()` on a mutex held in write mode.
238   // Mutex::ReaderTryLock()
240   // If the mutex can be acquired without blocking, acquires this mutex for
242   // `true` with high probability if the `Mutex` was free or shared.
245   // Mutex::AssertReaderHeld()
247   // Returns immediately if this thread holds the `Mutex` in at least shared
252   // Mutex::WriterLock()
253   // Mutex::WriterUnlock()
254   // Mutex::WriterTryLock()
256   // Aliases for `Mutex::Lock()`, `Mutex::Unlock()`, and `Mutex::TryLock()`.
259   // methods) to distingish simple exclusive `Mutex` usage (`Lock()`,
273   // Conditional usage of a `Mutex` can occur using two distinct paradigms:
275   //   * Use of `Mutex` member functions with `Condition` objects.
278   // In general, prefer use of `Condition` and the `Mutex` member functions
283   // `Mutex` contains member functions for performing lock operations only under
286   // the `Mutex`. The condition must be invariant w.r.t. environmental state
288   // always be invoked with the mutex held in at least read mode, so you should
301   // Mutex::Await()
303   // Unlocks this `Mutex` and blocks until simultaneously both `cond` is `true`
304   // and this `Mutex` can be reacquired, then reacquires this `Mutex` in the
308   // `Await()` requires that this thread holds this `Mutex` in some mode.
311   // Mutex::LockWhen()
312   // Mutex::ReaderLockWhen()
313   // Mutex::WriterLockWhen()
315   // Blocks until simultaneously both `cond` is `true` and this `Mutex` can
316   // be acquired, then atomically acquires this `Mutex`. `LockWhen()` is
328   // Mutex Variants with Timeouts/Deadlines
331   // Mutex::AwaitWithTimeout()
332   // Mutex::AwaitWithDeadline()
337   // If `cond` is initially false, unlock this `Mutex` and block until
341   //   - this `Mutex` can be reacquired,
342   // then reacquire this `Mutex` in the same mode in which it was previously
348   // This method requires that this thread holds this `Mutex` in some mode.
353   // Mutex::LockWhenWithTimeout()
354   // Mutex::ReaderLockWhenWithTimeout()
355   // Mutex::WriterLockWhenWithTimeout()
359   //   - this `Mutex` can be acquired,
360   // then atomically acquires this `Mutex`, returning `true` iff `cond` is
373   // Mutex::LockWhenWithDeadline()
374   // Mutex::ReaderLockWhenWithDeadline()
375   // Mutex::WriterLockWhenWithDeadline()
379   //   - this `Mutex` can be acquired,
380   // then atomically acquires this Mutex, returning `true` iff `cond` is `true`
397   // Mutex::EnableInvariantDebugging()
401   // this `Mutex` should hold (for example: just after acquire, just before
407   // substantially reduce `Mutex` performance; it should be set only for
412   // Mutex::EnableDebugLog()
414   // Cause all subsequent uses of this `Mutex` to be logged via
418   // Note: This method substantially reduces `Mutex` performance.
423   // Mutex::ForgetDeadlockInfo()
426   // about this `Mutex`. Call this method in debug mode when the lock ordering
427   // of a `Mutex` changes.
430   // Mutex::AssertNotHeld()
432   // Return immediately if this thread does not hold this `Mutex` in any
449   // Mutex::InternalAttemptToUseMutexInFatalSignalHandler()
451   // Causes the `Mutex` implementation to prepare itself for re-entry caused by
452   // future use of `Mutex` within a fatal signal handler. This method is
474   std::atomic<intptr_t> mu_;  // The Mutex state.
478   static inline void IncrementSynchSem(Mutex *mu,
481       Mutex *mu, base_internal::PerThreadSynch *w,
499   // Block a thread on mutex.
505   void Trans(MuHow how);  // used for CondVar->Mutex transfer
507       base_internal::PerThreadSynch *w);  // used for CondVar->Mutex transfer
510   // Catch the error of writing Mutex when intending MutexLock.
511   Mutex(const volatile Mutex * /*ignored*/) {}  // NOLINT(runtime/explicit)  in Mutex()  argument
513   Mutex(const Mutex&) = delete;
514   Mutex& operator=(const Mutex&) = delete;
518 // Mutex RAII Wrappers
523 // `MutexLock` is a helper class, which acquires and releases a `Mutex` via
537 //   Mutex lock_;
541   explicit MutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu) : mu_(mu) {  in MutexLock()
545   MutexLock(const MutexLock &) = delete;  // NOLINT(runtime/mutex)
546   MutexLock(MutexLock&&) = delete;  // NOLINT(runtime/mutex)
553   Mutex *const mu_;
559 // releases a shared lock on a `Mutex` via RAII.
562   explicit ReaderMutexLock(Mutex *mu) ABSL_SHARED_LOCK_FUNCTION(mu) : mu_(mu) {  in ReaderMutexLock()
574   Mutex *const mu_;
580 // releases a write (exclusive) lock on a `Mutex` via RAII.
583   explicit WriterMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)  in WriterMutexLock()
596   Mutex *const mu_;
603 // As noted above, `Mutex` contains a number of member functions which take a
605 // before attempting to acquire the mutex. These sections are known as
607 // construct it, and use within an appropriate `Mutex` member function;
615 // constant while the mutex is blocked on the condition (e.g. a stack variable),
616 // or objects of state protected explicitly by the mutex.
620 // exceptions. Correctness of `Mutex` / `Condition` is not guaranteed in
627 // appropriate `Mutex' member function, such as `Mutex::Await()`.
639 // + arg, or same pointer to object + method), so that the mutex implementation
741 // `Mutex` object, which can be signaled to wake callers.
742 // This class is not normally needed; use `Mutex` member functions such as
743 // `Mutex::Await()` and intrinsic `Condition` abstractions. In rare cases
753 // Usage for a thread waiting for some condition C protected by mutex mu:
778   // Atomically releases a `Mutex` and blocks on this condition variable.
780   // spurious wakeup), then reacquires the `Mutex` and returns.
782   // Requires and ensures that the current thread holds the `Mutex`.
783   void Wait(Mutex *mu);
787   // Atomically releases a `Mutex` and blocks on this condition variable.
790   // the `Mutex` and returns.
797   // Requires and ensures that the current thread holds the `Mutex`.
798   bool WaitWithTimeout(Mutex *mu, absl::Duration timeout);
802   // Atomically releases a `Mutex` and blocks on this condition variable.
805   // the `Mutex` and returns.
814   // Requires and ensures that the current thread holds the `Mutex`.
815   bool WaitWithDeadline(Mutex *mu, absl::Time deadline);
841   bool WaitCommon(Mutex *mutex, synchronization_internal::KernelTimeout t);
854 // Mutex::Unlock() and/or if-statements for clarity.
861   explicit MutexLockMaybe(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)  in MutexLockMaybe()
872   Mutex *const mu_;
882 // mutex before destruction. `Release()` may be called at most once.
885   explicit ReleasableMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)  in ReleasableMutexLock()
896   Mutex *mu_;
904 inline constexpr Mutex::Mutex(absl::ConstInitType) : impl_(absl::kConstInit) {}  in Mutex()  function
907 inline Mutex::Mutex() : mu_(0) {  in Mutex()  function
911 inline constexpr Mutex::Mutex(absl::ConstInitType) : mu_(0) {}  in Mutex()  function
960 // The function pointer registered here will be called whenever a mutex is
970 // Register a hook for Mutex tracing.
972 // The function pointer registered here will be called whenever a mutex is
973 // contended.  The callback is given an opaque handle to the contended mutex,
1018 // Enable or disable global support for Mutex invariant debugging.  If enabled,
1019 // then invariant predicates can be registered per-Mutex for debug checking.
1020 // See Mutex::EnableInvariantDebugging().
1037 // due to Mutex lock ordering inversions.  When set to 'kIgnore', tracking of