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/
36 //            - An RAII wrapper to acquire and release a `Mutex` for shared/read
43 // In addition to simple mutex locks, this file also defines ways to perform
47 //              depends on state protected by the `Mutex` to become true.
54 // Mutexes and mutex behavior can be quite complicated. The information within
55 // this header file is limited, as a result. Please consult the Mutex guide for
83 // Mutex
86 // A `Mutex` is a non-reentrant (aka non-recursive) Mutually Exclusive lock
91 // A `Mutex` has two basic operations: `Mutex::Lock()` and `Mutex::Unlock()`.
92 // The `Lock()` operation *acquires* a `Mutex` (in a state known as an
94 // Mutex. During the span of time between the Lock() and Unlock() operations,
95 // a mutex is said to be *held*. By design all mutexes support exclusive/write
96 // locks, as this is the most common way to use a mutex.
98 // The `Mutex` state machine for basic lock/unlock operations is quite simple:
108 // An "invalid" operation is disallowed by the API. The `Mutex` implementation
114 // `Mutex` is not guaranteed to be "fair" in prioritizing waiting threads; it
129 // See also `MutexLock`, below, for scoped `Mutex` acquisition.
131 class ABSL_LOCKABLE Mutex {
133   // Creates a `Mutex` that is not held by anyone. This constructor is
136   // To create `Mutex` instances with static storage duration
138   // `Mutex::Mutex(absl::kConstInit)` below instead.
139   Mutex();
141   // Creates a mutex with static storage duration.  A global variable
150   //   ABSL_CONST_INIT absl::Mutex mu(absl::kConstInit);
152   explicit constexpr Mutex(absl::ConstInitType);
154   ~Mutex();
156   // Mutex::Lock()
158   // Blocks the calling thread, if necessary, until this `Mutex` is free, and
162   // Mutex::Unlock()
164   // Releases this `Mutex` and returns it from the exclusive/write state to the
165   // free state. Calling thread must hold the `Mutex` exclusively.
168   // Mutex::TryLock()
170   // If the mutex can be acquired without blocking, does so exclusively and
172   // probability if the `Mutex` was free.
175   // Mutex::AssertHeld()
177   // Return immediately if this thread holds the `Mutex` exclusively (in write
186   // A Mutex can also be used as a starvation-free reader-writer lock.
190   // The Mutex API provides `Writer*()` aliases for the existing `Lock()`,
195   // Introducing reader locks necessarily complicates the `Mutex` state
197   // of a mutex in such cases. Note that ReaderLock() may block even if the lock
214   // Mutex::ReaderLock()
216   // Blocks the calling thread, if necessary, until this `Mutex` is either free,
219   // on the mutex.
223   // Mutex::ReaderUnlock()
225   // Releases a read share of this `Mutex`. `ReaderUnlock` may return a mutex to
226   // the free state if this thread holds the last reader lock on the mutex. Note
227   // that you cannot call `ReaderUnlock()` on a mutex held in write mode.
230   // Mutex::ReaderTryLock()
232   // If the mutex can be acquired without blocking, acquires this mutex for
234   // `true` with high probability if the `Mutex` was free or shared.
237   // Mutex::AssertReaderHeld()
239   // Returns immediately if this thread holds the `Mutex` in at least shared
244   // Mutex::WriterLock()
245   // Mutex::WriterUnlock()
246   // Mutex::WriterTryLock()
248   // Aliases for `Mutex::Lock()`, `Mutex::Unlock()`, and `Mutex::TryLock()`.
251   // methods) to distingish simple exclusive `Mutex` usage (`Lock()`,
265   // Conditional usage of a `Mutex` can occur using two distinct paradigms:
267   //   * Use of `Mutex` member functions with `Condition` objects.
270   // In general, prefer use of `Condition` and the `Mutex` member functions
275   // `Mutex` contains member functions for performing lock operations only under
278   // the `Mutex`. The condition must be invariant w.r.t. environmental state
280   // always be invoked with the mutex held in at least read mode, so you should
293   // Mutex::Await()
295   // Unlocks this `Mutex` and blocks until simultaneously both `cond` is `true`
296   // and this `Mutex` can be reacquired, then reacquires this `Mutex` in the
300   // `Await()` requires that this thread holds this `Mutex` in some mode.
303   // Mutex::LockWhen()
304   // Mutex::ReaderLockWhen()
305   // Mutex::WriterLockWhen()
307   // Blocks until simultaneously both `cond` is `true` and this `Mutex` can
308   // be acquired, then atomically acquires this `Mutex`. `LockWhen()` is
320   // Mutex Variants with Timeouts/Deadlines
323   // Mutex::AwaitWithTimeout()
324   // Mutex::AwaitWithDeadline()
326   // Unlocks this `Mutex` and blocks until simultaneously:
329   //   - this `Mutex` can be reacquired,
330   // then reacquire this `Mutex` in the same mode in which it was previously
339   // This method requires that this thread holds this `Mutex` in some mode.
344   // Mutex::LockWhenWithTimeout()
345   // Mutex::ReaderLockWhenWithTimeout()
346   // Mutex::WriterLockWhenWithTimeout()
350   //   - this `Mutex` can be acquired,
351   // then atomically acquires this `Mutex`, returning `true` iff `cond` is
364   // Mutex::LockWhenWithDeadline()
365   // Mutex::ReaderLockWhenWithDeadline()
366   // Mutex::WriterLockWhenWithDeadline()
370   //   - this `Mutex` can be acquired,
371   // then atomically acquires this Mutex, returning `true` iff `cond` is `true`
388   // Mutex::EnableInvariantDebugging()
392   // this `Mutex` should hold (for example: just after acquire, just before
398   // substantially reduce `Mutex` performance; it should be set only for
403   // Mutex::EnableDebugLog()
405   // Cause all subsequent uses of this `Mutex` to be logged via
409   // Note: This method substantially reduces `Mutex` performance.
414   // Mutex::ForgetDeadlockInfo()
417   // about this `Mutex`. Call this method in debug mode when the lock ordering
418   // of a `Mutex` changes.
421   // Mutex::AssertNotHeld()
423   // Return immediately if this thread does not hold this `Mutex` in any
440   // Mutex::InternalAttemptToUseMutexInFatalSignalHandler()
442   // Causes the `Mutex` implementation to prepare itself for re-entry caused by
443   // future use of `Mutex` within a fatal signal handler. This method is
456   std::atomic<intptr_t> mu_;  // The Mutex state.
460   static void IncrementSynchSem(Mutex *mu, base_internal::PerThreadSynch *w);
461   static bool DecrementSynchSem(Mutex *mu, base_internal::PerThreadSynch *w,
479   // Block a thread on mutex.
485   void Trans(MuHow how);  // used for CondVar->Mutex transfer
487       base_internal::PerThreadSynch *w);  // used for CondVar->Mutex transfer
489   // Catch the error of writing Mutex when intending MutexLock.
490   Mutex(const volatile Mutex * /*ignored*/) {}  // NOLINT(runtime/explicit)  in Mutex()  function
492   Mutex(const Mutex&) = delete;
493   Mutex& operator=(const Mutex&) = delete;
497 // Mutex RAII Wrappers
502 // `MutexLock` is a helper class, which acquires and releases a `Mutex` via
516 //   Mutex mu_;
525   explicit MutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu) : mu_(mu) {  in MutexLock()
532   explicit MutexLock(Mutex *mu, const Condition &cond)  in MutexLock()
538   MutexLock(const MutexLock &) = delete;  // NOLINT(runtime/mutex)
539   MutexLock(MutexLock&&) = delete;  // NOLINT(runtime/mutex)
546   Mutex *const mu_;
552 // releases a shared lock on a `Mutex` via RAII.
555   explicit ReaderMutexLock(Mutex *mu) ABSL_SHARED_LOCK_FUNCTION(mu) : mu_(mu) {  in ReaderMutexLock()
559   explicit ReaderMutexLock(Mutex *mu, const Condition &cond)  in ReaderMutexLock()
573   Mutex *const mu_;
579 // releases a write (exclusive) lock on a `Mutex` via RAII.
582   explicit WriterMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)  in WriterMutexLock()
587   explicit WriterMutexLock(Mutex *mu, const Condition &cond)  in WriterMutexLock()
601   Mutex *const mu_;
608 // As noted above, `Mutex` contains a number of member functions which take a
610 // before attempting to acquire the mutex. These sections are known as
612 // construct it, and use within an appropriate `Mutex` member function;
620 // constant while the mutex is blocked on the condition (e.g. a stack variable),
621 // or objects of state protected explicitly by the mutex.
625 // exceptions. Correctness of `Mutex` / `Condition` is not guaranteed in
632 // suitable `Mutex' member function, such as `Mutex::Await()`, or to the
654 // + arg, or same pointer to object + method), so that the mutex implementation
706   // the lambda as it may be called when the mutex is being unlocked from a
761 // `Mutex` object, which can be signaled to wake callers.
762 // This class is not normally needed; use `Mutex` member functions such as
763 // `Mutex::Await()` and intrinsic `Condition` abstractions. In rare cases
773 // Usage for a thread waiting for some condition C protected by mutex mu:
800   // Atomically releases a `Mutex` and blocks on this condition variable.
802   // spurious wakeup), then reacquires the `Mutex` and returns.
804   // Requires and ensures that the current thread holds the `Mutex`.
805   void Wait(Mutex *mu);
809   // Atomically releases a `Mutex` and blocks on this condition variable.
812   // the `Mutex` and returns.
819   // Requires and ensures that the current thread holds the `Mutex`.
820   bool WaitWithTimeout(Mutex *mu, absl::Duration timeout);
824   // Atomically releases a `Mutex` and blocks on this condition variable.
827   // the `Mutex` and returns.
836   // Requires and ensures that the current thread holds the `Mutex`.
837   bool WaitWithDeadline(Mutex *mu, absl::Time deadline);
857   bool WaitCommon(Mutex *mutex, synchronization_internal::KernelTimeout t);
869 // Mutex::Unlock() and/or if-statements for clarity.
876   explicit MutexLockMaybe(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)  in MutexLockMaybe()
883   explicit MutexLockMaybe(Mutex *mu, const Condition &cond)  in MutexLockMaybe()
896   Mutex *const mu_;
906 // mutex before destruction. `Release()` may be called at most once.
909   explicit ReleasableMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)  in ReleasableMutexLock()
914   explicit ReleasableMutexLock(Mutex *mu, const Condition &cond)  in ReleasableMutexLock()
927   Mutex *mu_;
934 inline Mutex::Mutex() : mu_(0) {  in Mutex()  function
938 inline constexpr Mutex::Mutex(absl::ConstInitType) : mu_(0) {}  in Mutex()  function
986 // The function pointer registered here will be called whenever a mutex is
996 // Register a hook for Mutex tracing.
998 // The function pointer registered here will be called whenever a mutex is
999 // contended.  The callback is given an opaque handle to the contended mutex,
1044 // Enable or disable global support for Mutex invariant debugging.  If enabled,
1045 // then invariant predicates can be registered per-Mutex for debug checking.
1046 // See Mutex::EnableInvariantDebugging().
1063 // due to Mutex lock ordering inversions.  When set to 'kIgnore', tracking of