// 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
//
// https://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
// * Shared/reader 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
// - Effectively an 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/const_init.h"
#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"
namespace absl {
ABSL_NAMESPACE_BEGIN
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 ABSL_LOCKABLE Mutex {
public:
// Creates a `Mutex` that is not held by anyone. This constructor is
// typically used for Mutexes allocated on the heap or the stack.
//
// To create `Mutex` instances with static storage duration
// (e.g. a namespace-scoped or global variable), see
// `Mutex::Mutex(absl::kConstInit)` below instead.
Mutex();
// Creates a mutex with static storage duration. A global variable
// constructed this way avoids the lifetime issues that can occur on program
// startup and shutdown. (See absl/base/const_init.h.)
//
// For Mutexes allocated on the heap and stack, instead use the default
// constructor, which can interact more fully with the thread sanitizer.
//
// Example usage:
// namespace foo {
// ABSL_CONST_INIT Mutex mu(absl::kConstInit);
// }
explicit constexpr Mutex(absl::ConstInitType);
~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() ABSL_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() ABSL_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() ABSL_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 ABSL_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() ABSL_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() ABSL_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() ABSL_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 ABSL_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() ABSL_EXCLUSIVE_LOCK_FUNCTION() { this->Lock(); }
void WriterUnlock() ABSL_UNLOCK_FUNCTION() { this->Unlock(); }
bool WriterTryLock() ABSL_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) ABSL_EXCLUSIVE_LOCK_FUNCTION();
void ReaderLockWhen(const Condition &cond) ABSL_SHARED_LOCK_FUNCTION();
void WriterLockWhen(const Condition &cond) ABSL_EXCLUSIVE_LOCK_FUNCTION() {
this->LockWhen(cond);
}
// ---------------------------------------------------------------------------
// Mutex Variants with Timeouts/Deadlines
// ---------------------------------------------------------------------------
// Mutex::AwaitWithTimeout()
// Mutex::AwaitWithDeadline()
//
// Unlocks this `Mutex` and blocks 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.
//
// If the condition is initially `true`, the implementation *may* skip the
// release/re-acquire step and return immediately.
//
// 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)
ABSL_EXCLUSIVE_LOCK_FUNCTION();
bool ReaderLockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
ABSL_SHARED_LOCK_FUNCTION();
bool WriterLockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
ABSL_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)
ABSL_EXCLUSIVE_LOCK_FUNCTION();
bool ReaderLockWhenWithDeadline(const Condition &cond, absl::Time deadline)
ABSL_SHARED_LOCK_FUNCTION();
bool WriterLockWhenWithDeadline(const Condition &cond, absl::Time deadline)
ABSL_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:
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
// 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 {
// public:
// Foo::Bar* Baz() {
// MutexLock lock(&mu_);
// ...
// return bar;
// }
//
// private:
// Mutex mu_;
// };
class ABSL_SCOPED_LOCKABLE MutexLock {
public:
// Constructors
// Calls `mu->Lock()` and returns when that call returns. That is, `*mu` is
// guaranteed to be locked when this object is constructed. Requires that
// `mu` be dereferenceable.
explicit MutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu) : mu_(mu) {
this->mu_->Lock();
}
// Like above, but calls `mu->LockWhen(cond)` instead. That is, in addition to
// the above, the condition given by `cond` is also guaranteed to hold when
// this object is constructed.
explicit MutexLock(Mutex *mu, const Condition &cond)
ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
: mu_(mu) {
this->mu_->LockWhen(cond);
}
MutexLock(const MutexLock &) = delete; // NOLINT(runtime/mutex)
MutexLock(MutexLock&&) = delete; // NOLINT(runtime/mutex)
MutexLock& operator=(const MutexLock&) = delete;
MutexLock& operator=(MutexLock&&) = delete;
~MutexLock() ABSL_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 ABSL_SCOPED_LOCKABLE ReaderMutexLock {
public:
explicit ReaderMutexLock(Mutex *mu) ABSL_SHARED_LOCK_FUNCTION(mu) : mu_(mu) {
mu->ReaderLock();
}
explicit ReaderMutexLock(Mutex *mu, const Condition &cond)
ABSL_SHARED_LOCK_FUNCTION(mu)
: mu_(mu) {
mu->ReaderLockWhen(cond);
}
ReaderMutexLock(const ReaderMutexLock&) = delete;
ReaderMutexLock(ReaderMutexLock&&) = delete;
ReaderMutexLock& operator=(const ReaderMutexLock&) = delete;
ReaderMutexLock& operator=(ReaderMutexLock&&) = delete;
~ReaderMutexLock() ABSL_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` via RAII.
class ABSL_SCOPED_LOCKABLE WriterMutexLock {
public:
explicit WriterMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
: mu_(mu) {
mu->WriterLock();
}
explicit WriterMutexLock(Mutex *mu, const Condition &cond)
ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
: mu_(mu) {
mu->WriterLockWhen(cond);
}
WriterMutexLock(const WriterMutexLock&) = delete;
WriterMutexLock(WriterMutexLock&&) = delete;
WriterMutexLock& operator=(const WriterMutexLock&) = delete;
WriterMutexLock& operator=(WriterMutexLock&&) = delete;
~WriterMutexLock() ABSL_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 an 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 to a
// suitable `Mutex' member function, such as `Mutex::Await()`, or to the
// constructor of one of the scope guard classes.
//
// Example using LockWhen/Unlock:
//
// // assume count_ is not internal reference count
// int count_ ABSL_GUARDED_BY(mu_);
// Condition count_is_zero(+[](int *count) { return *count == 0; }, &count_);
//
// mu_.LockWhen(count_is_zero);
// // ...
// mu_.Unlock();
//
// Example using a scope guard:
//
// {
// MutexLock lock(&mu_, count_is_zero);
// // ...
// }
//
// 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));
//
// NOTE: never use "mu_.AssertHeld()" instead of "mu_.AssertReaderHeld()" in
// the lambda as it may be called when the mutex is being unlocked from a
// scope holding only a reader lock, which will make the assertion not
// fulfilled and crash the binary.
// 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:
// A `CondVar` allocated on the heap or on the stack can use the this
// constructor.
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:
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.
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 ABSL_SCOPED_LOCKABLE MutexLockMaybe {
public:
explicit MutexLockMaybe(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
: mu_(mu) {
if (this->mu_ != nullptr) {
this->mu_->Lock();
}
}
explicit MutexLockMaybe(Mutex *mu, const Condition &cond)
ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
: mu_(mu) {
if (this->mu_ != nullptr) {
this->mu_->LockWhen(cond);
}
}
~MutexLockMaybe() ABSL_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;
};
// ReleasableMutexLock
//
// ReleasableMutexLock is like MutexLock, but permits `Release()` of its
// mutex before destruction. `Release()` may be called at most once.
class ABSL_SCOPED_LOCKABLE ReleasableMutexLock {
public:
explicit ReleasableMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
: mu_(mu) {
this->mu_->Lock();
}
explicit ReleasableMutexLock(Mutex *mu, const Condition &cond)
ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
: mu_(mu) {
this->mu_->LockWhen(cond);
}
~ReleasableMutexLock() ABSL_UNLOCK_FUNCTION() {
if (this->mu_ != nullptr) { this->mu_->Unlock(); }
}
void Release() ABSL_UNLOCK_FUNCTION();
private:
Mutex *mu_;
ReleasableMutexLock(const ReleasableMutexLock&) = delete;
ReleasableMutexLock(ReleasableMutexLock&&) = delete;
ReleasableMutexLock& operator=(const ReleasableMutexLock&) = delete;
ReleasableMutexLock& operator=(ReleasableMutexLock&&) = delete;
};
inline Mutex::Mutex() : mu_(0) {
ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);
}
inline constexpr Mutex::Mutex(absl::ConstInitType) : mu_(0) {}
inline CondVar::CondVar() : cv_(0) {}
// 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 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 NUL-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);
ABSL_NAMESPACE_END
} // 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_
