Source code

Revision control

Other Tools

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
// An interface for read-write locks.
#ifndef mozilla_RWLock_h
#define mozilla_RWLock_h
#include "mozilla/Assertions.h"
#include "mozilla/Atomics.h"
#include "mozilla/BlockingResourceBase.h"
#ifndef XP_WIN
# include <pthread.h>
namespace mozilla {
// A RWLock is similar to a Mutex, but whereas a Mutex permits only a single
// reader thread or a single writer thread to access a piece of data, a
// RWLock distinguishes between readers and writers: you may have multiple
// reader threads concurrently accessing a piece of data or a single writer
// thread. This difference should guide your usage of RWLock: if you are not
// reading the data from multiple threads simultaneously or you are writing
// to the data roughly as often as read from it, then Mutex will suit your
// purposes just fine.
// You should be using the AutoReadLock and AutoWriteLock classes, below,
// for RAII read locking and write locking, respectively. If you really must
// take a read lock manually, call the ReadLock method; to relinquish that
// read lock, call the ReadUnlock method. Similarly, WriteLock and WriteUnlock
// perform the same operations, but for write locks.
// It is unspecified what happens when a given thread attempts to acquire the
// same lock in multiple ways; some underlying implementations of RWLock do
// support acquiring a read lock multiple times on a given thread, but you
// should not rely on this behavior.
// It is unspecified whether RWLock gives priority to waiting readers or
// a waiting writer when unlocking.
class RWLock : public BlockingResourceBase {
explicit RWLock(const char* aName);
// Windows rwlocks don't need any special handling to be destroyed, but
// POSIX ones do.
#ifdef XP_WIN
~RWLock() = default;
#ifdef DEBUG
bool LockedForWritingByCurrentThread();
bool TryReadLock();
void ReadLock();
void ReadUnlock();
bool TryWriteLock();
void WriteLock();
void WriteUnlock();
bool TryReadLock() { return TryReadLockInternal(); }
void ReadLock() { ReadLockInternal(); }
void ReadUnlock() { ReadUnlockInternal(); }
bool TryWriteLock() { return TryWriteLockInternal(); }
void WriteLock() { WriteLockInternal(); }
void WriteUnlock() { WriteUnlockInternal(); }
bool TryReadLockInternal();
void ReadLockInternal();
void ReadUnlockInternal();
bool TryWriteLockInternal();
void WriteLockInternal();
void WriteUnlockInternal();
RWLock() = delete;
RWLock(const RWLock&) = delete;
RWLock& operator=(const RWLock&) = delete;
#ifndef XP_WIN
pthread_rwlock_t mRWLock;
// SRWLock is pointer-sized. We declare it in such a fashion here to
// avoid pulling in windows.h wherever this header is used.
void* mRWLock;
#ifdef DEBUG
// We record the owning thread for write locks only.
PRThread* mOwningThread;
template <typename T>
class MOZ_RAII BaseAutoTryReadLock {
explicit BaseAutoTryReadLock(T& aLock)
: mLock(aLock.TryReadLock() ? &aLock : nullptr) {}
~BaseAutoTryReadLock() {
if (mLock) {
explicit operator bool() const { return mLock; }
BaseAutoTryReadLock() = delete;
BaseAutoTryReadLock(const BaseAutoTryReadLock&) = delete;
BaseAutoTryReadLock& operator=(const BaseAutoTryReadLock&) = delete;
T* mLock;
template <typename T>
class MOZ_RAII BaseAutoReadLock {
explicit BaseAutoReadLock(T& aLock) : mLock(&aLock) {
MOZ_ASSERT(mLock, "null lock");
~BaseAutoReadLock() { mLock->ReadUnlock(); }
BaseAutoReadLock() = delete;
BaseAutoReadLock(const BaseAutoReadLock&) = delete;
BaseAutoReadLock& operator=(const BaseAutoReadLock&) = delete;
T* mLock;
template <typename T>
class MOZ_RAII BaseAutoTryWriteLock {
explicit BaseAutoTryWriteLock(T& aLock)
: mLock(aLock.TryWriteLock() ? &aLock : nullptr) {}
~BaseAutoTryWriteLock() {
if (mLock) {
explicit operator bool() const { return mLock; }
BaseAutoTryWriteLock() = delete;
BaseAutoTryWriteLock(const BaseAutoTryWriteLock&) = delete;
BaseAutoTryWriteLock& operator=(const BaseAutoTryWriteLock&) = delete;
T* mLock;
template <typename T>
class MOZ_RAII BaseAutoWriteLock final {
explicit BaseAutoWriteLock(T& aLock) : mLock(&aLock) {
MOZ_ASSERT(mLock, "null lock");
~BaseAutoWriteLock() { mLock->WriteUnlock(); }
BaseAutoWriteLock() = delete;
BaseAutoWriteLock(const BaseAutoWriteLock&) = delete;
BaseAutoWriteLock& operator=(const BaseAutoWriteLock&) = delete;
T* mLock;
// Read try-lock and unlock a RWLock with RAII semantics. Much preferred to
// bare calls to TryReadLock() and ReadUnlock().
typedef BaseAutoTryReadLock<RWLock> AutoTryReadLock;
// Read lock and unlock a RWLock with RAII semantics. Much preferred to bare
// calls to ReadLock() and ReadUnlock().
typedef BaseAutoReadLock<RWLock> AutoReadLock;
// Write try-lock and unlock a RWLock with RAII semantics. Much preferred to
// bare calls to TryWriteLock() and WriteUnlock().
typedef BaseAutoTryWriteLock<RWLock> AutoTryWriteLock;
// Write lock and unlock a RWLock with RAII semantics. Much preferred to bare
// calls to WriteLock() and WriteUnlock().
typedef BaseAutoWriteLock<RWLock> AutoWriteLock;
// XXX: normally we would define StaticRWLock as
// MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS, but the contexts in which it
// is used (e.g. member variables in a third-party library) are non-trivial
// to modify to properly declare everything at static scope. As those
// third-party libraries are the only clients, put it behind the detail
// namespace to discourage other (possibly erroneous) uses from popping up.
namespace detail {
class StaticRWLock {
// In debug builds, check that mLock is initialized for us as we expect by
// the compiler. In non-debug builds, don't declare a constructor so that
// the compiler can see that the constructor is trivial.
#ifdef DEBUG
StaticRWLock() { MOZ_ASSERT(!mLock); }
bool TryReadLock() { return Lock()->TryReadLock(); }
void ReadLock() { Lock()->ReadLock(); }
void ReadUnlock() { Lock()->ReadUnlock(); }
bool TryWriteLock() { return Lock()->TryWriteLock(); }
void WriteLock() { Lock()->WriteLock(); }
void WriteUnlock() { Lock()->WriteUnlock(); }
RWLock* Lock() {
if (mLock) {
return mLock;
RWLock* lock = new RWLock("StaticRWLock");
if (!mLock.compareExchange(nullptr, lock)) {
delete lock;
return mLock;
Atomic<RWLock*> mLock;
// Disallow copy constructor, but only in debug mode. We only define
// a default constructor in debug mode (see above); if we declared
// this constructor always, the compiler wouldn't generate a trivial
// default constructor for us in non-debug mode.
#ifdef DEBUG
StaticRWLock(const StaticRWLock& aOther);
// Disallow these operators.
StaticRWLock& operator=(StaticRWLock* aRhs);
static void* operator new(size_t) noexcept(true);
static void operator delete(void*);
typedef BaseAutoTryReadLock<StaticRWLock> StaticAutoTryReadLock;
typedef BaseAutoReadLock<StaticRWLock> StaticAutoReadLock;
typedef BaseAutoTryWriteLock<StaticRWLock> StaticAutoTryWriteLock;
typedef BaseAutoWriteLock<StaticRWLock> StaticAutoWriteLock;
} // namespace detail
} // namespace mozilla
#endif // mozilla_RWLock_h