gecko-dev/xpcom/threads/RWLock.h
Manish Goregaokar e104a8487b Bug 1367619 - Use RWLock when accessing font prefs service off main thread; r=heycam
MozReview-Commit-ID: Dxdq6Etbwxa

--HG--
extra : rebase_source : ec9f847c7dce6a101db6a3d157395f5b73d87ba1
2017-05-25 15:04:13 -07:00

151 lines
4.2 KiB
C++

/* -*- 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 http://mozilla.org/MPL/2.0/. */
// An interface for read-write locks.
#ifndef mozilla_RWLock_h
#define mozilla_RWLock_h
#include "mozilla/Assertions.h"
#include "mozilla/BlockingResourceBase.h"
#include "mozilla/GuardObjects.h"
#ifndef XP_WIN
#include <pthread.h>
#endif
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
{
public:
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;
#else
~RWLock();
#endif
#ifdef DEBUG
bool LockedForWritingByCurrentThread();
void ReadLock();
void ReadUnlock();
void WriteLock();
void WriteUnlock();
#else
void ReadLock() { ReadLockInternal(); }
void ReadUnlock() { ReadUnlockInternal(); }
void WriteLock() { WriteLockInternal(); }
void WriteUnlock() { WriteUnlockInternal(); }
#endif
private:
void ReadLockInternal();
void ReadUnlockInternal();
void WriteLockInternal();
void WriteUnlockInternal();
RWLock() = delete;
RWLock(const RWLock&) = delete;
RWLock& operator=(const RWLock&) = delete;
#ifndef XP_WIN
pthread_rwlock_t mRWLock;
#else
// 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;
#endif
#ifdef DEBUG
// We record the owning thread for write locks only.
PRThread* mOwningThread;
#endif
};
// Read lock and unlock a RWLock with RAII semantics. Much preferred to bare
// calls to ReadLock() and ReadUnlock().
class MOZ_RAII AutoReadLock final
{
public:
explicit AutoReadLock(RWLock& aLock MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: mLock(&aLock)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
MOZ_ASSERT(mLock, "null lock");
mLock->ReadLock();
}
~AutoReadLock()
{
mLock->ReadUnlock();
}
private:
AutoReadLock() = delete;
AutoReadLock(const AutoReadLock&) = delete;
AutoReadLock& operator=(const AutoReadLock&) = delete;
RWLock* mLock;
MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
};
// Write lock and unlock a RWLock with RAII semantics. Much preferred to bare
// calls to WriteLock() and WriteUnlock().
class MOZ_RAII AutoWriteLock final
{
public:
explicit AutoWriteLock(RWLock& aLock MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: mLock(&aLock)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
MOZ_ASSERT(mLock, "null lock");
mLock->WriteLock();
}
~AutoWriteLock()
{
mLock->WriteUnlock();
}
private:
AutoWriteLock() = delete;
AutoWriteLock(const AutoWriteLock&) = delete;
AutoWriteLock& operator=(const AutoWriteLock&) = delete;
RWLock* mLock;
MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
};
} // namespace mozilla
#endif // mozilla_RWLock_h