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// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_SYNCHRONIZATION_READ_WRITE_LOCK_H_
#define BASE_SYNCHRONIZATION_READ_WRITE_LOCK_H_
#include "base/base_export.h"
#include "base/macros.h"
#include "build/build_config.h"
#if defined(OS_NACL)
#include "base/synchronization/lock.h"
#endif
#if defined(OS_WIN)
#include <windows.h>
#elif defined(OS_POSIX)
#include <pthread.h>
#else
# error No reader-writer lock defined for this platform.
#endif
namespace base {
namespace subtle {
// An OS-independent wrapper around reader-writer locks. There's no magic here.
//
// You are strongly encouraged to use base::Lock instead of this, unless you
// can demonstrate contention and show that this would lead to an improvement.
// This lock does not make any guarantees of fairness, which can lead to writer
// starvation under certain access patterns. You should carefully consider your
// writer access patterns before using this lock.
class BASE_EXPORT ReadWriteLock {
public:
ReadWriteLock();
~ReadWriteLock();
// Reader lock functions.
void ReadAcquire();
void ReadRelease();
// Writer lock functions.
void WriteAcquire();
void WriteRelease();
private:
#if defined(OS_WIN)
using NativeHandle = SRWLOCK;
#elif defined(OS_NACL)
using NativeHandle = Lock;
#elif defined(OS_POSIX)
using NativeHandle = pthread_rwlock_t;
#endif
NativeHandle native_handle_;
#if defined(OS_NACL)
// Even though NaCl has a pthread_rwlock implementation, the build rules don't
// make it universally available. So instead, implement a slower and trivial
// reader-writer lock using a regular mutex.
// TODO(amistry): Remove this and use the posix implementation when it's
// available in all build configurations.
uint32_t readers_ = 0;
// base::Lock does checking to ensure the lock is acquired and released on the
// same thread. This is not the case for this lock, so use pthread mutexes
// directly here.
pthread_mutex_t writer_lock_ = PTHREAD_MUTEX_INITIALIZER;
#endif
DISALLOW_COPY_AND_ASSIGN(ReadWriteLock);
};
class AutoReadLock {
public:
explicit AutoReadLock(ReadWriteLock& lock) : lock_(lock) {
lock_.ReadAcquire();
}
~AutoReadLock() {
lock_.ReadRelease();
}
private:
ReadWriteLock& lock_;
DISALLOW_COPY_AND_ASSIGN(AutoReadLock);
};
class AutoWriteLock {
public:
explicit AutoWriteLock(ReadWriteLock& lock) : lock_(lock) {
lock_.WriteAcquire();
}
~AutoWriteLock() {
lock_.WriteRelease();
}
private:
ReadWriteLock& lock_;
DISALLOW_COPY_AND_ASSIGN(AutoWriteLock);
};
} // namespace subtle
} // namespace base
#endif // BASE_SYNCHRONIZATION_READ_WRITE_LOCK_H_
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