weex_core/Source/include/wtf/WordLock.h

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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
 *
 *   http://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.
 */
#ifndef WTF_WordLock_h
#define WTF_WordLock_h

#include <wtf/Atomics.h>
#include <wtf/Compiler.h>
#include <wtf/Locker.h>
#include <wtf/Noncopyable.h>

namespace TestWebKitAPI {
struct LockInspector;
};

namespace WTF {

// A WordLock is a fully adaptive mutex that uses sizeof(void*) storage. It has a fast path that is
// similar to a spinlock, and a slow path that is similar to std::mutex. In most cases, you should use
// Lock instead. WordLock sits lower in the stack and is used to implement Lock, so Lock is the main
// client of WordLock.

// NOTE: This is also a great lock to use if you are very low in the stack. For example,
// PrintStream uses this so that ParkingLot and Lock can use PrintStream. This means that if you
// try to use dataLog to debug this code, you will have a bad time.

struct WordLockBase {
    void lock()
    {
        if (LIKELY(m_word.compareExchangeWeak(0, isLockedBit, std::memory_order_acquire))) {
            // WordLock acquired!
            return;
        }

        lockSlow();
    }

    void unlock()
    {
        if (LIKELY(m_word.compareExchangeWeak(isLockedBit, 0, std::memory_order_release))) {
            // WordLock released, and nobody was waiting!
            return;
        }

        unlockSlow();
    }

    bool isHeld() const
    {
        return m_word.load(std::memory_order_acquire) & isLockedBit;
    }

    bool isLocked() const
    {
        return isHeld();
    }

protected:
    friend struct TestWebKitAPI::LockInspector;
    
    static const uintptr_t isLockedBit = 1;
    static const uintptr_t isQueueLockedBit = 2;
    static const uintptr_t queueHeadMask = 3;

    WTF_EXPORT_PRIVATE void lockSlow();
    WTF_EXPORT_PRIVATE void unlockSlow();

    // Method used for testing only.
    bool isFullyReset() const
    {
        return !m_word.load();
    }

    Atomic<uintptr_t> m_word;
};

class WordLock : public WordLockBase {
    WTF_MAKE_NONCOPYABLE(WordLock);
public:
    WordLock()
    {
        m_word.store(0, std::memory_order_relaxed);
    }
};

typedef WordLockBase StaticWordLock;
typedef Locker<WordLockBase> WordLockHolder;

} // namespace WTF

using WTF::WordLock;
using WTF::WordLockHolder;
using WTF::StaticWordLock;

#endif // WTF_WordLock_h