ReentrantLock的内存可见性语义

[seaswalker]

加锁

以NonfairSync为例:

final void lock() {
    if (compareAndSetState(0, 1))
        setExclusiveOwnerThread(Thread.currentThread());
     else
        acquire(1);
}

关键在于compareAndSetState方法:

/**
     * Atomically sets synchronization state to the given updated
     * value if the current state value equals the expected value.
     * This operation has memory semantics of a {@code volatile} read
     * and write.
     *
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that the actual
     *         value was not equal to the expected value.
     */
protected final boolean compareAndSetState(int expect, int update) {
    return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}

注释里写了，此方法有相当于volatile读写的内存语义。所以这个内存语义又是什么？
参考Doug lea的: The JSR-133 Cookbook for Compiler Writers

1. Issue a StoreStore barrier before each volatile store
2. Issue a StoreLoad barrier after each volatile store.(Alternatively, if available, you can implement volatile store as an atomic instruction (for example XCHG on x86) and omit the barrier. This may be more efficient if atomic instructions are cheaper than StoreLoad barriers.)
3. Issue LoadLoad and LoadStore barriers after each volatile load.

unsafe.compareAndSwapInt由Atomic::cmpxchg实现(Linux):

inline jlong Atomic::cmpxchg (jlong exchange_value, volatile jlong* dest, jlong compare_value) {
  bool mp = os::is_MP();
  __asm__ __volatile__ (LOCK_IF_MP(%4) "cmpxchgq %1,(%3)"
                        : "=a" (exchange_value)
                        : "r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp)
                        : "cc", "memory");
  return exchange_value;
}

所以这在x86上就是加了lock前缀的cmpxhgg指令，而lock前缀在intel上便充当了读写memory barrier的作用，来自书中的摘抄L:

On the other hand, x86 CPUs have traditionally given no ordering guarantees for loads, so the smp_mb() and smp_rmb() primitives expand to lock;addl. This atomic instruction acts as a barrier to both loads and stores.

解锁

只看一行:

protected final boolean tryRelease(int releases) {
    // ...
    setState(c);
    // ...
}

/**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a {@code volatile} write.
     * @param newState the new state value
     */
protected final void setState(int newState) {
    state = newState;
}

从前面Doug lea的文章中可以看出，volatile写会导致在后面追加一个StoreLoad屏障，而此屏障在x86上:

So，也许又是一条lock前缀指令。