jdk-17, StampedLock, 修改写锁计数有内存屏障，为什么修改读锁计数时没有？

Z时代
2024-02-20
分类：IT

是因为使用或操作吗？

    @ReservedStackAccess
    private long tryAcquireWrite() {
        long s, nextState;
        if (((s = state) & ABITS) == 0L && casState(s, nextState = s | WBIT)) {
            // ？
            U.storeStoreFence();
            return nextState;
        }
        return 0L;
    }
    @ReservedStackAccess
    private long tryAcquireRead() {
        for (long s, m, nextState;;) {
            if ((m = (s = state) & ABITS) < RFULL) {
                if (casState(s, nextState = s + RUNIT))
                    // ?
                    return nextState;
            }
            else if (m == WBIT)
                return 0L;
            else if ((nextState = tryIncReaderOverflow(s)) != 0L)
                return nextState;
        }
    }
    public long writeLock() {
        // try unconditional CAS confirming weak read
        long s = U.getLongOpaque(this, STATE) & ~ABITS, nextState;
        if (casState(s, nextState = s | WBIT)) {
            // ?
            U.storeStoreFence();
            return nextState;
        }
        return acquireWrite(false, false, 0L);
    }
    public long readLock() {
        // unconditionally optimistically try non-overflow case once
        long s = U.getLongOpaque(this, STATE) & RSAFE, nextState;
        if (casState(s, nextState = s + RUNIT))
            // ?
            return nextState;
        else
            return acquireRead(false, false, 0L);    }