ReentrantLock的unlock方法,将head节点状态设置为0的作用是啥?

Question

在java的ReentrantLock 的unlock()->release()->unparkSuccessor()方法中

private void unparkSuccessor(Node node) {
    int ws = node.waitStatus;
    if (ws < 0)
        compareAndSetWaitStatus(node, ws, 0);
    //................
}

关于compareAndSetWaitStatus这段的作用是什么呢? 我研究了下

假设AB两线程,A线程先执行,准备走unlock时切换到B,B线程第一次执行到lock() ->acquire()->acquireQueued()->shouldParkAfterFailedAcquire()方法时

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
    int ws = pred.waitStatus;
    if (ws == Node.SIGNAL)
        return true;
    if (ws > 0) {
        do {
            node.prev = pred = pred.prev;
        } while (pred.waitStatus > 0);
        pred.next = node;
    } else {
        compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
    }
    return false;
}

compareAndSetWaitStatus(pred, ws, Node.SIGNAL);当B线程走过到else代码块后,时间片用完此时head节点状态为-1

然后回到A线程执行unlock()->release()->unparkSuccessor()

private void unparkSuccessor(Node node) {

    int ws = node.waitStatus;
    if (ws < 0)
        compareAndSetWaitStatus(node, ws, 0);
    
    Node s = node.next;
    if (s == null || s.waitStatus > 0) {
        s = null;
        for (Node t = tail; t != null && t != node; t = t.prev)
            if (t.waitStatus <= 0)
                s = t;
    }
    if (s != null)
        LockSupport.unpark(s.thread);
}

释放了锁,然后将head节点的状态改为了0,然后回到B线程

final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            
           //B线程执行完方法后返回到这里
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

那么B线程继续for循环,之后就获取到了锁,并置空了head节点,所以A线程在unlock()中修改的head状态并没有什么作用

不是太明白这个步骤,而且注释上写了是允许这个cas失败的,这点也不是太明白