AQS unparkSuccessor 方法中for循环从tail开始而不是head

unparkSuccessor() 代码：

private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
			
			// 从tail节点往前遍历
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

这个问题，还需要看回 addWaiter() 往队列尾部添加节点代码：

private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {
			// 看这里（关键点）
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

新节点 pre 指向 tail，tail 指向新节点，这里后继指向前驱的指针是由 CAS 操作保证线程安全的。而cas操作之后 t.next=node 之前，可能会有其他线程进来。所以出现了问题，从尾部向前遍历是一定能遍历到所有的节点。