why there's a for(;;) in AbstractQueuedSynchronize enq(final Node node) method

Question

when I read the AbstractQueuedSynchronize source code, about the method

private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

I'm wondering why there's a for loop since it could be like this:

private Node enq(final Node node) {
        Node t = tail;
        if (t == null) { // Must initialize
            if (compareAndSetHead(new Node()))
                tail = head;
        } 
        node.prev = tail;
        if (compareAndSetTail(t, node)) {
            t.next = node;
            return t;
        }
}

dose this have something todo with the concurrent?

Answer 1

Your re-write is not equivalent code. In your version, if the call to compareAndSetHead(new Node()) evaluates to false, tail will still be null at node.prev = tail;

The original, wrapped in the for (;;) { ... } will keep trying until tail is non-null.