java - ConcurrentLinkedQueue代码解释

Question

http://www.java2s.com/Open-Source/Java-Open-Source-Library/7-JDK/java/java/util/concurrent/ConcurrentLinkedQueue.java.htm

以上是ConcurrentLinkedQueue的源码。我无法理解一种情况。

条件 (p == q) 将如何出现在下面的代码片段中，来自 offer 方法

  public boolean offer(E e) {
        checkNotNull(e);
        final Node<E> newNode = new Node<E>(e);

        for (Node<E> t = tail, p = t;;) {
            Node<E> q = p.next;
            if (q == null) {
                // p is last node
                if (p.casNext(null, newNode)) {
                    // Successful CAS is the linearization point
                    // for e to become an element of this queue,
                    // and for newNode to become "live".
                    if (p != t) // hop two nodes at a time
                        casTail(t, newNode);  // Failure is OK.
                    return true;
                }
                // Lost CAS race to another thread; re-read next
            }
            else if (p == q)
                // We have fallen off list.  If tail is unchanged, it
                // will also be off-list, in which case we need to
                // jump to head, from which all live nodes are always
                // reachable.  Else the new tail is a better bet.
                p = (t != (t = tail)) ? t : head;
            else
                // Check for tail updates after two hops.
                p = (p != t && t != (t = tail)) ? t : q;
        }
    }

还有作者所说的“我们已经从名单上掉下来了”是什么意思

Answer 1

ConcurrentLinkedQueue 允许在遍历内部列表时同时修改它。这意味着您正在查看的节点可能已被同时删除。为了检测这种情况，已删除节点的下一个指针更改为指向自身。查看 updateHead (L302) 了解详细信息。