What exactly does UNSAFE.compareAndSwapObject do

Question

I found UNSAFE.compareAndSwapObject very strange when reading the ConcurrentLinkedQueue JDK. (CLQ class is a copy from ConcurrentLinkedQueue for easy debugging ...)

When I suggest the first element of ConcurrentLinkedQueue.

• Before p.casNext(null, newNode) head == tail == p == t ref is the same object as this.

  http://i.stack.imgur.com/8dCO0.png

• go to casNext after UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);

  p.next refers to newNode, as expected, for example.

  http://i.stack.imgur.com/mNtUN.png

• go out to offer

  Everything is getting weird ... I can understand why p.next ref wound change to p, as head ref newNode automatically ...

  http://i.stack.imgur.com/Pvvcy.png

code: ConcurrentLinkedQueue.class offer ()

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;
    }
}