why ConcurrentLinkedQueue#addAll need set tail twice?

In JDK8's ConcurrentLinkedQueue, there is an addAll function, and it will set tail twice, why?


public boolean addAll(Collection<? extends E> c) {
if (c == this)
// As historically specified in AbstractQueue#addAll
throw new IllegalArgumentException();

// Copy c into a private chain of Nodes
Node<E> beginningOfTheEnd = null, last = null;
    for (E e : c) {
checkNotNull(e);
Node<E> newNode = new Node<E>(e);
        if (beginningOfTheEnd == null)
            beginningOfTheEnd = last = newNode;
        else {
            last.lazySetNext(newNode);
last = newNode;
}
    }
if (beginningOfTheEnd == null)
return false;

// Atomically append the chain at the tail of this collection
for (Node<E> t = tail, p = t;;) {
        Node<E> q = p.next;
        if (q == null) {
// p is last node
if (p.casNext(null, beginningOfTheEnd)) {
// Successful CAS is the linearization point
                // for all elements to be added to this queue.
if (!casTail(t, last)) {  // first set tail
// Try a little harder to update tail,
                    // since we may be adding many elements.
t = tail;
                    if (last.next == null)
                        casTail(t, last);  // second set tail
}
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;
}
}


I don't quite understand, I just think the second set operation is not necessary.
--------------------------------------------------------------------------------
Regards
Liu

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