In method acquire, node.prev is updated by node.setPrevRelaxed(t).
It actually calls unsafe.putReference which does not ensure the visibility.
final void setPrevRelaxed(Node p) { // for off-queue assignment
U.putReference(this, PREV, p);
}
How dose it ensure the visibility of node.prev?
In Java, the happened before semantic ensure that visibility.
For example, node.setPrevRelaxed(t) is used in two place, the first one is in the enqueue method
final void enqueue(Node node) {
if (node != null) {
for (;;) {
Node t = tail;
node.setPrevRelaxed(t); // avoid unnecessary fence
if (t == null) // initialize
tryInitializeHead();
else if (casTail(t, node)) {
t.next = node;
if (t.status < 0) // wake up to clean link
LockSupport.unpark(node.waiter);
break;
}
}
}
}
node != null iff euqueue is call in Condition object which ensure this object has acquired a lock.
Another place is in acquire method
...
node.waiter = current;
Node t = tail;
node.setPrevRelaxed(t); // avoid unnecessary fence
if (t == null)
tryInitializeHead();
else if (!casTail(t, node))
node.setPrevRelaxed(null); // back out
else
t.next = node;
...
In theses two case, the node.prev will be seen after the lock is released or casTail successfully.