code: private boolean findNodeFromTail(Node node) {
          // We check for node first, since it's likely to be at or near tail.
          // tail is known to be non-null, so we could re-order to "save"
          // one null check, but we leave it this way to help the VM.
          for (Node p = tail;;) {
              if (p == node)
                  return true;
              if (p == null)
                  return false;
              p = p.prev;
          }
      }
docs: Returns true if node is on sync queue by searching backwards from tail.
  Called only when needed by isOnSyncQueue.
  @return true if present