newHead.execute();
          return;
        }
        // Try to make newHead the new head of the stack at runnables.
        newHead.next = oldHead;
      } while (!UNSAFE.compareAndSwapObject(this, HEAD_OFFSET, oldHead, newHead));
    }

    public void execute() {
      RunnableExecutorPair stack;
      do {
        stack = head;
        if (stack == null) {