}
  }

  private abstract static class AtomicHelper {
    /** Non-volatile write of the thread to the {@link Waiter#thread} field. */
    abstract void putThread(Waiter waiter, Thread newValue);

    /** Non-volatile write of the waiter to the {@link Waiter#next} field. */
    abstract void putNext(Waiter waiter, @CheckForNull Waiter newValue);

    /** Performs a CAS operation on the {@link #waiters} field. */