}
  }

  /**
   * {@link AtomicHelper} based on {@code synchronized} and volatile writes.
   *
   * <p>This is an implementation of last resort for when certain basic VM features are broken (like
   * AtomicReferenceFieldUpdater).
   */
  private static final class SynchronizedHelper extends AtomicHelper {
    @Override
    void putThread(Waiter waiter, Thread newValue) {