* simple approach would be for each caller CAS it from null to a Set populated with its
     * exception. But there's another race: If the first thread fails with an exception and a second
     * thread immediately fails with the same exception:
     *
     * Thread1: calls setException(), which returns true, context switch before it can CAS
     * seenExceptionsField to its exception
     *

// Some Android 5.0.x Samsung devices have bugs in JDK reflection APIs that cause
// getDeclaredField to throw a NoSuchFieldException when the field is definitely there.
// Since this class only needs CAS on one field, we can avoid this bug by extending AtomicReference
// instead of using an AtomicReferenceFieldUpdater. This reference stores Thread instances
// and DONE/INTERRUPTED - they have a common ancestor of Runnable.

     */
    Waiter(boolean unused) {}

    Waiter() {
      // avoid volatile write, write is made visible by subsequent CAS on waitersField field
      putThread(this, Thread.currentThread());
    }

    // non-volatile write to the next field. Should be made visible by a subsequent CAS on
    // waitersField.
    void setNext(@Nullable Waiter next) {
      putNext(this, next);
    }

    void unpark() {

     */
    Waiter(boolean unused) {}

    Waiter() {
      // avoid volatile write, write is made visible by subsequent CAS on waitersField field
      putThread(this, Thread.currentThread());
    }

    // non-volatile write to the next field. Should be made visible by a subsequent CAS on
    // waitersField.
    void setNext(@Nullable Waiter next) {
      putNext(this, next);
    }

    void unpark() {

      boolean uncontended = true;
      if ((hc = threadHashCode.get()) == null
          || as == null
          || (n = as.length) < 1
          || (a = as[(n - 1) & hc[0]]) == null
          || !(uncontended = a.cas(v = a.value, v + x))) retryUpdate(x, hc, uncontended);
    }
  }

  /** Equivalent to {@code add(1)}. */
  @Override
  public void increment() {
    add(1L);
  }

  /** Equivalent to {@code add(-1)}. */