final boolean casBase(long cmp, long val) {
    return UNSAFE.compareAndSwapLong(this, BASE_OFFSET, cmp, val);
  }

  /** CASes the busy field from 0 to 1 to acquire lock. */
  final boolean casBusy() {
    return UNSAFE.compareAndSwapInt(this, BUSY_OFFSET, 0, 1);
  }

  /**
   * Computes the function of current and new value. Subclasses should open-code this update