}
          oldHead = waitersField; // re-read and loop.
        } while (oldHead != Waiter.TOMBSTONE);
      }
      // re-read valueField, if we get here then we must have observed a TOMBSTONE while trying to
      // add a waiter.
      // requireNonNull is safe because valueField is always set before TOMBSTONE.
      return getDoneValue(requireNonNull(valueField));
    }

          }
          oldHead = waitersField; // re-read and loop.
        } while (oldHead != Waiter.TOMBSTONE);
      }
      // re-read valueField, if we get here then we must have observed a TOMBSTONE while trying to
      // add a waiter.
      // requireNonNull is safe because valueField is always set before TOMBSTONE.
      return getDoneValue(requireNonNull(valueField));
    }