code: public final int decrementPendingCountUnlessZero() {
      int c;
      do {
      } while ((c = pending) != 0 && !U.compareAndSwapInt(this, PENDING, c, c - 1));
      return c;
    }
docs: If the pending count is nonzero, (atomically) decrements it.
  
  @return the initial (undecremented) pending count holding on entry to this method