BTW,
do you understand why there is a test in (*)?

    void
    upgrade_mutex::unlock_shared()
    {
      boost::lock_guard<mutex_t> _(mut_);
      count_t num_readers = (state_ & n_readers_) - 1;
      state_ &= ~n_readers_;
      state_ |= num_readers;
      if (state_ & write_entered_)
      {
        if (num_readers == 0)
          gate2_.notify_one();
      }
      else
      {
        if (num_readers == n_readers_ - 1) // (*)
          gate1_.notify_one();
      }
    }