code: func (m *RWMutex) RUnlock() {
  	m.mu.RUnlock()
  	if !Opts.Disable {
  		postUnlock(m)
  	}
  }
docs: // RUnlock undoes a single RLock call;
  // it does not affect other simultaneous readers.
  // It is a run-time error if rw is not locked for reading
  // on entry to RUnlock.