code: func (m *Mutex) Unlock() {
  	m.mu.Unlock()
  	if !Opts.Disable {
  		postUnlock(m)
  	}
  }
docs: // Unlock unlocks the mutex.
  // It is a run-time error if m is not locked on entry to Unlock.
  //
  // A locked Mutex is not associated with a particular goroutine.
  // It is allowed for one goroutine to lock a Mutex and then
  // arrange for another goroutine to unlock it.