code: func (devices *DeviceSet) lookupDevice(hash string) (*devInfo, error) {
  	info := devices.Devices[hash]
  	if info == nil {
  		info = devices.loadMetadata(hash)
  		if info == nil {
  			return nil, fmt.Errorf("devmapper: Unknown device %s", hash)
  		}
  
  		devices.Devices[hash] = info
  	}
  	return info, nil
  }
docs: // Should be called with devices.Lock() held.