Comment # 14 on bug 1080485 from Ronnie Bailey

  In my case, the last suggestion in comment 11 may work. I used to get
shutdown time-outs of 90 seconds while the system was waiting for TeamViewer
(3rd party) to shut down. I no longer get that error (possibly due to updates),
but it is possible that TeamViewer is what is causing the delay in releasing
root. 
  I another forum, someone suggested allowing the first device-mapper error to
post, then to remain silent unless there is a total failure in unmounting. 
  Or if there were a way to add a 2 second delay to allow more time to complete
the release of root, that may help.