I have an ext4 filesystem mounted on /var/lib/docker, using Ubuntu 14.04.  I ran umount /var/lib/docker and got:umount: /var/lib/docker: device is busy.I had already shut down docker with service docker stop and ps aux showed no processes.The strange thing is that both fuser -M /var/lib/docker and lsof +f -- /var/lib/docker show no processes with files open.  This doesn't happen on 40+ other servers with same setup, and I have just noticed this.  A reboot fixed this - curious if anyone has some idea what's going on, or any other ways to investigate if it recurs.In case it helps: Docker (1.11.2) had been using aufs as the overlay filesystem for its containers, on top of this FS - but all containers and Docker processes were killed before the umount.Kernel details: Linux node05 4.4.0-38-generic #57~14.04.1-Ubuntu SMP Tue Sep 6 17:20:43 UTC 2016 x86_64 x86_64 x86_64 GNU/LinuxPossibly related questions:Can't unmount a loop filesystem but no open filesWhat's preventing unmount of a bind mount?UPDATE - recurrence on another serverThis recurred on a different server with very similar configuration, in same situation - have shut down docker, and can't umount /var/lib/docker because it's busy.As before, fuser and lsof show no processes with files open on the docker FS.  mount | grep aufs showed no aufs filesystemsdf -h| grep docker showed two Docker managed filesystems mounted underneath the /var/lib/docker directory (lsof doesn't show these, it seems).  However, unmounting these two filesystems didn't help.WorkaroundThe only workaround remains to edit /etc/fstab to temporarily comment out the /var/lib/docker filesystem, then reboot the server so /var/lib/docker is not mounted. 
Docker filesystem has no open files but cannot unmount
linux;ubuntu;filesystems;mount;docker