Comment 17 for bug 151579

All the ones before (and including) /

That is required in situations where / is inside another device (e.g. / is inside /host/path/to/root.containing.loopfile where /dev/sdXY is mounted as /host), otherwise unmounting /host would be equivalent to unmounting /. Host devices will appear before / in /proc/mounts. /etc/init.d/umountroot has also been modified to remount r/o all such devices.