Comment 4 for bug 974374

This problem is not limited to stale mounts.

It's an error in the interaction between mount, umount and mount details in /proc/mounts vs /etc/mtab

In /etc/mtab, nfs4 mounts seem to have a problem with leading slashes.

For example, on my 12.04 client machine, mounting a zfs volume on a 12.04 host, the following works fine:

me@client:/$ sudo mount -t nfs bighost:/remotevol /mnt/target
me@client:/$ sudo umount /mnt/target
me@client:/$

No issues.

However, the mount command will accept the volume spec without the leading slash and mount it:

me@client:/$ sudo mount -t nfs bighost:remotevol /mnt/target
me@client:/$ sudo umount /mnt/target
/mnt/target was not found in /proc/mounts
/mnt/target was not found in /proc/mounts
me@client:/$

And the the remotevol is not unmounted.

umount fails to identify the mounted volume in /proc/mounts because, when mounted without the leading slash, it is denoted differently between /proc/mounts and the entry in /etc/mtab

me@client:/$ grep /mnt/target /etc/mtab
bighost:remotevol /mnt/target nfs rw,vers=4,addr=192.168.1.19,clientaddr=192.168.1.128 0 0
me@client:/$ grep /mnt/target /proc/mounts
bighost:/remotevol/ /mnt/target nfs4 rw,relatime,vers=4,rsize=262144,wsize=262144,namlen=255,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=192.168.1.128,minorversion=0,local_lock=none,addr=192.168.1.19 0 0

To fix at time of the error, edit mtab for the un-umountable volume and add a leading slash to the volume path specifier... then unmount and it will work.

To prevent the error occurring, always specify volume paths with a leading slash on the command line or in fstab...

Real fix... I guess mount should require a leading slash on the volume name, and it's a bug that it doesn't?