Umount: device busy

Hello,

I have a problem with a file system that I cannot unmount:
[s1: ~]# umount /root/backup
umount: /root/backup: device is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))
[s1: ~]#

I have tried to find who is using it, but neither lsof nor fuser does not show any process using the file system:
[s1: ~]# fuser /root/backup
[s1: ~]# fuser -c /root/backup
[s1: ~]# lsof | grep backup
[s1: ~]#
I have checked the /proc/mounts to see if there is any mount which would use a mount point below this one, but there is none.

Now I can reboot the system to have that mount gone, but what can be done if reboot is not an option? Where should I look for its usage.
The device that is mounted here is an LVM snapshot which I'm using to backup a volume, I don't know if this makes any difference.

Thank you in advance for your answers.