Stable Debian + bonnie -> unmount problems

[GCS <gcs@xxxxxxxxxxxxxxxx>]

Hello all!

I have a stable Debian (2.2 rev2), only comes from people.debian.org. This is
the way to install 4.0.2 on Potato. Kernel 2.4.2-XFS from CVS, a week ago.
Created a 400 megs file, and xfs on it. Mounted on loopback, and I have
tried 'bonnie -s 380', which caused a segmentation fault. It should not be
a problem, but the 'filesystem' looks full, without any file.
# umount /mnt
umount: /mnt: device is busy

# fuser -v /mnt
           USER        PID ACCESS COMMAND
/mnt       root     kernel mount  /mnt
What can I do?
Thanks, Laszlo