trying to umount a chroot /dev

[Ross Boylan <ross@biostat.ucsf.edu>]

I setup a chroot on a snapshot.  Part of the setup was
mount --rbind /dev /mnt/chrtest/dev

I have exited the chroot and, I believe, ended the processes I started.
umount /mnt/chrtest/dev
gives umount: /mnt/chrtest/dev: device is busy

How can I get this to work?

After reviewing the output of mount I umounted the mounts below /dev,
which seems to be the main advice on the net for undoing rbinds.

The net also mentions using lsof to see what is accessing the file
system.  lsof isn't much help because it shows everything that is
touching /dev (I presume--it's a long list), and I can't shut all that
down without a reboot.

My ultimate goal is to destroy the snapshot, but neither lvremove nor
lvchange -an work with the volume "in use".

linux kernel 2.6.26-2-686.

Thanks.
Ross