Comment 7 for bug 805886

I think that this is related to this behaviour on debootstrap exit, when it tries to umount the chrooted /proc:

root@oneiric:/var/cache/pbuilder/build/26972# umount /var/cache/pbuilder/build//26972/./proc
umount: /proc: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))

but the very odd behaviour is that if i try to do (changing working dir):

root@oneiric:/var/cache/pbuilder/build/26972# cd ..
root@oneiric:/var/cache/pbuilder/build# umount /var/cache/pbuilder/build//26972/./proc
root@oneiric:/var/cache/pbuilder/build#

Using mount 2.17.2-9.1ubuntu4 there are no issues.

The mounting is done via chroot, so the entry in /etc/mtab isn't present.

I think that it's more related to http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=634107