Linux refuses to remove a directory that is a mount point, so
<chroot>/proc should still exist. And Linux refuses to remove a directory that is not empty, so
<chroot> should still exist.
Taking what you wrote at face value, you tried to unmount
<chroot>/proc. So the kernel tried to unmount the
none filesystem mounted on
/proc. You should unmount the
none filesystem mounted at
umount <chroot>/proc then
rmdir <chroot>/proc <chroot>.
Note that if you moved
<chroot>, you must pass the new name to
umount. You can check what the kernel thinks is mounted by looking in
umount complains because your
/etc/mtab has somehow gone out of sync, use
umount -n <chroot>/proc doesn't work, it's probably because some process is still running inside the chroot to access it. Use
lsof to locate the process and kill it.