umount: /mnt/tmp: device is busy

If you are trying to unmount a device, you get the error "device is busy", but lsof/fuser don't show any process holding the device open, it could be that a file on the device is associated with a loop device.

try

# losetup -a
/dev/loop/0: [0821]:25280514 (image) offset=55273881600
# losetup  -d /dev/loop0 
# umount /mnt/tmp

dmsetup info | grep loop
dmsetup remove loop0p2
dmsetup remove loop0p3