can't delete device /dev/loop3: Device or resource busy

[Ruben]

losetup cant unmount the loop device.

# sudo losetup -a
/dev/loop3: [fc00]:12451874 (/var/lib/libvirt/images/server-nagios.img)

# sudo losetup -d /dev/loop3
loop: can't delete device /dev/loop3: Device or resource busy

# sudo umount /dev/loop3
umount: /dev/loop3: not mounted

Distributor ID: Ubuntu
Description: Ubuntu 12.04.3 LTS
Release: 12.04
Codename: precise

Kernel: 3.2.0-57-generic
x86_64 x86_64 x86_64 GNU/Linux

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[amias (amias)]

still present in 14.04 , this looks like an upstream bug as i've seen it reported for fedora as well.

this is a very annoying bug because it means you cannot reuse loopback devices so are limited
to 8 loopbacks untill you have to reboot , its quite easy to use that up , this also makes scripting
with loopbacks much harder because you can't clear and remake them in a simple manner.