Was playing around with using the loopback devices to mount some OS images on CentOS and following the losetup followed by a mount command would indeed mount the image, but umount wouldn’t unmount it and would leave it stagnant, leaving me with this error:

ioctl: LOOP_CLR_FD: Device or resource busy

Interestingly enough though the lomount command

lomount -diskimage OS.img -partition 1  /MOUNTFOLDER

and then using umount to unmount the mountfolder would unmount the loopback device properly and free it for re-use.

Hope this helps someone!