Unmounting problems

umount: device is busy. Why?
There are several reasons why a file system cannot be unmounted. Here are a few of the reasons.

Working directory or file open of a running process

# umount /mnt/test
umount: /mnt/test: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
# lsof /mnt/test/
COMMAND   PID  USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
bash    15819 tange  cwd    DIR   8,81    16384 6931 /mnt/test/testdir
cat     24932 root    1w   REG   8,81        0 6944 /mnt/test/testdir/a
# kill -9 15819 24932
# lsof /mnt/test/
# umount /mnt/test
nfs-kernel-server is exporting the directory (and the directory is mounted by a client)

# umount /mnt/test
umount: /mnt/test: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
# showmount -e
Export list for alpha:
/mnt/test 192.168.1.203
# /etc/init.d/nfs-kernel-server stop
# umount /mnt/test
A loopback device backing file exists in the dir

# umount /mnt/test
umount: /mnt/test: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
# losetup -a
/dev/loop0: [0851]:6983 (/mnt/test/testdir/disk.img)
/dev/loop1: [0851]:6986 (/mnt/test/testdir/disk2.img)
# umount /mnt/test/testdir/disk.img
# umount /mnt/test/testdir/disk2.img
umount: /mnt/test/testdir/disk2.img: not mounted
# losetup -d /dev/loop1
# umount /mnt/test
A swapfile exists in the dir

# umount /mnt/test
umount: /mnt/test: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
# cat /proc/swaps
Filename                                Type            Size    Used    Priority
/mnt/test/testdir/swapfile              file            95480   0       -2
# swapoff /mnt/test/testdir/swapfile
# umount /mnt/test
 
The device failed

# umount /mnt/test
umount: /mnt/test: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1)) 
# dmesg
:
[75620.618404] lost page write due to I/O error on md0
[75620.618408] Buffer I/O error on device md0, logical block 16804
:
[75890.015507] INFO: task cp:27736 blocked for more than 120 seconds.
[75890.015513] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[75890.015516] cp              D ffff88053fdd39c0     0 27736  27734 0x00000004
## Here I have not found a solution, so:
# umount -l /mnt/test
The dir contains an umounted encfs


normaluser$ encfs /mnt/test/dir.enc /mnt/test/dir
EncFS Password:
# umount /mnt/test/dir
# umount /mnt/test
umount: /mnt/test: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
## Here I have not found a solution, so:
# umount -l /mnt/test