umount, 老是提示：device is busy， 服務又不能停止的。可以用"umount -fl"解決!
掛載：
mount - mount a filesystem
mount [options] device dir
如：mount --bind /dev /home/tmp/dev
解掛載：
umount - unmount a filesystem
umount [options] dir
如：umount /home/tmp/dev
Note that a file system cannot be unmounted when it is 'busy' - for example, when there are open files on it, or when some process has its working directory there, or when a swap file on it is in use. The offending process could even be umount itself - it opens libc, and libc in its turn may open for example locale files. A lazy unmount avoids this problem。
-f, --force
Force an unmount (in case of an unreachable NFS system).
Note that this option does not guarantee that umount command
does not hang. It's strongly recommended to use absolute
paths without symlinks to avoid unwanted readlink and stat
system calls on unreachable NFS in umount.
-l, --lazy
Lazy unmount. Detach the filesystem from the file hierarchy
now, and clean up all references to this filesystem as soon as
it is not busy anymore.