执行df hang住

$ df -h
Filesystem              Size  Used Avail Use% Mounted on
/dev/sda3               221G  100G  121G  46% /
devtmpfs                 16G     0   16G   0% /dev
tmpfs                    16G     0   16G   0% /dev/shm
tmpfs                    16G  1.6G   14G  11% /run
tmpfs                    16G     0   16G   0% /sys/fs/cgroup
/dev/sda1               494M  204M  291M  42% /boot

umount -l -f /mnt

$ umount -h

Usage:
 umount [-hV]
 umount -a [options]
 umount [options] <source> | <directory>

Options:
 -a, --all               unmount all filesystems
 -A, --all-targets       unmount all mountpoins for the given device
                         in the current namespace
 -c, --no-canonicalize   don‘t canonicalize paths
 -d, --detach-loop       if mounted loop device, also free this loop device
     --fake              dry run; skip the umount(2) syscall
 -f, --force             force unmount (in case of an unreachable NFS system)
 -i, --internal-only     don‘t call the umount.<type> helpers
 -n, --no-mtab           don‘t write to /etc/mtab
 -l, --lazy              detach the filesystem now, and cleanup all later
 -O, --test-opts <list>  limit the set of filesystems (use with -a)
 -R, --recursive         recursively unmount a target with all its children
 -r, --read-only         In case unmounting fails, try to remount read-only
 -t, --types <list>      limit the set of filesystem types
 -v, --verbose           say what is being done