Running df or ls in a NFS share it simply hangs.
See if df or ls hangs. Run strace df -h or strace ls -l and see if it finishes
root@linux:~ # strace df -h
root@linux:~ # strace ls -l
Umount with -f or -l
Force an unmount (in case of an unreachable NFS system). (Requires kernel 2.1.116 or later.)
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 syscalls on unreachable NFS in umount.
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. (Requires kernel 2.4.11 or later.)