I previously mounted one NFS volume from the NFS server, but now the NFS server is down, then I have problem with “df” command to see the disk space usage information from my computer, now the only thing I want to do is to unmount the unreachable NFS volume. The NFS volume is mounted at /mnt/nfs, when I try to unmount the NFS volume, I am told that it is busy, so I use lsof /mnt/nfs to find out the process using it normally, but lsof gets stuck and it never return the shell prompt to me.