umount NFS filesystem fails in Linux

there is a good example how to umount dead NFS share (let's assume that umount -f / doesn't work.

Quick fix to this problem is editing /etc/mtab file and removing the nfs filesystem mount point . In Linux /etc/mtab file contains all local and nfs mounted filesystems.By editing /etc/mtab i was able to umount hanged nfs filesystem on my Linux machine without doing a reboot.

Finally to resolve
failing nfs mount issue i commented the entry in /etc/mtab file :
#sun:/export /mnt/nfs nfs rw,addr=192.168.0.135 0 0.