Can someone advise me of a way to unmount nfs devices without
rebooting after an nfs server has become unavailable.

Every time one of my nfs servers locks up, the computers the
terminals that had been using the mounts become hung. If I type in df
on one of the other machine, that terminal will hang. If I try to
umount the device it'll give the error:

umount: /mnt: device is busy

It's not possible to unmount the device because something had be
using it. Since the terminals were telnet sessions, I kill all the
login sessions that had had access to the device. It's still not
possible to unmount the device. The only way I can recover the ability
to use the df command (as well as others that might test that mounted
device), the terminal will hang.

Grep'ing ps for "df" shows:

root 22764 0.0 0.3 1052 468 ? D 13:55 0:00 df -v
root 23352 0.0 0.3 1052 468 ? D 14:07 0:00 df -v

An attemp to kill the tasks 22764 and 23352 fails. (using kill -9 tasks)

Thanks in advance for any suggestions or comments.

-- L. James

umount -f should work.

I should have included that I had already used:

umount -f
umount -r (to change it to read-only which I was hoping would notify the system that everything was flushed and updated then allow the umount to proceed)
umount -aft nfs (to umount everything)

Also, the nfs system that had failed is back up. I would have thought that the nfs becoming available would have wokeup the hung processes.

I keep locking up terminals because I have a habit of typing df -v for no reason and unconscienciously. It's like a fill-in while I'm thinking of another command to type when I'm working.

-- L. James

hmm...that's strange. According to the umount man page
Did you trying running this as root?