File locking quits

[Steve Gaarder]

I am running an NFS (version 3 and 4) server on Red Hat Enterprise 5, upgraded 
to kernel version 2.6.18-92.el5 a couple weeks ago.  A couple days ago NFS file 
locking quit completely.  Any program that tried to lock a file would hang, 
including this piece of Python code:

import fcntl
fp = open("lock-test4", "a")
fcntl.lockf(fp.fileno(), fcntl.LOCK_EX|fcntl.LOCK_NB)

A packet sniff showed periodic retransmissions of requests to the lock manager 
port, and no replies.  I am running iptables with that port (among others) 
allowed through.  Restarting iptables did not help.  The only thing that did 
help was a reboot.  The next day the problem happened again; this time, when I 
rebooted, I reverted to the older kernel, 2.6.18-53.1.4.el5.  So far, it's run 
a bit more than 24 hours without incident.

- any idea what might be going on?
- am I correct that this locking is handled in the kernel?
- is there a way of restarting locking short of rebooting?
- how would I go about debugging this further?

thanks,

Steve Gaarder
System Administrator, Dept of Mathematics
Cornell University, Ithaca, NY, USA
gaarder1 at math.cornell.edu