We have a NFS server here with a fairly high load. The clients are
Linux, FreeBSD and Solaris. The exported filesystem is XFS, which is onb
a LVM drive. After between 3 and 30 days it seems that locking
completely stops working, clients generally either error or simply lock
up when they try to lock a file. The only way to fix it seems to be a
reboot.

Last time it happened was on 2.6.17-rc2, it started around 2.6.15.

There is nothing in the dmesg on the server, the (Linux) clients are
printing this in the dmesg when something tries to create a lock:

lockd: server xxx.xxx.xxx.xxx not responding, still trying
lockd: server xxx.xxx.xxx.xxx not responding, still trying
lockd: server xxx.xxx.xxx.xxx not responding, still trying
lockd: server xxx.xxx.xxx.xxx not responding, still trying

On FreeBSD machines, there are no messages, the process just hangs in
the D state until the system is either rebooted, or we stop the nfs
lockd on the client.

I am attaching the .config from the server.