I've been pursuing a simple reproducer for an NFS lockup that shows up 
under stress. There is a bunch of info (some of it extraneous) in 
http://bugs.launchpad.net/bugs/561210. I can reproduce it by writing 
loop mounted NFS exports:

/etc/fstab: 127.0.0.1:/srv /mnt/srv nfs rw 0 2
/etc/exports: /srv 127.0.0.1(rw,insecure,no_subtree_check)

See the attached scripts test_master.sh and test_client.sh. I simply 
repeat './test_master.sh wait' until nfsd locks up, typically within 1-3 
cycles, e.g.,

cd /mnt/srv
while true; do ./test_master.sh wait; done

Note that this test will run indefinitely if invoked from /srv, e.g.,

cd /srv
while true; do ./test_master.sh wait; done

This issue, or something like it, appears to exist as far back as I've 
tested (Ubuntu Lucid 2.6.32.21). For now I'm assuming that, since the 
symptoms are similar, any lockup bug found in -rc3 is the likely culprit.

See attached dmesg and config. Debug options of interest that I've 
enabled are CONFIG_DEBUG_SLAB, CONFIG_DEBUG_SLAB_LEAK, 
CONFIG_DEBUG_SPINLOCK, CONFIG_DEBUG_MUTEXES.

dmesg.txt contains the initial 'INFO: task nfsd:1263 blocked for more 
than 120 seconds.' complaints as well as information dumped from

echo d | sudo tee /proc/sysrq-trigger
echo w | sudo tee /proc/sysrq-trigger

Anything else I can provide?

rtg
-- 
Tim Gardner tim.gardner@canonical.com