NFS deadlock on 9.2-Beta1

[Rick Macklem]

Michael Tratz wrote:
> Two machines (NFS Server: running ZFS / Client: disk-less), both are
> running FreeBSD r253506. The NFS client starts to deadlock processes
> within a few hours. It usually gets worse from there on. The
> processes stay in "D" state. I haven't been able to reproduce it
> when I want it to happen. I only have to wait a few hours until the
> deadlocks occur when traffic to the client machine starts to pick
> up. The only way to fix the deadlocks is to reboot the client. Even
> an ls to the path which is deadlocked, will deadlock ls itself. It's
> totally random what part of the file system gets deadlocked. The NFS
> server itself has no problem at all to access the files/path when
> something is deadlocked on the client.
> 
> Last night I decided to put an older kernel on the system r252025
> (June 20th). The NFS server stayed untouched. So far 0 deadlocks on
> the client machine (it should have deadlocked by now). FreeBSD is
> working hard like it always does. :-) There are a few changes to the
> NFS code from the revision which seems to work until Beta1. I
> haven't tried to narrow it down if one of those commits are causing
> the problem. Maybe someone has an idea what could be wrong and I can
> test a patch or if it's something else, because I'm not a kernel
> expert. :-)
> 
Well, the only NFS client change committed between r252025 and r253506
is r253124. It fixes a file corruption problem caused by a previous
commit that delayed the vnode_pager_setsize() call until after the
nfs node mutex lock was unlocked.

If you can test with only r253124 reverted to see if that gets rid of
the hangs, it would be useful, although from the procstats, I doubt it.

> I have run several procstat -kk on the processes including the ls
> which deadlocked. You can see them here:
> 
> http://pastebin.com/1RPnFT6r

All the processes you show seem to be stuck waiting for a vnode lock
or in __utmx_op_wait. (I`m not sure what the latter means.)

What is missing is what processes are holding the vnode locks and
what they are stuck on.

A starting point might be ``ps axhl``, to see what all the threads
are doing (particularily the WCHAN for them all). If you can drop into
the debugger when the NFS mounts are hung and do a ```show alllocks``
that could help. See:
http://www.freebsd.org/doc/en_US.ISO8859-1/books/developers-handbook/kerneldebug-deadlocks.html

I`ll admit I`d be surprised if r253124 caused this, but who knows.

If there have been changes to your network device driver between
r252025 and r253506, I`d try reverting those. (If an RPC gets stuck
waiting for a reply while holding a vnode lock, that would do it.)

Good luck with it and maybe someone else can think of a commit
between r252025 and r253506 that could cause vnode locking or network
problems.

rick

> 
> I have tried to mount the file system with and without nolockd. It
> didn't make a difference. Other than that it is mounted with:
> 
> rw,nfsv3,tcp,noatime,rsize=32768,wsize=32768
> 
> Let me know if you need me to do something else or if some other
> output is required. I would have to go back to the problem kernel
> and wait until the deadlock occurs to get that information.
> 
> Thanks for your help,
> 
> Michael
> 
> 
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