NFS deadlock (unkillable nfsd and no mounts work)

[Rick Macklem]

> >> I'm having a problem with nfsd hanging and not serving mount
> >> points,
> >> during which time it can not not be killed. This problem started
> >> happening sometime after November 2nd, since kernel from 11/2
> >> sources
> >> does not exhibit this problem.
> >
> > Please try the attached patch, rick
> 
> Thanks! I had to manually patch for some reason, but I can confirmed
> that nfsd is now well-behaved with your patch applied. I tested a
> couple of different mounts and played two separate files on the
> Popcorn Hour (one lower bitrate, the other higher bitrate) and both
> played without a hiccup. While those were playing I also was able to
> automount my home directory on the macbook and move around my home
> directory.
> 
> So it looks like this patch did the trick. Thanks Rick, really
> appreciate the fast response. Is there a reason why this doesn't seem
> to be getting reported a lot? What is particular in my setup that
> broke it?
> 
Well, the commit that broke things just hit stable/8 on Nov. 3. Also,
I'm not sure what scenarios would have caused the breakage. I think it
would be something like a file system where vget worked that dropped
out of the loop just after looking up "." or ".." at the root, so that
the nvp remained locked. But I'm not sure what the exact scenarios are?
(Holding the shared lock shouldn't have stopped further VFS_VGET()s from
 succeeding, I think?)

> > ps: Starting about Monday I won't be able to do commits for about 3
> > weeks
> >    so, if this patch works, could someone else please commit it,
> > thanks,
> >    rick
> >
> 
> If someone can commit this, I'd really appreciate it. I will report
> back if I notice any problems, but I imagine this would probably get
> fixed in HEAD first, then MFC'd anyway, right? Unless this is already
> fixed in HEAD.
> 
The patch isn't in head, but hopefully someone like kib@ or jhb@ can do it,
since I won't be able to MFC it before code freeze. They might have a
better patch?

Anyhow, good to hear it fixes the problem. Thanks for reporting the problem
and testing the patch, rick