[nfs] process locks in "bo_wwait" on 6.4

[Kostik Belousov]

On Mon, Jun 29, 2009 at 05:18:03PM +0400, pluknet wrote:
> 2009/6/29 Attilio Rao <attilio at freebsd.org>:
> > 2009/6/29 pluknet <pluknet at gmail.com>:
> >> 2009/6/29 Attilio Rao <attilio at freebsd.org>:
> >>> 2009/6/29 pluknet <pluknet at gmail.com>:
> >>>> 2009/6/26 pluknet <pluknet at gmail.com>:
> >>>>> 2009/6/26 pluknet <pluknet at gmail.com>:
> >>>>>> Hello.
> >>>>>>
> >>>>>> While building a module on nfs mounted /usr/src
> >>>>>> I got an unkillable process waiting forever in bo_wwait.
> >>>>>
> >>>>> Small note: iface on NFS server has mtu changed from 1500 to 1450.
> >>>>> Can this be a source of the problem?
> >>>>
> >>>> This is 100% reproducible. Lock in the same place. Any hints?
> >>>
> >>> Can you also show the value of ps?
> >>> A precise map of what processes are doing would give an help.
> >>> Also would be useful to printout traces for other threads and not only
> >>> the stucked one.
> >>>
> >>
> >> >From another run:
> >
> > I'm unable to see who would be locking the buffer object in question.
> > Do you have INVARIANT_SUPPORT/INVARIANTS on?
> 
> Yes, I do both.
> 
> > What revision of /usr/src/sys/kern/vfs_bio.c are you running with?
> >
> 
> As of 6.4-R: CVS rev 1.491.2.12.4.1 / SVN rev 183531.

It seems that your changes of MTU cause nfs requests to never reach
network. bo_wwait is the state where thread waits for all outstanding
i/o on bufobj to drain.
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