Re: Soft-Lockup/Race in networking in 2.6.31-rc1+195 (possibly caused by netem)

[Andres Freund]

On 07/01/2009 11:22 PM, Andres Freund wrote:
> On 07/01/2009 08:39 PM, Jarek Poplawski wrote:
> > Andres Freund wrote, On 07/01/2009 01:20 AM:
> > > While playing around with netem (time, not packet count based loss-
> > > bursts) I experienced soft lockups several times - to exclude it was my
> > > modifications causing this I recompiled with the original and it is
> > > still locking up.
> > > I captured several of those traces via the thankfully
> > > still working netconsole.
> > > The simplest policy I could reproduce the error with was:
> > > tc qdisc add dev eth0 root handle 1: netem delay 10ms loss 0
> > >
> > > I could not reproduce the error without delay - but that may only be a
> > > timing issue, as the host I was mainly transferring data to was on a
> > > local network.
> > > I could not reproduce the issue on lo.
> > >
> > > The time to reproduce the error varied from seconds after executing tc
> > > to several minutes.
> > >
> > > Traces 5+6 are made with vanilla
> > > 52989765629e7d182b4f146050ebba0abf2cb0b7
> > >
> > > The earlier traces are made with parts of my patches applied, and only
> > > included for completeness as I don't believe my modifications were
> > > causing this and all traces are different, so it may give some clues.
> > >
> > > Lockdep was enabled but did not diagnose anything relevant (one dvb
> > > warning during bootup).
> > >
> > > Any ideas for debugging?
> >
> > Maybe these traces will be enough, but lockdep report could save time.
> > If dvb warning triggers every time then lockdep probably turns off
> > just after (it works this way, unless something was changed). So,
> > could you try to repeat this without dvb? Btw., did you try this on
> > some earlier kernel?
> Yes. Today I could not manage to reproduce it on 2.6.30 but could on
> current git...
> Will try without dvb.
So I tried - and I did not catch any lockdep output before the crash. 
Unfortunately I do not have another machine on the same local network to 
catch any messages after the crash... So I could be missing some warning 
(I did synchronous logging though).
Will check with netconsole tomorrow.

Andres
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