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

[Andres Freund]

Hi,

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...

I *think* I could also provoke the same issue on lo, but I am not 
completely sure, as the host I was redirecting netconsole to 
unfortunately was not up, so I could not check if it was a similar trace.
It could also have been triggered by some random traffic on eth0... Hard 
to say.

Will try without dvb.

Andres
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