linux-kernel - Re: kmemcheck in linux-next causes NULL pointer dereference at task_rq

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Message-Id: <1250877875.2168.56.camel@dhcp231-106.rdu.redhat.com>
Date:	Fri, 21 Aug 2009 14:04:35 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Vegard Nossum <vegard.nossum@...il.com>
Cc:	linux-kernel@...r.kernel.org, penberg@...helsinki.fi, mingo@...e.hu
Subject: Re: kmemcheck in linux-next causes NULL pointer dereference at 
 task_rq_lock

On Fri, 2009-08-21 at 19:20 +0200, Vegard Nossum wrote:
> 2009/8/10 Eric Paris <eparis@...hat.com>:
> > On Mon, 2009-08-10 at 20:56 +0200, Vegard Nossum wrote:
> >> 2009/8/10 Eric Paris <eparis@...hat.com>:
> >> > I'm using 2.6.31-rc5-next-20090810 on a vmware server.  Originally I saw
> >> > messages about setting to one cpu, so I booted with maxcpu=1.  I get
> >> > this same panic with and without maxpu.  Booting with kmemcheck=0 boots
> >> > just fine.
> >> >
> >> > I have not tested kmemcheck in linus' tree but will start looking for a
> >> > working version now.  Any suggestions or things I should try?
> >>
> >> Hi, thanks for the report, and for trying it out.
> >>
> >> Does it crash with "kmemcheck=0 maxcpus=1" (it should be maxcpus, not
> >> maxcpu). This should be a simple way of determining whether it was
> >> kmemcheck or the existing maxcpus code, which we use, that broke.
> >
> > I boots fine with kmemcheck=0
> >
> > I did get the command line right.  maxcpus=1 kmemcheck=1 was the panic I
> > showed in the last message.
> >
> >> If you can send config (off-list, probably), I will try to reproduce
> >> and investigate more tomorrow.
> >
> > Will send it now.
> 
> Thanks. I've tried your config with next-20090810, next-20090820, and
> current mainline, and wasn't able to reproduce it anywhere :-/
> 
> Does it still occur for you with the most recent linux-next or current mainline?
> 
> In any case, I think the kmemcheck SMP-handling code has to be fixed.
> It seems that we currently allow CPU hotplug while kmemcheck is
> enabled, which could lead to, erm, interesting crashes. Maybe we could
> do something like what mmiotrace does.

I just tried it again with 0821.  The machine doesn't boot with
kmemcheck=1 but it doesn't look like it's directly kmemcheck's fault.  I
poked others about their problems.  I'm going to be giving it a shot on
a different box.

-Eric


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