linux-kernel - Re: Null pointer crash at find_idlest

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Message-ID: <20191204094216.u7yld5r3zelp22lf@e107158-lin.cambridge.arm.com>
Date:   Wed, 4 Dec 2019 09:42:17 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     John Stultz <john.stultz@...aro.org>,
        Quentin Perret <qperret@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Patrick Bellasi <Patrick.Bellasi@....com>,
        Ingo Molnar <mingo@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Null pointer crash at find_idlest_group on db845c w/ linus/master

On 12/04/19 09:06, Vincent Guittot wrote:
> Hi John,
> 
> On Tue, 3 Dec 2019 at 20:15, John Stultz <john.stultz@...aro.org> wrote:
> >
> > With today's linus/master on db845c running android, I'm able to
> > fairly easily reproduce the following crash. I've not had a chance to
> > bisect it yet, but I'm suspecting its connected to Vincent's recent
> > rework.
> 
> Does the crash happen randomly or after a specific action ?
> I have a db845 so I can try to reproduce it locally.

Isn't there a chance we use local_sgs without initializing it in that function?
AFAICS we define local_sgs on the stack but not always could be populated with
the right values. I can't see tmp_sgs being used in the function too. Could
this cause the/a problem?

 8377         struct sg_lb_stats local_sgs, tmp_sgs;
.
.
.
 8399                 if (local_group) {
 8400                         sgs = &local_sgs;
 8401                         local = group;
 8402                 } else {
 8403                         sgs = &tmp_sgs;
 8404                 }
 8405
 8406                 update_sg_wakeup_stats(sd, group, sgs, p);

Cheers

--
Qais Youef