linux-kernel - Re: NULL pointer dereference in pick_next_task

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Message-ID: <20191106130838.GL5671@hirez.programming.kicks-ass.net>
Date:   Wed, 6 Nov 2019 14:08:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     linux-kernel@...r.kernel.org, aaron.lwe@...il.com,
        valentin.schneider@....com, mingo@...nel.org, pauld@...hat.com,
        jdesfossez@...italocean.com, naravamudan@...italocean.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        juri.lelli@...hat.com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, kernel-team@...roid.com, john.stultz@...aro.org
Subject: Re: NULL pointer dereference in pick_next_task_fair

On Wed, Nov 06, 2019 at 01:05:25PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 28, 2019 at 05:46:03PM +0000, Quentin Perret wrote:
> > 
> > After digging a bit, the offending commit seems to be:
> > 
> >     67692435c411 ("sched: Rework pick_next_task() slow-path")
> > 
> > By 'offending' I mean that reverting it makes the issue go away. The
> > issue comes from the fact that pick_next_entity() returns a NULL se in
> > the 'simple' path of pick_next_task_fair(), which causes obvious
> > problems in the subsequent call to set_next_entity().
> > 
> > I'll dig more, but if anybody understands the issue in the meatime feel
> > free to send me a patch to try out :)
> 
> So for all those who didn't follow along on IRC, the below seems to cure
> things.
> 
> The only thing I'm now considering is if we shouldn't be setting
> ->on_cpu=2 _before_ calling put_prev_task(). I'll go audit the RT/DL
> cases.

So I think it all works, but that's more by accident than anything else.
I'll move the ->on_cpu=2 assignment earlier. That clearly avoids calling
put_prev_task() while we're in put_prev_task().