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Date:   Thu, 7 Nov 2019 19:27:53 +0000
From:   Quentin Perret <qperret@google.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kirill Tkhai <ktkhai@virtuozzo.com>, linux-kernel@vger.kernel.org,
        aaron.lwe@gmail.com, valentin.schneider@arm.com, mingo@kernel.org,
        pauld@redhat.com, jdesfossez@digitalocean.com,
        naravamudan@digitalocean.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, juri.lelli@redhat.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        kernel-team@android.com, john.stultz@linaro.org
Subject: Re: NULL pointer dereference in pick_next_task_fair
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On Thursday 07 Nov 2019 at 19:43:56 (+0100), Peter Zijlstra wrote:
> But you mean something like:
> 
> 	for (class = prev->sched_class; class; class = class->next) {
> 		if (class->balance(rq, rf))
> 			break;
> 	}
> 
> 	put_prev_task(rq, prev);
> 
> 	for_each_class(class) {
> 		p = class->pick_next_task(rq);
> 		if (p)
> 			return p;
> 	}
> 
> 	BUG();
> 
> like?

Right, something like that, though what I had was basically doing the
pull from within the pick_next_task_*() functions directly, like we were
doing before. I'm now seeing how easy it is to get this wrong, and that
even good-looking code in this area can be broken in very subtle ways,
so I didn't feel comfortable refactoring again so close to rc7. If you
feel more confident, I'm more than happy to test a patch implemeting the
above :)

> I had convinced myself we didn't need that, but that DL to RT case is
> pesky and might require it after all.

Yep, I don't see a way to avoid iterating all classes to do the balance,
one way or another ...

Thanks,
Quentin