linux-kernel - Re: Re: NULL pointer dereference in pick_next_task

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Message-ID: <20191106172737.GM5671@hirez.programming.kicks-ass.net>
Date:   Wed, 6 Nov 2019 18:27:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Quentin Perret <qperret@...gle.com>, 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: Re: NULL pointer dereference in pick_next_task_fair

On Wed, Nov 06, 2019 at 05:54:37PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 06, 2019 at 06:51:40PM +0300, Kirill Tkhai wrote:
> > > +#ifdef CONFIG_SMP
> > > +	if (!rq->nr_running) {
> > > +		/*
> > > +		 * Make sure task_on_rq_curr() fails, such that we don't do
> > > +		 * put_prev_task() + set_next_task() on this task again.
> > > +		 */
> > > +		prev->on_cpu = 2;
> > >  		newidle_balance(rq, rf);
> > 
> > Shouldn't we restore prev->on_cpu = 1 after newidle_balance()? Can't prev
> > become pickable again after newidle_balance() releases rq->lock, and we
> > take it again, so this on_cpu == 2 never will be cleared?
> 
> Indeed so.

Oh wait, the way it was written this is not possible. Because
rq->nr_running == 0 and prev->on_cpu > 0 it means the current task is
going to sleep and cannot be woken back up.

But if I move the ->on_cpu=2 thing earlier, as I wrote I'd do, then yes,
we have to set it back to 1. Because in that case we can get here for a
spurious schedule and we'll pick the same task again.