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Date:   Wed, 6 Nov 2019 17:54:37 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     Quentin Perret <qperret@google.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: Re: NULL pointer dereference in pick_next_task_fair
Message-ID: <20191106165437.GX4114@hirez.programming.kicks-ass.net>
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 <33643a5b-1b83-8605-2347-acd1aea04f93@virtuozzo.com>
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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.