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Date:   Fri, 8 Nov 2019 13:52:09 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Quentin Perret <qperret@google.com>
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 Fri, Nov 08, 2019 at 12:55:57PM +0100, Peter Zijlstra wrote:
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3929,13 +3929,14 @@ pick_next_task(struct rq *rq, struct tas
>  	}
>  
>  restart:
> -	/*
> -	 * Ensure that we put DL/RT tasks before the pick loop, such that they
> -	 * can PULL higher prio tasks when we lower the RQ 'priority'.
> -	 */
> -	prev->sched_class->put_prev_task(rq, prev, rf);
> -	if (!rq->nr_running)
> -		newidle_balance(rq, rf);
> +#ifdef CONFIG_SMP
	/*
	 * We must do the balancing pass before put_next_task(), such
	 * that when we release the rq->lock the task is in the same
	 * state as before we took rq->lock.
	 *
	 * We can terminate the balance pass as soon as we know there is
	 * a runnable task of @class priority or higher.
	 */
> +	for_class_range(class, prev->sched_class, &idle_sched_class) {
> +		if (class->balance(rq, prev, rf))
> +			break;
> +	}
> +#endif
> +
> +	put_prev_task(rq, prev);
>  
>  	for_each_class(class) {
>  		p = class->pick_next_task(rq, NULL, NULL);