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Subject: Re: NULL pointer dereference in pick_next_task_fair
To:     Peter Zijlstra <peterz@infradead.org>
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
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <831c2cd4-40a4-31b2-c0aa-b5f579e770d6@virtuozzo.com>
Date:   Thu, 7 Nov 2019 11:36:50 +0300
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On 06.11.2019 20:27, Peter Zijlstra wrote:
> 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.

I mostly mean throttling. AFAIR, tasks of throttled rt_rq are not accounted
in rq->nr_running. But it seems rt_rq may become unthrottled again after
newidle_balance() unlocks rq lock, and prev will become pickable again.
 
> 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.