linux-kernel - Re: NULL pointer dereference in pick_next_task

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Message-ID: <07d30588-22e6-e098-b591-29c7cd3c8054@arm.com>
Date:   Fri, 8 Nov 2019 11:47:44 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Quentin Perret <qperret@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.com>, linux-kernel@...r.kernel.org,
        aaron.lwe@...il.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: NULL pointer dereference in pick_next_task_fair

On 08/11/2019 11:02, Quentin Perret wrote:
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index a14487462b6c..6b983214e00f 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6746,10 +6746,18 @@ done: __maybe_unused;
>>  	return NULL;
>>  }
>>
>> +static int balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>> +{
>> +	if (rq->cfs.nr_running)
>> +		return 1;
>> +
>> +	return newidle_balance(rq, rf) != 0;
> 
> And you can ignore the RETRY_TASK case here under the assumption that
> we must have tried to pull from RT/DL before ending up here ?
> 

I think we can ignore RETRY_TASK because this happens before the picking loop,
so we'll observe any new DL/RT task that got enqueued while newidle released
the lock. This also means we can safely break the balance loop in
pick_next_task() when we get RETRY_TASK, because we've got something to pick
(some new RT/DL task). This wants a comment though, methinks.

Other than that I agree with Quentin, it's a much cleaner approach and I quite
like it.

> Thanks,
> Quentin
>