linux-kernel - Re: [PATCH] sched: fix NULL pointer issue in pick_next

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Date:   Tue, 1 Aug 2017 17:20:11 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity()

Hi Peter,

2017-08-01 17:12 GMT+08:00 Peter Zijlstra <peterz@...radead.org>:
> On Tue, Aug 01, 2017 at 04:57:43PM +0800, Yafang Shao wrote:
>> > And how would that happen? We only call pick_next_entity(.curr=NULL)
>> > when we _know_ cfs_rq->nr_running.
>>
>> It crashed my machine when I did hadoop test, and after I made this change
>> it works now.
>> On SMP system, cfs_rq->nr_running isn't protected well, although we _know_
>> cfs_rq->nr_running,
>> but it is modified by other thread running on other CPU and the
>> sched_entity is set NULL as well.
>> Then this thread broken here as accessed the NULL pointer here.
>
> cfs_rq->nr_running should be protected by the rq->lock. If it is not,
> something else is buggered.

Yes, I admit that something else is buggered, but unfortunately I
haven't understood the
scheduler deeply so can't find the root cause.

But from my understanding,  it is obviously  a bug here as we can find
that it may occurs that both 'se'
and 'curr' are NULL. That's why I submit this patch to fix it.