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

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Date:   Tue, 1 Aug 2017 11:12:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity()

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.