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

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Date:   Tue, 1 Aug 2017 12:23:55 +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 05:20:11PM +0800, Yafang Shao wrote:
> 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.

Thing is, it doesn't fix a bug, it hides one. The real bug is nr_running
changing while you own the rq->lock. That should _never_ happen.