Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity()

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity()
Date	2017-08-01 12:30 +0200
Message-ID	<u9CB3-2At-3@gated-at.bofh.it> (permalink)
References	<u9uWS-5OL-7@gated-at.bofh.it> <u9ASC-TK-25@gated-at.bofh.it> <u9Bvj-1zO-5@gated-at.bofh.it> <u9Bvj-1zO-3@gated-at.bofh.it> <u9BF0-1GX-17@gated-at.bofh.it>
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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@infradead.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.

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[PATCH] sched: fix NULL pointer issue in pick_next_entity() Yafang Shao <laoar.shao@gmail.com> - 2017-08-01 04:20 +0200
  Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity() Peter Zijlstra <peterz@infradead.org> - 2017-08-01 10:40 +0200
    Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity() Yafang Shao <laoar.shao@gmail.com> - 2017-08-01 11:00 +0200
    Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity() Peter Zijlstra <peterz@infradead.org> - 2017-08-01 11:20 +0200
      Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity() Yafang Shao <laoar.shao@gmail.com> - 2017-08-01 11:30 +0200
        Re: [PATCH] sched: fix NULL pointer issue in pick_next_entity() Peter Zijlstra <peterz@infradead.org> - 2017-08-01 12:30 +0200

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