[PATCH] sched: Fix infinite loop in update_blocked_averages() by reverting a9e7f6544b9c

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> 
> * Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> 
> > > Reported-by: Zhipeng Xie <xiezhipeng1@xxxxxxxxxx>
> > > Cc: Bin Li <huawei.libin@xxxxxxxxxx>
> > > Cc: <stable@xxxxxxxxxxxxxxx>        [4.10+]
> > > Fixes: 9c2791f936ef (sched/fair: Fix hierarchical order in rq->leaf_cfs_rq_list)
> > 
> > If it only happens in update_blocked_averages(), the del leaf has been added by:
> > a9e7f6544b9c (sched/fair: Fix O(nr_cgroups) in load balance path)
> 
> So I think until we are confident in the proposed fixes, how about 
> applying Linus's patch that reverts a9e7f6544b9c and simplifies the list 
> manipulation?
> 
> That way we can re-introduce the O(nr_cgroups) optimization without 
> pressure.
> 
> I'll prepare a commit for sched/urgent that does this, please holler if 
> any of you disagrees!

I've applied the patch below to tip:sched/urgent and I'll push it out if 
all goes well in testing:

  1e2adc76e619: ("sched: Fix infinite loop in update_blocked_averages() by reverting a9e7f6544b9c")

I've preemptively added the Tested-by tags of the gents who found and 
analyzed this bug:

  Tested-by: Zhipeng Xie <xiezhipeng1@xxxxxxxxxx>
  Tested-by: Sargun Dhillon <sargun@xxxxxxxxx>

... in the assumption that you'll do the testing of Linus's fix to make 
sure it's all good!

[ Will probably update the commit with acks and any other feedback before
  sending it to Linus tomorrow-ish. We don't want to end 2018 with a
  known scheduler bug in the upstream tree! ;-) ]

Thanks,

	Ingo

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