Re: mm: deadlock between get_online_cpus/pcpu_alloc

[Michal Hocko]

On Tue 07-02-17 12:03:19, Tejun Heo wrote:
> Hello,
> 
> Sorry about the delay.
> 
> On Tue, Feb 07, 2017 at 04:34:59PM +0100, Michal Hocko wrote:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index c3358d4f7932..b6411816787a 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2343,7 +2343,16 @@ void drain_local_pages(struct zone *zone)
> >  
> >  static void drain_local_pages_wq(struct work_struct *work)
> >  {
> > +	/*
> > +	 * drain_all_pages doesn't use proper cpu hotplug protection so
> > +	 * we can race with cpu offline when the WQ can move this from
> > +	 * a cpu pinned worker to an unbound one. We can operate on a different
> > +	 * cpu which is allright but we also have to make sure to not move to
> > +	 * a different one.
> > +	 */
> > +	preempt_disable();
> >  	drain_local_pages(NULL);
> > +	preempt_enable();
> >  }
> >  
> >  /*
> > @@ -2379,12 +2388,6 @@ void drain_all_pages(struct zone *zone)
> >  	}
> >  
> >  	/*
> > -	 * As this can be called from reclaim context, do not reenter reclaim.
> > -	 * An allocation failure can be handled, it's simply slower
> > -	 */
> > -	get_online_cpus();
> > -
> > -	/*
> >  	 * We don't care about racing with CPU hotplug event
> >  	 * as offline notification will cause the notified
> >  	 * cpu to drain that CPU pcps and on_each_cpu_mask
> > @@ -2423,7 +2426,6 @@ void drain_all_pages(struct zone *zone)
> >  	for_each_cpu(cpu, &cpus_with_pcps)
> >  		flush_work(per_cpu_ptr(&pcpu_drain, cpu));
> >  
> > -	put_online_cpus();
> >  	mutex_unlock(&pcpu_drain_mutex);
> 
> I think this would work; however, a more canonical way would be
> something along the line of...
> 
>   drain_all_pages()
>   {
> 	  ...
> 	  spin_lock();
> 	  for_each_possible_cpu() {
> 		  if (this cpu should get drained) {
> 			  queue_work_on(this cpu's work);
> 		  }
> 	  }
> 	  spin_unlock();
> 	  ...
>   }
> 
>   offline_hook()
>   {
> 	  spin_lock();
> 	  this cpu should get drained = false;
> 	  spin_unlock();
> 	  queue_work_on(this cpu's work);
> 	  flush_work(this cpu's work);
>   }

I see

> I think what workqueue should do is automatically flush in-flight CPU
> work items on CPU offline and erroring out on queue_work_on() on
> offline CPUs.  And we now actually can do that because we have lifted
> the guarantee that queue_work() is local CPU affine some releases ago.
> I'll look into it soonish.
> 
> For the time being, either approach should be fine.  The more
> canonical one might be a bit less surprising but the
> preempt_disable/enable() change is short and sweet and completely fine
> for the case at hand.

Thanks for double checking!
 
> Please feel free to add
> 
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks!
-- 
Michal Hocko
SUSE Labs