Re: mm: deadlock between get_online_cpus/pcpu_alloc

[Tejun Heo]

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 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.

Please feel free to add

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun