Re: [PATCH] mm/slub: fix a deadlock in show_slab_objects()

[David Rientjes <rientjes@google.com>]

On Thu, 3 Oct 2019, Qian Cai wrote:

> diff --git a/mm/slub.c b/mm/slub.c
> index 42c1b3af3c98..922cdcf5758a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4838,7 +4838,15 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
>  		}
>  	}
>  
> -	get_online_mems();
> +/*
> + * It is not possible to take "mem_hotplug_lock" here, as it has already held
> + * "kernfs_mutex" which could race with the lock order:
> + *
> + * mem_hotplug_lock->slab_mutex->kernfs_mutex
> + *
> + * In the worest case, it might be mis-calculated while doing NUMA node
> + * hotplug, but it shall be corrected by later reads of the same files.
> + */
>  #ifdef CONFIG_SLUB_DEBUG
>  	if (flags & SO_ALL) {
>  		struct kmem_cache_node *n;

No objection to removing the {get,put}_online_mems() but the comment 
doesn't match the kernel style.  I actually don't think we need the 
comment at all, actually.

> @@ -4879,7 +4887,6 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
>  			x += sprintf(buf + x, " N%d=%lu",
>  					node, nodes[node]);
>  #endif
> -	put_online_mems();
>  	kfree(nodes);
>  	return x + sprintf(buf + x, "\n");
>  }