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Date:   Fri, 4 Oct 2019 10:13:58 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, cl@linux.com, penberg@kernel.org,
        rientjes@google.com, tj@kernel.org, vdavydov.dev@gmail.com,
        hannes@cmpxchg.org, guro@fb.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/slub: fix a deadlock in show_slab_objects()
Message-ID: <20191004081358.GA9578@dhcp22.suse.cz>
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On Thu 03-10-19 15:44:29, Qian Cai wrote:
> Long time ago, there fixed a similar deadlock in show_slab_objects()
> [1]. However, it is apparently due to the commits like 01fb58bcba63
> ("slab: remove synchronous synchronize_sched() from memcg cache
> deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
> kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
> just reading files in /sys/kernel/slab will generate a lockdep splat
> below.
> 
> Since the "mem_hotplug_lock" here is only to obtain a stable online node
> mask while racing with NUMA node hotplug, it is probably fine to do
> without it.

"It is probably fine" is not a proper justification. Please have a look
at my older email where I've exaplained why I believe it is safe.

> WARNING: possible circular locking dependency detected
> ------------------------------------------------------

I pressume the deadlock is real. If that is the case then Cc: stable and
Fixes tag would be really appreciated.

> Signed-off-by: Qian Cai <cai@lca.pw>

Anyway, I do agree that this is the right thing to do. With the improved
changelog, fixed up the comment alignment feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/slub.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> 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;
> @@ -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");
>  }
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs