linux-kernel - Re: [PATCH v2] mm/slub: fix a deadlock in show_slab

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Date:   Fri, 4 Oct 2019 14:57:01 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, tj@...nel.org, vdavydov.dev@...il.com,
        hannes@...xchg.org, guro@...com, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] mm/slub: fix a deadlock in show_slab_objects()

On Fri 04-10-19 08:31:49, 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 which 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, in the worst case, the results
> may me miscalculated while doing NUMA node hotplug, but they shall be
> corrected by later reads of the same files.

I think it is important to mention that this doesn't expose the
show_slab_objects to use-after-free. There is only a single path that
might really race here and that is the slab hotplug notifier callback
__kmem_cache_shrink (via slab_mem_going_offline_callback) but that path
doesn't really destroy kmem_cache_node data structures.

Thanks!
-- 
Michal Hocko
SUSE Labs