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From: Michal Hocko <>
To: Qian Cai <>
Subject: Re: [PATCH v2] mm/slub: fix a deadlock in show_slab_objects()
Date: Fri, 4 Oct 2019 14:57:01 +0200
Message-ID: <> (raw)
In-Reply-To: <>

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.

Michal Hocko

  reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-04 12:31 Qian Cai
2019-10-04 12:57 ` Michal Hocko [this message]
2019-10-07  8:16   ` Michal Hocko
2019-10-07 21:59     ` Andrew Morton
2019-10-08  8:47       ` Michal Hocko

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