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Date:   Sat, 16 May 2020 21:57:58 -0400
From:   Qian Cai <cai@....pw>
To:     Waiman Long <longman@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v2 4/4] mm/slub: Fix sysfs shrink circular locking dependency



> On Apr 27, 2020, at 7:56 PM, Waiman Long <longman@...hat.com> wrote:
> 
> A lockdep splat is observed by echoing "1" to the shrink sysfs file
> and then shutting down the system:
> 
> [  167.473392] Chain exists of:
> [  167.473392]   kn->count#279 --> mem_hotplug_lock.rw_sem --> slab_mutex
> [  167.473392]
> [  167.484323]  Possible unsafe locking scenario:
> [  167.484323]
> [  167.490273]        CPU0                    CPU1
> [  167.494825]        ----                    ----
> [  167.499376]   lock(slab_mutex);
> [  167.502530]                                lock(mem_hotplug_lock.rw_sem);
> [  167.509356]                                lock(slab_mutex);
> [  167.515044]   lock(kn->count#279);
> [  167.518462]
> [  167.518462]  *** DEADLOCK ***
> 
> It is because of the get_online_cpus() and get_online_mems() calls in
> kmem_cache_shrink() invoked via the shrink sysfs file. To fix that, we
> have to use trylock to get the memory and cpu hotplug read locks. Since
> hotplug events are rare, it should be fine to refuse a kmem caches
> shrink operation when some hotplug events are in progress.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>

Feel free to use,

Reviewed-by: Qian Cai <cai@....pw>

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