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Subject: Re: [PATCH v2 4/4] mm/slub: Fix sysfs shrink circular locking
 dependency
To:     Qian Cai <cai@lca.pw>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Juri Lelli <juri.lelli@redhat.com>
References: <20200427235621.7823-5-longman@redhat.com>
 <55509F31-A503-4148-B209-B4D062AD0ED7@lca.pw>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <dbbfe685-7374-9a96-b7c2-684142746e30@redhat.com>
Date:   Mon, 27 Apr 2020 21:39:44 -0400
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On 4/27/20 8:13 PM, Qian Cai wrote:
>
>> On Apr 27, 2020, at 7:56 PM, Waiman Long <longman@redhat.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_mu=
tex
>> [  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. Sinc=
e
>> hotplug events are rare, it should be fine to refuse a kmem caches
>> shrink operation when some hotplug events are in progress.
> I don=E2=80=99t understand how trylock could prevent a splat. The funda=
mental issue is that in sysfs slab store case, the locking order (once tr=
ylock succeed) is,
>
> kn->count =E2=80=94> cpu/memory_hotplug
>
> But we have the existing reverse chain everywhere.
>
> cpu/memory_hotplug =E2=80=94> slab_mutex =E2=80=94> kn->count
>
The sequence that was prevented by this patch is "kn->count -->=20
mem_hotplug_lock.rwsem". This sequence isn't directly in the splat. Once=20
this link is broken, the 3-lock circular loop cannot be formed. Maybe I=20
should modify the commit log to make this point more clear.

Cheers,
Longman