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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: <dbbfe685-7374-9a96-b7c2-684142746e30@redhat.com>
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <56327de0-fa44-d5f3-2409-69cf2b97a209@redhat.com>
Date:   Tue, 28 Apr 2020 10:06:49 -0400
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On 4/27/20 10:11 PM, Qian Cai wrote:
>
>> On Apr 27, 2020, at 9:39 PM, Waiman Long <longman@redhat.com> wrote:
>>
>> The sequence that was prevented by this patch is "kn->count --> mem_ho=
tplug_lock.rwsem". This sequence isn't directly in the splat. Once this l=
ink is broken, the 3-lock circular loop cannot be formed. Maybe I should =
modify the commit log to make this point more clear.
> I don=E2=80=99t know what you are talking about. Once trylock succeed o=
nce, you will have kn->count =E2=80=94> cpu/memory_hotplug_lock.
>
Trylock is handled differently from lockdep's perspective as trylock can=20
failed. When trylock succeeds, the critical section is executed. As long=20
as it doesn't try to acquire another lock in the circular chain, the=20
execution will finish at some point and release the lock. On the other=20
hand, if another task has already held all those locks, the trylock will=20
fail and held locks should be released. Again, no deadlock will happen.

Regards,
Longman