linux-kernel - Re: [PATCH v2 4/4] mm/slub: Fix sysfs shrink circular locking dependency

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Date:   Tue, 28 Apr 2020 10:06:49 -0400
From:   Waiman Long <longman@...hat.com>
To:     Qian Cai <cai@....pw>
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 4/27/20 10:11 PM, Qian Cai wrote:
>
>> On Apr 27, 2020, at 9:39 PM, Waiman Long <longman@...hat.com> wrote:
>>
>> The sequence that was prevented by this patch is "kn->count --> mem_hotplug_lock.rwsem". This sequence isn't directly in the splat. Once this link 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’t know what you are talking about. Once trylock succeed once, you will have kn->count —> cpu/memory_hotplug_lock.
>
Trylock is handled differently from lockdep's perspective as trylock can 
failed. When trylock succeeds, the critical section is executed. As long 
as it doesn't try to acquire another lock in the circular chain, the 
execution will finish at some point and release the lock. On the other 
hand, if another task has already held all those locks, the trylock will 
fail and held locks should be released. Again, no deadlock will happen.

Regards,
Longman