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

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Date:   Sat, 16 May 2020 21:46:05 -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 28, 2020, at 10:07 AM, Waiman Long <longman@...hat.com> wrote:
> 
> 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.

Ok, I can see that in validate_chain() especially mentioned,

“Trylock needs to maintain the stack of held locks, but it does not add new dependencies, because trylock can be done in any order.”

So, I agree this trylock trick could really work. Especially, I don’t know any other better way to fix this.