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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH v2 4/4] mm/slub: Fix sysfs shrink circular locking dependency
Date:   Sat, 16 May 2020 21:46:05 -0400
Message-Id: <62C1A69E-A14F-42EE-970F-ABAEA2782256@lca.pw>
References: <56327de0-fa44-d5f3-2409-69cf2b97a209@redhat.com>
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>
In-Reply-To: <56327de0-fa44-d5f3-2409-69cf2b97a209@redhat.com>
To:     Waiman Long <longman@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 28, 2020, at 10:07 AM, Waiman Long <longman@redhat.com> wrote:
>=20
> Trylock is handled differently from lockdep's perspective as trylock can f=
ailed. When trylock succeeds, the critical section is executed. As long as i=
t doesn't try to acquire another lock in the circular chain, the execution w=
ill 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,

=E2=80=9CTrylock needs to maintain the stack of held locks, but it does not a=
dd new dependencies, because trylock can be done in any order.=E2=80=9D

So, I agree this trylock trick could really work. Especially, I don=E2=80=99=
t know any other better way to fix this.=