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[71.184.117.43]) by smtp.gmail.com with ESMTPSA id 11sm2439712qkv.92.2020.04.27.17.13.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Apr 2020 17:13:36 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable From: Qian Cai Mime-Version: 1.0 (1.0) Subject: Re: [PATCH v2 4/4] mm/slub: Fix sysfs shrink circular locking dependency Date: Mon, 27 Apr 2020 20:13:35 -0400 Message-Id: <55509F31-A503-4148-B209-B4D062AD0ED7@lca.pw> References: <20200427235621.7823-5-longman@redhat.com> Cc: Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Johannes Weiner , Michal Hocko , Vladimir Davydov , linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Juri Lelli In-Reply-To: <20200427235621.7823-5-longman@redhat.com> To: Waiman Long X-Mailer: iPhone Mail (17D50) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > On Apr 27, 2020, at 7:56 PM, Waiman Long wrote: >=20 > A lockdep splat is observed by echoing "1" to the shrink sysfs file > and then shutting down the system: >=20 > [ 167.473392] Chain exists of: > [ 167.473392] kn->count#279 --> mem_hotplug_lock.rw_sem --> slab_mutex > [ 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 *** >=20 > 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. Since > 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 fundamenta= l issue is that in sysfs slab store case, the locking order (once trylock su= cceed) 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