Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
From:   Qian Cai <cai@lca.pw>
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 <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: <20200427235621.7823-5-longman@redhat.com>
To:     Waiman Long <longman@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 27, 2020, at 7:56 PM, Waiman Long <longman@redhat.com> 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