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Subject: Re: [PATCH v2 4/4] mm/slub: Fix sysfs shrink circular locking
 dependency
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <56327de0-fa44-d5f3-2409-69cf2b97a209@redhat.com>
Date:   Tue, 28 Apr 2020 22:52:48 -0400
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>
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References: <dbbfe685-7374-9a96-b7c2-684142746e30@redhat.com>
 <28BB8A1C-CFDF-4F5A-8686-537C3CE04818@lca.pw>
 <56327de0-fa44-d5f3-2409-69cf2b97a209@redhat.com>
To:     Waiman Long <longman@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Apr 28, 2020, at 10:06 AM, Waiman Long <longman@redhat.com> wrote:
>=20
> On 4/27/20 10:11 PM, Qian Cai wrote:
>>=20
>>> On Apr 27, 2020, at 9:39 PM, Waiman Long <longman@redhat.com> wrote:
>>>=20
>>> 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=E2=80=99t know what you are talking about. Once trylock succeed =
once, you will have kn->count =E2=80=94> cpu/memory_hotplug_lock.
>>=20
> 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.

So once,

CPU0 (trylock succeed):
kn->count =E2=80=94> cpu/memory_hotplug_lock.

Did you mean that lockdep will not record this existing chain?

If it did. Then later, are you still sure that CPU1 (via memcg path =
below) will still be impossible to trigger a splat just because lockdep =
will be able to tell that those arennon-exclusive =
(cpu/memory_hotplug_lock) locks instead?

 cpu/memory_hotplug_lock -> kn->count

[  290.805818] -> #3 (kn->count#86){++++}-{0:0}:
[  290.811954]        __kernfs_remove+0x455/0x4c0
[  290.816428]        kernfs_remove+0x23/0x40
[  290.820554]        sysfs_remove_dir+0x74/0x80
[  290.824947]        kobject_del+0x57/0xa0
[  290.828905]        sysfs_slab_unlink+0x1c/0x20
[  290.833377]        shutdown_cache+0x15d/0x1c0
[  290.837964]        kmemcg_cache_shutdown_fn+0xe/0x20
[  290.842963]        kmemcg_workfn+0x35/0x50   <=E2=80=94=E2=80=94 =
cpu/memory_hotplug_lock
[  290.847095]        process_one_work+0x57e/0xb90
[  290.851658]        worker_thread+0x63/0x5b0
[  290.855872]        kthread+0x1f7/0x220
[  290.859653]        ret_from_fork+0x27/0x50=