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Date:   Tue, 28 Apr 2020 22:52:48 -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 10:06 AM, Waiman Long <longman@...hat.com> wrote:
> 
> On 4/27/20 10:11 PM, Qian Cai wrote:
>> 
>>> On Apr 27, 2020, at 9:39 PM, Waiman Long <longman@...hat.com> wrote:
>>> 
>>> 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’t know what you are talking about. Once trylock succeed once, you will have kn->count —> cpu/memory_hotplug_lock.
>> 
> 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 —> 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   <—— 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

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