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Date:   Tue, 18 Jan 2022 12:07:21 +0100
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Subject: Re: [PATCH] mm: slub: fix a deadlock warning in kmem_cache_destroy
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To:     Xin Long <lucien.xin@gmail.com>, Juri Lelli <juri.lelli@redhat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Antoine Tenart <atenart@kernel.org>,
        Clark Williams <williams@redhat.com>
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From:   Vlastimil Babka <vbabka@suse.cz>
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On 1/18/22 09:00, Xin Long wrote:
> On Mon, Jan 17, 2022 at 9:13 PM Juri Lelli <juri.lelli@redhat.com> wrote:
>> >
>> > RHEL-8 kernel seems to be 4.18, unless RT uses a newer one. Could be some
>> > silently relevant backport is missing? How about e.g. 59450bbc12be ("mm,
>> > slab, slub: stop taking cpu hotplug lock") ?
>>
>> Hummm, looks like we have backported commit 59450bbc12be in RHEL-8.
>>
>> Xin Long, would you be able to check if you still see the lockdep splat
>> with latest upstream RT?
>>
>> git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git linux-5.16.y-rt
> Hi, Juri,
> 
> Thanks for sharing the RT kernel repo.
> 
> I just tried with this kernel, and I couldn't reproduce it on my env.
> But I don't see how the upstream RT kernel can avoid the call trace.
> 
> As this warning was triggered when the system was shutting down, it might
> not be reproduced on it due to some timing change.

As it was caught by lockdep and not as a real deadlock, I think it should be
indepenedent of a timing change. Lockdep will correlate potentially deadlock
scenarios even if they don't really occur in the same time, AFAIK.

But let's go back to:

> Although cpu_hotplug_lock is a RWSEM, [a] will not block in there. But as
> lockdep annotations are added for cpu_hotplug_lock, a deadlock warning
> would be detected:

Is it possible that upstream lockdep handles this RWSEM scenario properly
and doesn't report it, but the RHEL kernel is missing some relevant lockdep fix?

>>
>> Thanks!
>> Juri
>>