netdev - Re: [PATCH 06/17] batman-adv: replace call_rcu by kfree_rcu for simple kmem_cache

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Message-ID: <cb49d58e-6221-4117-b0b6-bd8ded715749@suse.cz>
Date: Mon, 14 Oct 2024 09:19:57 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Julia Lawall <julia.lawall@...ia.fr>, Sven Eckelmann <sven@...fation.org>
Cc: linus.luessing@...3.blue, Marek Lindner <mareklindner@...mailbox.ch>,
 kernel-janitors@...r.kernel.org, paulmck@...nel.org,
 Simon Wunderlich <sw@...onwunderlich.de>, Antonio Quartulli <a@...table.cc>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 b.a.t.m.a.n@...ts.open-mesh.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/17] batman-adv: replace call_rcu by kfree_rcu for
 simple kmem_cache_free callback

On 10/14/24 09:08, Julia Lawall wrote:
> 
> 
> On Mon, 14 Oct 2024, Sven Eckelmann wrote:
> 
>> On Sunday, 13 October 2024 22:16:53 CEST Julia Lawall wrote:
>> > Since SLOB was removed and since
>> > commit 6c6c47b063b5 ("mm, slab: call kvfree_rcu_barrier() from kmem_cache_destroy()"),
>> > it is not necessary to use call_rcu when the callback only performs
>> > kmem_cache_free. Use kfree_rcu() directly.
>> >
>> > The changes were made using Coccinelle.
>> >
>> > Signed-off-by: Julia Lawall <Julia.Lawall@...ia.fr>
>> >
>> > ---
>> >  net/batman-adv/translation-table.c |   47 ++-----------------------------------
>> >  1 file changed, 3 insertions(+), 44 deletions(-)
>>
>>
>> This was tried and we noticed that it is not safe [1]. So, I would get
>> confirmation that commit 6c6c47b063b5 ("mm, slab: call kvfree_rcu_barrier()
>> from kmem_cache_destroy()") is fixing the problem which we had at that time.
>> The commit message sounds like it but I just want to be sure.
> 
> Thanks for the feedback. I think that Vlastimil Babka can help with that.

Hi, yeah the batman-adv issue was how we learned about the problem and the
series of commits leading to and including 6c6c47b063b5 was done exactly to
address the kmem_cache_destroy() on module unload issue, and unblock the
conversion to kfree_rcu().

Thanks, Vlastimil

> julia