netdev - Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache

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Date: Mon, 17 Jun 2024 23:19:00 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Uladzislau Rezki <urezki@...il.com>, "Paul E. McKenney"
 <paulmck@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
 Julia Lawall <Julia.Lawall@...ia.fr>, linux-block@...r.kernel.org,
 kernel-janitors@...r.kernel.org, bridge@...ts.linux.dev,
 linux-trace-kernel@...r.kernel.org,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, kvm@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Nicholas Piggin <npiggin@...il.com>, netdev@...r.kernel.org,
 wireguard@...ts.zx2c4.com, linux-kernel@...r.kernel.org,
 ecryptfs@...r.kernel.org, Neil Brown <neilb@...e.de>,
 Olga Kornievskaia <kolga@...app.com>, Dai Ngo <Dai.Ngo@...cle.com>,
 Tom Talpey <tom@...pey.com>, linux-nfs@...r.kernel.org,
 linux-can@...r.kernel.org, Lai Jiangshan <jiangshanlai@...il.com>,
 netfilter-devel@...r.kernel.org, coreteam@...filter.org
Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple
 kmem_cache_free callback

On 6/17/24 7:04 PM, Jason A. Donenfeld wrote:
>>> Vlastimil, this is just checking a boolean (which could be
>>> unlikely()'d), which should have pretty minimal overhead. Is that
>>> alright with you?
>>
>> Well I doubt we can just set and check it without any barriers? The
>> completion of the last pending kfree_rcu() might race with
>> kmem_cache_destroy() in a way that will leave the cache there forever, no?
>> And once we add barriers it becomes a perf issue?
> 
> Hm, yea you might be right about barriers being required. But actually,
> might this point toward a larger problem with no matter what approach,
> polling or event, is chosen? If the current rule is that
> kmem_cache_free() must never race with kmem_cache_destroy(), because

Yes calling alloc/free operations that race with destroy is a bug and we
can't prevent that.

> users have always made diligent use of call_rcu()/rcu_barrier() and

But the issue we are solving here is a bit different - the users are not
buggy, they do kfree_rcu() and then kmem_cache_destroy() and no more
operations on the cache afterwards. We need to ensure that the handling
of kfree_rcu() (which ultimately is basically kmem_cache_free() but
internally to rcu/slub) doesn't race with kmem_cache_destroy().

> such, but now we're going to let those race with each other - either by
> my thing above or by polling - so we're potentially going to get in trouble
> and need some barriers anyway. 

The barrier in the async part of kmem_cache_destroy() should be enough
to make sure all kfree_rcu() have finished before we proceed with the
potentially racy parts of destroying, and we should be able to avoid
changes in kmem_cache_free().

> I think?
> 
> Jason