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

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Date: Wed, 19 Jun 2024 11:51:58 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Uladzislau Rezki <urezki@...il.com>, Vlastimil Babka <vbabka@...e.cz>,
	"Jason A. Donenfeld" <Jason@...c4.com>,
	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,
	kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple
 kmem_cache_free callback

On Tue, Jun 18, 2024 at 09:48:49AM -0700, Paul E. McKenney wrote:
> On Tue, Jun 18, 2024 at 11:31:00AM +0200, Uladzislau Rezki wrote:
> > > On 6/17/24 8:42 PM, Uladzislau Rezki wrote:
> > > >> +
> > > >> +	s = container_of(work, struct kmem_cache, async_destroy_work);
> > > >> +
> > > >> +	// XXX use the real kmem_cache_free_barrier() or similar thing here
> > > > It implies that we need to introduce kfree_rcu_barrier(), a new API, which i
> > > > wanted to avoid initially.
> > > 
> > > I wanted to avoid new API or flags for kfree_rcu() users and this would
> > > be achieved. The barrier is used internally so I don't consider that an
> > > API to avoid. How difficult is the implementation is another question,
> > > depending on how the current batching works. Once (if) we have sheaves
> > > proven to work and move kfree_rcu() fully into SLUB, the barrier might
> > > also look different and hopefully easier. So maybe it's not worth to
> > > invest too much into that barrier and just go for the potentially
> > > longer, but easier to implement?
> > > 
> > Right. I agree here. If the cache is not empty, OK, we just defer the
> > work, even we can use a big 21 seconds delay, after that we just "warn"
> > if it is still not empty and leave it as it is, i.e. emit a warning and
> > we are done.
> > 
> > Destroying the cache is not something that must happen right away. 
> 
> OK, I have to ask...
> 
> Suppose that the cache is created and destroyed by a module and
> init/cleanup time, respectively.  Suppose that this module is rmmod'ed
> then very quickly insmod'ed.
> 
> Do we need to fail the insmod if the kmem_cache has not yet been fully
> cleaned up?  If not, do we have two versions of the same kmem_cache in
> /proc during the overlap time?
> 
No fail :) If same cache is created several times, its s->refcount gets
increased, so, it does not create two entries in the "slabinfo". But i
agree that your point is good! We need to be carefully with removing and
simultaneous creating.

>From the first glance, there is a refcounter and a global "slab_mutex"
which is used to protect a critical section. Destroying is almost fully
protected(as noted above, by a global mutex) with one exception, it is:

static void kmem_cache_release(struct kmem_cache *s)
{
	if (slab_state >= FULL) {
		sysfs_slab_unlink(s);
		sysfs_slab_release(s);
	} else {
		slab_kmem_cache_release(s);
	}
}

this one can race, IMO.

--
Uladzislau Rezki