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

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Message-ID: <baee4d58-17b4-4918-8e45-4d8068a23e8c@paulmck-laptop>
Date: Wed, 12 Jun 2024 15:37:55 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: 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,
	Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple
 kmem_cache_free callback

On Wed, Jun 12, 2024 at 02:33:05PM -0700, Jakub Kicinski wrote:
> On Sun,  9 Jun 2024 10:27:12 +0200 Julia Lawall wrote:
> > Since SLOB was removed, it is not necessary to use call_rcu
> > when the callback only performs kmem_cache_free. Use
> > kfree_rcu() directly.
> > 
> > The changes were done using the following Coccinelle semantic patch.
> > This semantic patch is designed to ignore cases where the callback
> > function is used in another way.
> 
> How does the discussion on:
>   [PATCH] Revert "batman-adv: prefer kfree_rcu() over call_rcu() with free-only callbacks"
>   https://lore.kernel.org/all/20240612133357.2596-1-linus.luessing@c0d3.blue/
> reflect on this series? IIUC we should hold off..

We do need to hold off for the ones in kernel modules (such as 07/14)
where the kmem_cache is destroyed during module unload.

OK, I might as well go through them...

[PATCH 01/14] wireguard: allowedips: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	Needs to wait, see wg_allowedips_slab_uninit().

[PATCH 02/14] net: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	I don't immediately see the rcu_barrier(), but if there isn't
	one in there somewhere there probably should be.  Caution
	suggests a need to wait.

[PATCH 03/14] KVM: PPC: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	I don't immediately see the rcu_barrier(), but if there isn't
	one in there somewhere there probably should be.  Caution
	suggests a need to wait.

[PATCH 04/14] xfrm6_tunnel: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	Needs to wait, see xfrm6_tunnel_fini().

[PATCH 05/14] tracefs: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	This one is fine because the tracefs_inode_cachep kmem_cache
	is created at boot and never destroyed.

[PATCH 06/14] eCryptfs: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	I don't see a kmem_cache_destroy(), but then again, I also don't
	see the kmem_cache_create().  Unless someone can see what I am
	not seeing, let's wait.

[PATCH 07/14] net: bridge: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	Needs to wait, see br_fdb_fini() and br_deinit().

[PATCH 08/14] nfsd: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	I don't immediately see the rcu_barrier(), but if there isn't
	one in there somewhere there probably should be.  Caution
	suggests a need to wait.

[PATCH 09/14] block: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	I don't see a kmem_cache_destroy(), but then again, I also don't
	see the kmem_cache_create().  Unless someone can see what I am
	not seeing, let's wait.

[PATCH 10/14] can: gw: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	Needs to wait, see cgw_module_exit().

[PATCH 11/14] posix-timers: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	This one is fine because the posix_timers_cache kmem_cache is
	created at boot and never destroyed.

[PATCH 12/14] workqueue: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	This one is fine because the pwq_cache kmem_cache is created at
	boot and never destroyed.

[PATCH 13/14] kcm: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	I don't immediately see the rcu_barrier(), but if there isn't
	one in there somewhere there probably should be.  Caution
	suggests a need to wait.

[PATCH 14/14] netfilter: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
	Needs to wait, see hashlimit_mt_exit().

So 05/14, 11/14 and 12/14 are OK and can go ahead.  The rest need some
help.

Apologies for my having gotten overly enthusiastic about this change!

							Thanx, Paul