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Date: Thu, 13 Jun 2024 13:58:59 +0200
From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>, Julia Lawall <Julia.Lawall@inria.fr>,
	linux-block@vger.kernel.org, kernel-janitors@vger.kernel.org,
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	"Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
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	Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple
 kmem_cache_free callback
Message-ID: <ZmrfA1p2zSVIaYam@zx2c4.com>
References: <20240609082726.32742-1-Julia.Lawall@inria.fr>
 <20240612143305.451abf58@kernel.org>
 <baee4d58-17b4-4918-8e45-4d8068a23e8c@paulmck-laptop>
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In-Reply-To: <baee4d58-17b4-4918-8e45-4d8068a23e8c@paulmck-laptop>

On Wed, Jun 12, 2024 at 03:37:55PM -0700, Paul E. McKenney wrote:
> 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().

Also, notably, this patch needs additionally:

diff --git a/drivers/net/wireguard/allowedips.c b/drivers/net/wireguard/allowedips.c
index e4e1638fce1b..c95f6937c3f1 100644
--- a/drivers/net/wireguard/allowedips.c
+++ b/drivers/net/wireguard/allowedips.c
@@ -377,7 +377,6 @@ int __init wg_allowedips_slab_init(void)

 void wg_allowedips_slab_uninit(void)
 {
-	rcu_barrier();
 	kmem_cache_destroy(node_cache);
 }

Once kmem_cache_destroy has been fixed to be deferrable.

I assume the other patches are similar -- an rcu_barrier() can be
removed. So some manual meddling of these might be in order.

Jason