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Date: Thu, 13 Jun 2024 05:47:08 -0700
From: "Paul E. McKenney" <paulmck@kernel.org>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
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>,
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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: <80e03b02-7e24-4342-af0b-ba5117b19828@paulmck-laptop>
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 <ZmrfA1p2zSVIaYam@zx2c4.com>
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On Thu, Jun 13, 2024 at 01:58:59PM +0200, Jason A. Donenfeld wrote:
> 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.

Assuming that the deferrable kmem_cache_destroy() is the option chosen,
agreed.

							Thanx, Paul