Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback

Jason A. Donenfeld
2024-06-13T11:59:12Z

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