Re: [PATCH net-next v3] wireguard: allowedips: Use kfree_rcu() instead of call_rcu()

[Eric Dumazet]

On Thu, Jan 15, 2026 at 3:33 PM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
>
> Hi Eric,
>
> On Thu, Jan 15, 2026 at 10:15 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> > > > The existing cleanup path is:
> > > >   wg_allowedips_slab_uninit() -> rcu_barrier() -> kmem_cache_destroy()
> > > >
> > > > With kfree_rcu(), this sequence could destroy the slab cache while
> > > > kfree_rcu_work() still has pending frees queued. The proper barrier for
> > > > kfree_rcu() is kvfree_rcu_barrier() which also calls flush_rcu_work()
> > > > on all pending batches.
> > >
> > > We do not need to add an explict kvfree_rcu_barrier(), becasue the commit
> > > 6c6c47b063b5 ("mm, slab: call kvfree_rcu_barrier() from kmem_cache_destroy()")
> > > already does it.
> >
> > It was doing it, but got replaced recently with a plain rcu_barrier()
> >
> > commit 0f35040de59371ad542b915d7b91176c9910dadc
> > Author: Harry Yoo <harry.yoo@xxxxxxxxxx>
> > Date:   Mon Dec 8 00:41:47 2025 +0900
> >
> >     mm/slab: introduce kvfree_rcu_barrier_on_cache() for cache destruction
> >
> > We would like explicit +2 from mm _and_ rcu experts on this wireguard patch.
>
> I'll take this through the wireguard tree.
>
> But just a question on your comment, "It was doing it, but got
> replaced recently with a plain rcu_barrier()". Are you suggesting I
> need a kvfree_rcu_barrier() instead? The latest net-next has a
> kvfree_rcu_barrier_on_cache() called from kmem_cache_destroy()
> still... But are you suggesting I add this anyway?
>
> diff --git a/drivers/net/wireguard/allowedips.c
> b/drivers/net/wireguard/allowedips.c
> index 5ece9acad64d..aee39a0303b0 100644
> --- a/drivers/net/wireguard/allowedips.c
> +++ b/drivers/net/wireguard/allowedips.c
> @@ -417,7 +417,7 @@ int __init wg_allowedips_slab_init(void)
>
>  void wg_allowedips_slab_uninit(void)
>  {
> - rcu_barrier();
> + kvfree_rcu_barrier();

It seems kmem_cache_destroy() should take care of needed barriers,
at least this is what is claimed. An rcu_barrier() or kvfree_rcu_barrier()
should not be needed in wg_allowedips_slab_uninit() ?

Probably boring/distracting, I do not expect anyone needing to unload
this module in a loop and expect this to be ultra fast ?

>   kmem_cache_destroy(node_cache);
>  }
>
> Let me know.
>
> Thanks,
> Jason