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

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Thu, Jan 15, 2026 at 03:40:17PM +0100, Eric Dumazet wrote:
> On Thu, Jan 15, 2026 at 3:33 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >
> > Hi Eric,
> >
> > On Thu, Jan 15, 2026 at 10:15 AM Eric Dumazet <edumazet@google.com> 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@oracle.com>
> > > 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 ?

Okay so I'll take this patch, and perhaps amend to it the _removal_ of
that rcu_barrier()?

Jason