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

From: Uladzislau Rezki <[email-masked]>
Date: Thu, 13 Jun 2024 15:06:54 +0200

On Thu, Jun 13, 2024 at 05:47:08AM -0700, Paul E. McKenney wrote:
> 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/[email-masked]/
> > > > 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.
>
<snip>
void kmem_cache_destroy(struct kmem_cache *s)
{
	int err = -EBUSY;
	bool rcu_set;

	if (unlikely(!s) || !kasan_check_byte(s))
		return;

	cpus_read_lock();
	mutex_lock(&slab_mutex);

	rcu_set = s->flags & SLAB_TYPESAFE_BY_RCU;

	s->refcount--;
	if (s->refcount)
		goto out_unlock;

	err = shutdown_cache(s);
	WARN(err, "%s %s: Slab cache still has objects when called from %pS",
	     __func__, s->name, (void *)_RET_IP_);
...
	cpus_read_unlock();
	if (!err && !rcu_set)
		kmem_cache_release(s);
}
<snip>

so we have SLAB_TYPESAFE_BY_RCU flag that defers freeing slab-pages
and a cache by a grace period. Similar flag can be added, like
SLAB_DESTROY_ONCE_FULLY_FREED, in this case a worker rearm itself
if there are still objects which should be freed.

Any thoughts here?

--
Uladzislau Rezki