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

Paul E. McKenney
2024-06-12T22:37:56Z

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().

[PATCH 02/14] net: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
I don't immediately see the rcu_barrier(), but if there isn't
one in there somewhere there probably should be. Caution
suggests a need to wait.

[PATCH 03/14] KVM: PPC: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
I don't immediately see the rcu_barrier(), but if there isn't
one in there somewhere there probably should be. Caution
suggests a need to wait.

[PATCH 04/14] xfrm6_tunnel: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
Needs to wait, see xfrm6_tunnel_fini().

[PATCH 05/14] tracefs: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
This one is fine because the tracefs_inode_cachep kmem_cache
is created at boot and never destroyed.

[PATCH 06/14] eCryptfs: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
I don't see a kmem_cache_destroy(), but then again, I also don't
see the kmem_cache_create(). Unless someone can see what I am
not seeing, let's wait.

[PATCH 07/14] net: bridge: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
Needs to wait, see br_fdb_fini() and br_deinit().

[PATCH 08/14] nfsd: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
I don't immediately see the rcu_barrier(), but if there isn't
one in there somewhere there probably should be. Caution
suggests a need to wait.

[PATCH 09/14] block: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
I don't see a kmem_cache_destroy(), but then again, I also don't
see the kmem_cache_create(). Unless someone can see what I am
not seeing, let's wait.

[PATCH 10/14] can: gw: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
Needs to wait, see cgw_module_exit().

[PATCH 11/14] posix-timers: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
This one is fine because the posix_timers_cache kmem_cache is
created at boot and never destroyed.

[PATCH 12/14] workqueue: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
This one is fine because the pwq_cache kmem_cache is created at
boot and never destroyed.

[PATCH 13/14] kcm: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
I don't immediately see the rcu_barrier(), but if there isn't
one in there somewhere there probably should be. Caution
suggests a need to wait.

[PATCH 14/14] netfilter: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
Needs to wait, see hashlimit_mt_exit().

So 05/14, 11/14 and 12/14 are OK and can go ahead. The rest need some
help.

Apologies for my having gotten overly enthusiastic about this change!

Thanx, Paul