Re: [PATCH v2 10/10] nf_conntrack: Use rcu_barrier() and fix kmem_cache_create flags

Patrick McHardy
2009-06-24T13:58:46Z

Jesper Dangaard Brouer wrote:
> Adjusting SLAB_DESTROY_BY_RCU flags.
>
> kmem_cache_create("nf_conntrack", ...) does not need the
> SLAB_DESTROY_BY_RCU flag.

It does need it. We're using it instead of call_rcu() for conntracks.

> But the
> kmem_cache_create("nf_conntrack_expect", ...) should use the
> SLAB_DESTROY_BY_RCU flag, because it uses a call_rcu() callback to
> invoke kmem_cache_free().

No, using call_rcu() means we don't need SLAB_DESTROY_BY_RCU.
Please see the note in include/linux/slab.h.

> RCU barriers, rcu_barrier(), is inserted two places.
>
> In nf_conntrack_expect.c nf_conntrack_expect_fini() before the
> kmem_cache_destroy(), even though the use of the SLAB_DESTROY_BY_RCU
> flag, because slub does not (currently) handle rcu sync correctly.

I think that should be fixed in slub then.

> And in nf_conntrack_extend.c nf_ct_extend_unregister(), inorder to
> wait for completion of callbacks to __nf_ct_ext_free_rcu(), which is
> invoked by __nf_ct_ext_add(). It might be more efficient to call
> rcu_barrier() in nf_conntrack_core.c nf_conntrack_cleanup_net(), but
> thats make it more difficult to read the code (as the callback code
> in located in nf_conntrack_extend.c).

This one looks fine.

> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index 5f72b94..438ce84 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -1242,7 +1242,7 @@ static int nf_conntrack_init_init_net(void)
>
> nf_conntrack_cachep = kmem_cache_create("nf_conntrack",
> sizeof(struct nf_conn),
> - 0, SLAB_DESTROY_BY_RCU, NULL);
> + 0, 0, NULL);
> if (!nf_conntrack_cachep) {
> printk(KERN_ERR "Unable to create nf_conn slab cache\n");
> ret = -ENOMEM;
> diff --git a/net/netfilter/nf_conntrack_expect.c b/net/netfilter/nf_conntrack_expect.c
> index afde8f9..56227c2 100644
> --- a/net/netfilter/nf_conntrack_expect.c
> +++ b/net/netfilter/nf_conntrack_expect.c
> @@ -593,7 +593,7 @@ int nf_conntrack_expect_init(struct net *net)
> if (net_eq(net, &init_net)) {
> nf_ct_expect_cachep = kmem_cache_create("nf_conntrack_expect",
> sizeof(struct nf_conntrack_expect),
> - 0, 0, NULL);
> + 0, SLAB_DESTROY_BY_RCU, NULL);
> if (!nf_ct_expect_cachep)
> goto err2;
> }
> @@ -617,8 +617,15 @@ err1:
> void nf_conntrack_expect_fini(struct net *net)
> {
> exp_proc_remove(net);
> - if (net_eq(net, &init_net))
> + if (net_eq(net, &init_net)) {
> + /* hawk@comx.dk 2009-06-24: The rcu_barrier() can be
> + * removed once the sl*b allocators has been fixed
> + * regarding handling the SLAB_DESTROY_BY_RCU flag
> + * correctly.
> + */
> + rcu_barrier(); /* Wait for call_rcu() before destroy */
> kmem_cache_destroy(nf_ct_expect_cachep);
> + }
> nf_ct_free_hashtable(net->ct.expect_hash, net->ct.expect_vmalloc,
> nf_ct_expect_hsize);
> }
> diff --git a/net/netfilter/nf_conntrack_extend.c b/net/netfilter/nf_conntrack_extend.c
> index 4b2c769..fef95be 100644
> --- a/net/netfilter/nf_conntrack_extend.c
> +++ b/net/netfilter/nf_conntrack_extend.c
> @@ -186,6 +186,6 @@ void nf_ct_extend_unregister(struct nf_ct_ext_type *type)
> rcu_assign_pointer(nf_ct_ext_types[type->id], NULL);
> update_alloc_size(type);
> mutex_unlock(&nf_ct_ext_type_mutex);
> - synchronize_rcu();
> + rcu_barrier(); /* Wait for completion of call_rcu()'s */
> }
> EXPORT_SYMBOL_GPL(nf_ct_extend_unregister);
>