netfilter: nf_tables: asynchronous release

Release the committed transaction log from a work queue, moving
expensive synchronize_rcu out of the locked section and providing
opportunity to batch this.

On my test machine this cuts runtime of nft-test.py in half.
Based on earlier patch from Pablo Neira Ayuso.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -1298,12 +1298,14 @@ static inline void nft_set_elem_clear_busy(struct nft_set_ext *ext)
  *
  *	@list: used internally
  *	@msg_type: message type
+ *	@put_net: ctx->net needs to be put
  *	@ctx: transaction context
  *	@data: internal information related to the transaction
  */
 struct nft_trans {
 	struct list_head		list;
 	int				msg_type;
+	bool				put_net;
 	struct nft_ctx			ctx;
 	char				data[0];
 };

net/netfilter/nf_tables_api.c

@@ -27,6 +27,8 @@
 static LIST_HEAD(nf_tables_expressions);
 static LIST_HEAD(nf_tables_objects);
 static LIST_HEAD(nf_tables_flowtables);
+static LIST_HEAD(nf_tables_destroy_list);
+static DEFINE_SPINLOCK(nf_tables_destroy_list_lock);
 static u64 table_handle;
 
 enum {
@@ -64,6 +66,8 @@ static void nft_validate_state_update(struct net *net, u8 new_validate_state)
 
 	net->nft.validate_state = new_validate_state;
 }
+static void nf_tables_trans_destroy_work(struct work_struct *w);
+static DECLARE_WORK(trans_destroy_work, nf_tables_trans_destroy_work);
 
 static void nft_ctx_init(struct nft_ctx *ctx,
 			 struct net *net,
@@ -2453,7 +2457,6 @@ static void nf_tables_rule_destroy(const struct nft_ctx *ctx,
 {
 	struct nft_expr *expr;
 
-	lockdep_assert_held(&ctx->net->nft.commit_mutex);
 	/*
 	 * Careful: some expressions might not be initialized in case this
 	 * is called on error from nf_tables_newrule().
@@ -6224,19 +6227,28 @@ static void nft_commit_release(struct nft_trans *trans)
 		nf_tables_flowtable_destroy(nft_trans_flowtable(trans));
 		break;
 	}
+
+	if (trans->put_net)
+		put_net(trans->ctx.net);
+
 	kfree(trans);
 }
 
-static void nf_tables_commit_release(struct net *net)
+static void nf_tables_trans_destroy_work(struct work_struct *w)
 {
 	struct nft_trans *trans, *next;
+	LIST_HEAD(head);
 
-	if (list_empty(&net->nft.commit_list))
+	spin_lock(&nf_tables_destroy_list_lock);
+	list_splice_init(&nf_tables_destroy_list, &head);
+	spin_unlock(&nf_tables_destroy_list_lock);
+
+	if (list_empty(&head))
 		return;
 
 	synchronize_rcu();
 
-	list_for_each_entry_safe(trans, next, &net->nft.commit_list, list) {
+	list_for_each_entry_safe(trans, next, &head, list) {
 		list_del(&trans->list);
 		nft_commit_release(trans);
 	}
@@ -6367,6 +6379,37 @@ static void nft_chain_del(struct nft_chain *chain)
 	list_del_rcu(&chain->list);
 }
 
+static void nf_tables_commit_release(struct net *net)
+{
+	struct nft_trans *trans;
+
+	/* all side effects have to be made visible.
+	 * For example, if a chain named 'foo' has been deleted, a
+	 * new transaction must not find it anymore.
+	 *
+	 * Memory reclaim happens asynchronously from work queue
+	 * to prevent expensive synchronize_rcu() in commit phase.
+	 */
+	if (list_empty(&net->nft.commit_list)) {
+		mutex_unlock(&net->nft.commit_mutex);
+		return;
+	}
+
+	trans = list_last_entry(&net->nft.commit_list,
+				struct nft_trans, list);
+	get_net(trans->ctx.net);
+	WARN_ON_ONCE(trans->put_net);
+
+	trans->put_net = true;
+	spin_lock(&nf_tables_destroy_list_lock);
+	list_splice_tail_init(&net->nft.commit_list, &nf_tables_destroy_list);
+	spin_unlock(&nf_tables_destroy_list_lock);
+
+	mutex_unlock(&net->nft.commit_mutex);
+
+	schedule_work(&trans_destroy_work);
+}
+
 static int nf_tables_commit(struct net *net, struct sk_buff *skb)
 {
 	struct nft_trans *trans, *next;
@@ -6528,9 +6571,8 @@ static int nf_tables_commit(struct net *net, struct sk_buff *skb)
 		}
 	}
 
-	nf_tables_commit_release(net);
 	nf_tables_gen_notify(net, skb, NFT_MSG_NEWGEN);
-	mutex_unlock(&net->nft.commit_mutex);
+	nf_tables_commit_release(net);
 
 	return 0;
 }
@@ -7304,6 +7346,7 @@ static int __init nf_tables_module_init(void)
 {
 	int err;
 
+	spin_lock_init(&nf_tables_destroy_list_lock);
 	err = register_pernet_subsys(&nf_tables_net_ops);
 	if (err < 0)
 		return err;
@@ -7343,6 +7386,7 @@ static void __exit nf_tables_module_exit(void)
 	unregister_netdevice_notifier(&nf_tables_flowtable_notifier);
 	nft_chain_filter_fini();
 	unregister_pernet_subsys(&nf_tables_net_ops);
+	cancel_work_sync(&trans_destroy_work);
 	rcu_barrier();
 	nf_tables_core_module_exit();
 }