netfilter: nft_set_rbtree: use seqcount to avoid lock in most cases

switch to lockless lockup. write side now also increments sequence
counter.  On lookup, sample counter value and only take the lock
if we did not find a match and the counter has changed.

This avoids need to write to private area in normal (lookup) cases.

In case we detect a writer (seqretry is true) we fall back to taking
the readlock.

The readlock is also used during dumps to ensure we get a consistent
tree walk.

Similar technique (rbtree+seqlock) was used by David Howells in rxrpc.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_set_rbtree.c

@@ -19,8 +19,9 @@
 #include <net/netfilter/nf_tables.h>
 
 struct nft_rbtree {
-	rwlock_t		lock;
 	struct rb_root		root;
+	rwlock_t		lock;
+	seqcount_t		count;
 };
 
 struct nft_rbtree_elem {
@@ -40,8 +41,9 @@ static bool nft_rbtree_equal(const struct nft_set *set, const void *this,
 	return memcmp(this, nft_set_ext_key(&interval->ext), set->klen) == 0;
 }
 
-static bool nft_rbtree_lookup(const struct net *net, const struct nft_set *set,
-			      const u32 *key, const struct nft_set_ext **ext)
+static bool __nft_rbtree_lookup(const struct net *net, const struct nft_set *set,
+				const u32 *key, const struct nft_set_ext **ext,
+				unsigned int seq)
 {
 	struct nft_rbtree *priv = nft_set_priv(set);
 	const struct nft_rbtree_elem *rbe, *interval = NULL;
@@ -50,15 +52,17 @@ static bool nft_rbtree_lookup(const struct net *net, const struct nft_set *set,
 	const void *this;
 	int d;
 
-	read_lock_bh(&priv->lock);
-	parent = priv->root.rb_node;
+	parent = rcu_dereference_raw(priv->root.rb_node);
 	while (parent != NULL) {
+		if (read_seqcount_retry(&priv->count, seq))
+			return false;
+
 		rbe = rb_entry(parent, struct nft_rbtree_elem, node);
 
 		this = nft_set_ext_key(&rbe->ext);
 		d = memcmp(this, key, set->klen);
 		if (d < 0) {
-			parent = parent->rb_left;
+			parent = rcu_dereference_raw(parent->rb_left);
 			if (interval &&
 			    nft_rbtree_equal(set, this, interval) &&
 			    nft_rbtree_interval_end(this) &&
@@ -66,15 +70,14 @@ static bool nft_rbtree_lookup(const struct net *net, const struct nft_set *set,
 				continue;
 			interval = rbe;
 		} else if (d > 0)
-			parent = parent->rb_right;
+			parent = rcu_dereference_raw(parent->rb_right);
 		else {
 			if (!nft_set_elem_active(&rbe->ext, genmask)) {
-				parent = parent->rb_left;
+				parent = rcu_dereference_raw(parent->rb_left);
 				continue;
 			}
 			if (nft_rbtree_interval_end(rbe))
 				goto out;
-			read_unlock_bh(&priv->lock);
 
 			*ext = &rbe->ext;
 			return true;
@@ -84,15 +87,32 @@ static bool nft_rbtree_lookup(const struct net *net, const struct nft_set *set,
 	if (set->flags & NFT_SET_INTERVAL && interval != NULL &&
 	    nft_set_elem_active(&interval->ext, genmask) &&
 	    !nft_rbtree_interval_end(interval)) {
-		read_unlock_bh(&priv->lock);
 		*ext = &interval->ext;
 		return true;
 	}
 out:
-	read_unlock_bh(&priv->lock);
 	return false;
 }
 
+static bool nft_rbtree_lookup(const struct net *net, const struct nft_set *set,
+			      const u32 *key, const struct nft_set_ext **ext)
+{
+	struct nft_rbtree *priv = nft_set_priv(set);
+	unsigned int seq = read_seqcount_begin(&priv->count);
+	bool ret;
+
+	ret = __nft_rbtree_lookup(net, set, key, ext, seq);
+	if (ret || !read_seqcount_retry(&priv->count, seq))
+		return ret;
+
+	read_lock_bh(&priv->lock);
+	seq = read_seqcount_begin(&priv->count);
+	ret = __nft_rbtree_lookup(net, set, key, ext, seq);
+	read_unlock_bh(&priv->lock);
+
+	return ret;
+}
+
 static int __nft_rbtree_insert(const struct net *net, const struct nft_set *set,
 			       struct nft_rbtree_elem *new,
 			       struct nft_set_ext **ext)
@@ -130,7 +150,7 @@ static int __nft_rbtree_insert(const struct net *net, const struct nft_set *set,
 			}
 		}
 	}
-	rb_link_node(&new->node, parent, p);
+	rb_link_node_rcu(&new->node, parent, p);
 	rb_insert_color(&new->node, &priv->root);
 	return 0;
 }
@@ -144,7 +164,9 @@ static int nft_rbtree_insert(const struct net *net, const struct nft_set *set,
 	int err;
 
 	write_lock_bh(&priv->lock);
+	write_seqcount_begin(&priv->count);
 	err = __nft_rbtree_insert(net, set, rbe, ext);
+	write_seqcount_end(&priv->count);
 	write_unlock_bh(&priv->lock);
 
 	return err;
@@ -158,7 +180,9 @@ static void nft_rbtree_remove(const struct net *net,
 	struct nft_rbtree_elem *rbe = elem->priv;
 
 	write_lock_bh(&priv->lock);
+	write_seqcount_begin(&priv->count);
 	rb_erase(&rbe->node, &priv->root);
+	write_seqcount_end(&priv->count);
 	write_unlock_bh(&priv->lock);
 }
 
@@ -264,6 +288,7 @@ static int nft_rbtree_init(const struct nft_set *set,
 	struct nft_rbtree *priv = nft_set_priv(set);
 
 	rwlock_init(&priv->lock);
+	seqcount_init(&priv->count);
 	priv->root = RB_ROOT;
 	return 0;
 }