There a two issues with traversal:
1. Key lookup (tree search) cannot detect concurrent modifications and may
   not find a result in case of parallel modification.

2. Worker does a lockless iteration.  This is never safe.

Add a sequence counter and re-do the lookup under lock in case the
tree was modified / seqcount changed.

gc_worker bugs are addressed in the next patch.

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 v2: new in this iteration (split from last patch)
 net/netfilter/nf_conncount.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/net/netfilter/nf_conncount.c b/net/netfilter/nf_conncount.c
index 8d9c24b69dc9..3c88fb206fb4 100644
--- a/net/netfilter/nf_conncount.c
+++ b/net/netfilter/nf_conncount.c
@@ -58,6 +58,7 @@ static spinlock_t nf_conncount_locks[CONNCOUNT_SLOTS] __cacheline_aligned_in_smp
 
 struct nf_conncount_root {
 	struct rb_root root;
+	seqcount_t count;
 };
 
 struct nf_conncount_data {
@@ -382,8 +383,10 @@ static void tree_nodes_free(struct nf_conncount_root *root,
 		rbconn = gc_nodes[--gc_count];
 		spin_lock(&rbconn->list.list_lock);
 		if (!rbconn->list.count) {
+			write_seqcount_begin(&root->count);
 			rb_erase(&rbconn->node, &root->root);
 			call_rcu(&rbconn->rcu_head, __tree_nodes_free);
+			write_seqcount_end(&root->count);
 		}
 		spin_unlock(&rbconn->list.list_lock);
 	}
@@ -478,8 +481,10 @@ insert_tree(struct net *net,
 		count = 1;
 		rbconn->list.count = count;
 
+		write_seqcount_begin(&root->count);
 		rb_link_node_rcu(&rbconn->node, parent, rbnode);
 		rb_insert_color(&rbconn->node, &root->root);
+		write_seqcount_end(&root->count);
 	}
 out_unlock:
 	if (refcounted)
@@ -492,6 +497,7 @@ static struct nf_conncount_rb *
 find_tree_node(struct nf_conncount_root *root, struct nf_conncount_data *data,
 	       const u32 *key)
 {
+	unsigned int seq = read_seqcount_begin(&root->count);
 	struct rb_node *parent;
 
 	parent = rcu_dereference_raw(root->root.rb_node);
@@ -509,6 +515,9 @@ find_tree_node(struct nf_conncount_root *root, struct nf_conncount_data *data,
 		} else {
 			return rbconn;
 		}
+
+		if (read_seqcount_retry(&root->count, seq))
+			return ERR_PTR(-EAGAIN);
 	}
 
 	return ERR_PTR(-ENOENT);
@@ -531,6 +540,12 @@ count_tree(struct net *net,
 
 	rbconn = find_tree_node(root, data, key);
 	if (IS_ERR(rbconn)) {
+		if (PTR_ERR(rbconn) == -EAGAIN) {
+			spin_lock_bh(&nf_conncount_locks[hash]);
+			rbconn = find_tree_node(root, data, key);
+			spin_unlock_bh(&nf_conncount_locks[hash]);
+		}
+
 		if (PTR_ERR(rbconn) == -ENOENT) {
 			if (!skb)
 				return 0;
@@ -647,6 +662,7 @@ EXPORT_SYMBOL_GPL(nf_conncount_count_skb);
 static void nf_conncount_root_init(struct nf_conncount_root *r)
 {
 	r->root = RB_ROOT;
+	seqcount_init(&r->count);
 }
 
 struct nf_conncount_data *nf_conncount_init(struct net *net, unsigned int keylen)
-- 
2.54.0