vxlan: Convert FDB flushing to RCU

Instead of holding the FDB hash lock when traversing the FDB linked list
during flushing, use RCU and only acquire the lock for entries that need
to be flushed.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250415121143.345227-11-idosch@nvidia.com
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: idosch

drivers/net/vxlan/vxlan_core.c

@@ -3037,27 +3037,32 @@ static void vxlan_flush(struct vxlan_dev *vxlan,
 			const struct vxlan_fdb_flush_desc *desc)
 {
 	bool match_remotes = vxlan_fdb_flush_should_match_remotes(desc);
-	struct hlist_node *n;
 	struct vxlan_fdb *f;
 
-	spin_lock_bh(&vxlan->hash_lock);
-	hlist_for_each_entry_safe(f, n, &vxlan->fdb_list, fdb_node) {
+	rcu_read_lock();
+	hlist_for_each_entry_rcu(f, &vxlan->fdb_list, fdb_node) {
 		if (!vxlan_fdb_flush_matches(f, vxlan, desc))
 			continue;
 
+		spin_lock_bh(&vxlan->hash_lock);
+		if (hlist_unhashed(&f->fdb_node))
+			goto unlock;
+
 		if (match_remotes) {
 			bool destroy_fdb = false;
 
 			vxlan_fdb_flush_match_remotes(f, vxlan, desc,
 						      &destroy_fdb);
 
 			if (!destroy_fdb)
-				continue;
+				goto unlock;
 		}
 
 		vxlan_fdb_destroy(vxlan, f, true, true);
+unlock:
+		spin_unlock_bh(&vxlan->hash_lock);
 	}
-	spin_unlock_bh(&vxlan->hash_lock);
+	rcu_read_unlock();
 }
 
 static const struct nla_policy vxlan_del_bulk_policy[NDA_MAX + 1] = {