net_ns: add __rcu annotations

add __rcu annotation to (struct net)->gen, and use
rcu_dereference_protected() in net_assign_generic()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -88,7 +88,7 @@ struct net {
 #ifdef CONFIG_WEXT_CORE
 	struct sk_buff_head	wext_nlevents;
 #endif
-	struct net_generic	*gen;
+	struct net_generic __rcu	*gen;
 
 	/* Note : following structs are cache line aligned */
 #ifdef CONFIG_XFRM

net/core/net_namespace.c

@@ -42,7 +42,9 @@ static int net_assign_generic(struct net *net, int id, void *data)
 	BUG_ON(!mutex_is_locked(&net_mutex));
 	BUG_ON(id == 0);
 
-	ng = old_ng = net->gen;
+	old_ng = rcu_dereference_protected(net->gen,
+					   lockdep_is_held(&net_mutex));
+	ng = old_ng;
 	if (old_ng->len >= id)
 		goto assign;