ipv4: convert fib_num_tclassid_users to atomic_t

Before commit faa041a40b9f ("ipv4: Create cleanup helper for fib_nh")
changes to net->ipv4.fib_num_tclassid_users were protected by RTNL.

After the change, this is no longer the case, as free_fib_info_rcu()
runs after rcu grace period, without rtnl being held.

Fixes: faa041a40b9f ("ipv4: Create cleanup helper for fib_nh")
Signed-off-by: Eric Dumazet <[e-mail]>
Cc: David Ahern <[e-mail]>
Reviewed-by: David Ahern <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/ip_fib.h

···
 #ifdef CONFIG_IP_ROUTE_CLASSID
 static inline int fib_num_tclassid_users(struct net *net)
 {
-	return net->ipv4.fib_num_tclassid_users;
+	return atomic_read(&net->ipv4.fib_num_tclassid_users);
 }
 #else
 static inline int fib_num_tclassid_users(struct net *net)

include/net/netns/ipv4.h

···
 	bool			fib_has_custom_local_routes;
 	bool			fib_offload_disabled;
 #ifdef CONFIG_IP_ROUTE_CLASSID
-	int			fib_num_tclassid_users;
+	atomic_t		fib_num_tclassid_users;
 #endif
 	struct hlist_head	*fib_table_hash;
 	struct sock		*fibnl;

net/ipv4/fib_frontend.c

···
 	int error;
 
 #ifdef CONFIG_IP_ROUTE_CLASSID
-	net->ipv4.fib_num_tclassid_users = 0;
+	atomic_set(&net->ipv4.fib_num_tclassid_users, 0);
 #endif
 	error = ip_fib_net_init(net);
 	if (error < 0)

net/ipv4/fib_rules.c

···
 	if (tb[FRA_FLOW]) {
 		rule4->tclassid = nla_get_u32(tb[FRA_FLOW]);
 		if (rule4->tclassid)
-			net->ipv4.fib_num_tclassid_users++;
+			atomic_inc(&net->ipv4.fib_num_tclassid_users);
 	}
 #endif
 
···
 
 #ifdef CONFIG_IP_ROUTE_CLASSID
 	if (((struct fib4_rule *)rule)->tclassid)
-		net->ipv4.fib_num_tclassid_users--;
+		atomic_dec(&net->ipv4.fib_num_tclassid_users);
 #endif
 	net->ipv4.fib_has_custom_rules = true;
 

net/ipv4/fib_semantics.c

···
 {
 #ifdef CONFIG_IP_ROUTE_CLASSID
 	if (fib_nh->nh_tclassid)
-		net->ipv4.fib_num_tclassid_users--;
+		atomic_dec(&net->ipv4.fib_num_tclassid_users);
 #endif
 	fib_nh_common_release(&fib_nh->nh_common);
 }
···
 #ifdef CONFIG_IP_ROUTE_CLASSID
 	nh->nh_tclassid = cfg->fc_flow;
 	if (nh->nh_tclassid)
-		net->ipv4.fib_num_tclassid_users++;
+		atomic_inc(&net->ipv4.fib_num_tclassid_users);
 #endif
 #ifdef CONFIG_IP_ROUTE_MULTIPATH
 	nh->fib_nh_weight = nh_weight;