tcp: batch tcp_net_metrics_exit

When dealing with a list of dismantling netns, we can scan
tcp_metrics once, saving cpu cycles.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_metrics.c

@@ -892,10 +892,14 @@ static void tcp_metrics_flush_all(struct net *net)
 
 	for (row = 0; row < max_rows; row++, hb++) {
 		struct tcp_metrics_block __rcu **pp;
+		bool match;
+
 		spin_lock_bh(&tcp_metrics_lock);
 		pp = &hb->chain;
 		for (tm = deref_locked(*pp); tm; tm = deref_locked(*pp)) {
-			if (net_eq(tm_net(tm), net)) {
+			match = net ? net_eq(tm_net(tm), net) :
+				!atomic_read(&tm_net(tm)->count);
+			if (match) {
 				*pp = tm->tcpm_next;
 				kfree_rcu(tm, rcu_head);
 			} else {
@@ -1018,14 +1022,14 @@ static int __net_init tcp_net_metrics_init(struct net *net)
 	return 0;
 }
 
-static void __net_exit tcp_net_metrics_exit(struct net *net)
+static void __net_exit tcp_net_metrics_exit_batch(struct list_head *net_exit_list)
 {
-	tcp_metrics_flush_all(net);
+	tcp_metrics_flush_all(NULL);
 }
 
 static __net_initdata struct pernet_operations tcp_net_metrics_ops = {
-	.init	=	tcp_net_metrics_init,
-	.exit	=	tcp_net_metrics_exit,
+	.init		=	tcp_net_metrics_init,
+	.exit_batch	=	tcp_net_metrics_exit_batch,
 };
 
 void __init tcp_metrics_init(void)