[TCP]: Export accept queue len of a TCP listening socket via rx_queue

While debugging a TCP server hang issue, we noticed that currently there is
no way for a user to get the acceptq backlog value for a TCP listen socket.

All the standard networking utilities that display socket info like netstat,
ss and /proc/net/tcp have 2 fields called rx_queue and tx_queue. These
fields do not mean much for listening sockets. This patch uses one of these
unused fields(rx_queue) to export the accept queue len for listening sockets.
Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_diag.c

@@ -26,6 +26,9 @@ static void tcp_diag_get_info(struct sock *sk, struct inet_diag_msg *r,
 	const struct tcp_sock *tp = tcp_sk(sk);
 	struct tcp_info *info = _info;
 
+	if (sk->sk_state == TCP_LISTEN)
+		r->idiag_rqueue = sk->sk_ack_backlog;
+	else
 		r->idiag_rqueue = tp->rcv_nxt - tp->copied_seq;
 	r->idiag_wqueue = tp->write_seq - tp->snd_una;
 	if (info != NULL)

net/ipv4/tcp_ipv4.c

@@ -1726,7 +1726,8 @@ static void get_tcp4_sock(struct sock *sp, char *tmpbuf, int i)
 	sprintf(tmpbuf, "%4d: %08X:%04X %08X:%04X %02X %08X:%08X %02X:%08lX "
 			"%08X %5d %8d %lu %d %p %u %u %u %u %d",
 		i, src, srcp, dest, destp, sp->sk_state,
-		tp->write_seq - tp->snd_una, tp->rcv_nxt - tp->copied_seq,
+		tp->write_seq - tp->snd_una,
+		(sp->sk_state == TCP_LISTEN) ? sp->sk_ack_backlog : (tp->rcv_nxt - tp->copied_seq),
 		timer_active,
 		jiffies_to_clock_t(timer_expires - jiffies),
 		icsk->icsk_retransmits,

net/ipv6/tcp_ipv6.c

@@ -1469,7 +1469,8 @@ static void get_tcp6_sock(struct seq_file *seq, struct sock *sp, int i)
 		   dest->s6_addr32[0], dest->s6_addr32[1],
 		   dest->s6_addr32[2], dest->s6_addr32[3], destp,
 		   sp->sk_state, 
-		   tp->write_seq-tp->snd_una, tp->rcv_nxt-tp->copied_seq,
+		   tp->write_seq-tp->snd_una,
+		   (sp->sk_state == TCP_LISTEN) ? sp->sk_ack_backlog : (tp->rcv_nxt - tp->copied_seq),
 		   timer_active,
 		   jiffies_to_clock_t(timer_expires - jiffies),
 		   icsk->icsk_retransmits,