tcp: set TCP_SYNCNT locklessly

icsk->icsk_syn_retries can safely be set without locking the socket.

We have to add READ_ONCE() annotations in tcp_fastopen_synack_timer()
and tcp_write_timeout().

Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/tcp.c

···
 	if (val < 1 || val > MAX_TCP_SYNCNT)
 		return -EINVAL;
 
-	lock_sock(sk);
 	WRITE_ONCE(inet_csk(sk)->icsk_syn_retries, val);
-	release_sock(sk);
 	return 0;
 }
 EXPORT_SYMBOL(tcp_sock_set_syncnt);
···
 	if (copy_from_sockptr(&val, optval, sizeof(val)))
 		return -EFAULT;
 
+	/* Handle options that can be set without locking the socket. */
+	switch (optname) {
+	case TCP_SYNCNT:
+		return tcp_sock_set_syncnt(sk, val);
+	}
+
 	sockopt_lock_sock(sk);
 
 	switch (optname) {
···
 		else
 			WRITE_ONCE(tp->keepalive_probes, val);
 		break;
-	case TCP_SYNCNT:
-		if (val < 1 || val > MAX_TCP_SYNCNT)
-			err = -EINVAL;
-		else
-			WRITE_ONCE(icsk->icsk_syn_retries, val);
-		break;
-
 	case TCP_SAVE_SYN:
 		/* 0: disable, 1: enable, 2: start from ether_header */
 		if (val < 0 || val > 2)

net/ipv4/tcp_timer.c

···
 	if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
 		if (icsk->icsk_retransmits)
 			__dst_negative_advice(sk);
-		retry_until = icsk->icsk_syn_retries ? :
+		/* Paired with WRITE_ONCE() in tcp_sock_set_syncnt() */
+		retry_until = READ_ONCE(icsk->icsk_syn_retries) ? :
 			READ_ONCE(net->ipv4.sysctl_tcp_syn_retries);
 
 		max_retransmits = retry_until;
···
 
 	req->rsk_ops->syn_ack_timeout(req);
 
-	/* add one more retry for fastopen */
-	max_retries = icsk->icsk_syn_retries ? :
+	/* Add one more retry for fastopen.
+	 * Paired with WRITE_ONCE() in tcp_sock_set_syncnt()
+	 */
+	max_retries = READ_ONCE(icsk->icsk_syn_retries) ? :
 		READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_synack_retries) + 1;
 
 	if (req->num_timeout >= max_retries) {