tcp: Change SYN ACK retransmit behaviour to account for rehash

Disabling rehash behavior did not affect SYN ACK retransmits because hash
was forcefully changed bypassing the sk_rethink_hash function. This patch
adds a condition which checks for rehash mode before resetting hash.
Signed-off-by: NAkhmat Karakotov <hmukos@yandex-team.ru>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -1452,7 +1452,8 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
 			ret = -EINVAL;
 			break;
 		}
-		sk->sk_txrehash = (u8)val;
+		/* Paired with READ_ONCE() in tcp_rtx_synack() */
+		WRITE_ONCE(sk->sk_txrehash, (u8)val);
 		break;
 
 	default:

net/ipv4/tcp_output.c

@@ -4092,7 +4092,9 @@ int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
 	struct flowi fl;
 	int res;
 
-	tcp_rsk(req)->txhash = net_tx_rndhash();
+	/* Paired with WRITE_ONCE() in sock_setsockopt() */
+	if (READ_ONCE(sk->sk_txrehash) == SOCK_TXREHASH_ENABLED)
+		tcp_rsk(req)->txhash = net_tx_rndhash();
 	res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL,
 				  NULL);
 	if (!res) {