tcp: annotate data-races around tp->tcp_tx_delay

do_tcp_getsockopt() reads tp->tcp_tx_delay while another cpu
might change its value.

Fixes: a842fe1 ("tcp: add optional per socket transmit delay")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230719212857.3943972-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv4/tcp.c

@@ -3674,7 +3674,7 @@ int do_tcp_setsockopt(struct sock *sk, int level, int optname,
 	case TCP_TX_DELAY:
 		if (val)
 			tcp_enable_tx_delay();
-		tp->tcp_tx_delay = val;
+		WRITE_ONCE(tp->tcp_tx_delay, val);
 		break;
 	default:
 		err = -ENOPROTOOPT;
@@ -4154,7 +4154,7 @@ int do_tcp_getsockopt(struct sock *sk, int level,
 		break;
 
 	case TCP_TX_DELAY:
-		val = tp->tcp_tx_delay;
+		val = READ_ONCE(tp->tcp_tx_delay);
 		break;
 
 	case TCP_TIMESTAMP: