net: annotate data-race around sk->sk_txrehash

sk_getsockopt() runs locklessly. This means sk->sk_txrehash
can be read while other threads are changing its value.

Other locations were handled in commit cb6cd2ce
("tcp: Change SYN ACK retransmit behaviour to account for rehash")

Fixes: 26859240 ("txhash: Add socket option to control TX hash rethink behavior")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Akhmat Karakotov <hmukos@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -1534,7 +1534,9 @@ int sk_setsockopt(struct sock *sk, int level, int optname,
 		}
 		if ((u8)val == SOCK_TXREHASH_DEFAULT)
 			val = READ_ONCE(sock_net(sk)->core.sysctl_txrehash);
-		/* Paired with READ_ONCE() in tcp_rtx_synack() */
+		/* Paired with READ_ONCE() in tcp_rtx_synack()
+		 * and sk_getsockopt().
+		 */
 		WRITE_ONCE(sk->sk_txrehash, (u8)val);
 		break;
 
@@ -1978,7 +1980,8 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
 		break;
 
 	case SO_TXREHASH:
-		v.val = sk->sk_txrehash;
+		/* Paired with WRITE_ONCE() in sk_setsockopt() */
+		v.val = READ_ONCE(sk->sk_txrehash);
 		break;
 
 	default: