tcp: annotate data races in __tcp_oow_rate_limited()

request sockets are lockless, __tcp_oow_rate_limited() could be called
on the same object from different cpus. This is harmless.

Add READ_ONCE()/WRITE_ONCE() annotations to avoid a KCSAN report.

Fixes: 4ce7e93c ("tcp: rate limit ACK sent by SYN_RECV request sockets")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -3590,8 +3590,11 @@ static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32
 static bool __tcp_oow_rate_limited(struct net *net, int mib_idx,
 				   u32 *last_oow_ack_time)
 {
-	if (*last_oow_ack_time) {
-		s32 elapsed = (s32)(tcp_jiffies32 - *last_oow_ack_time);
+	/* Paired with the WRITE_ONCE() in this function. */
+	u32 val = READ_ONCE(*last_oow_ack_time);
+
+	if (val) {
+		s32 elapsed = (s32)(tcp_jiffies32 - val);
 
 		if (0 <= elapsed &&
 		    elapsed < READ_ONCE(net->ipv4.sysctl_tcp_invalid_ratelimit)) {
@@ -3600,7 +3603,10 @@ static bool __tcp_oow_rate_limited(struct net *net, int mib_idx,
 		}
 	}
 
-	*last_oow_ack_time = tcp_jiffies32;
+	/* Paired with the prior READ_ONCE() and with itself,
+	 * as we might be lockless.
+	 */
+	WRITE_ONCE(*last_oow_ack_time, tcp_jiffies32);
 
 	return false;	/* not rate-limited: go ahead, send dupack now! */
 }