selftests/net: fix out-of-order delivery of FIN in gro:tcp test

Due to the gro_sender sending data packets and FIN packets
in very quick succession, these are received almost simultaneously
by the gro_receiver. FIN packets are sometimes processed before the
data packets leading to intermittent (~1/100) test failures.

This change adds a delay of 100ms before sending FIN packets
in gro:tcp test to avoid the out-of-order delivery. The same
mitigation already exists for the gro:ip test.

Fixes: 7d15750 ("selftests/net: GRO coalesce test")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Anubhav Singh <anubhavsinggh@google.com>
Link: https://patch.msgid.link/20251030062818.1562228-1-anubhavsinggh@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Anubhav Singh

tools/testing/selftests/net/gro.c

@@ -989,6 +989,7 @@ static void check_recv_pkts(int fd, int *correct_payload,
 
 static void gro_sender(void)
 {
+	const int fin_delay_us = 100 * 1000;
 	static char fin_pkt[MAX_HDR_LEN];
 	struct sockaddr_ll daddr = {};
 	int txfd = -1;
@@ -1032,15 +1033,22 @@ static void gro_sender(void)
 		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
 	} else if (strcmp(testname, "tcp") == 0) {
 		send_changed_checksum(txfd, &daddr);
+		/* Adding sleep before sending FIN so that it is not
+		 * received prior to other packets.
+		 */
+		usleep(fin_delay_us);
 		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
 
 		send_changed_seq(txfd, &daddr);
+		usleep(fin_delay_us);
 		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
 
 		send_changed_ts(txfd, &daddr);
+		usleep(fin_delay_us);
 		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
 
 		send_diff_opt(txfd, &daddr);
+		usleep(fin_delay_us);
 		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
 	} else if (strcmp(testname, "ip") == 0) {
 		send_changed_ECN(txfd, &daddr);