selftests/net: Synchronize client/server before counters checks

On tests that are expecting failure the timeout value is
TEST_RETRANSMIT_SEC == 1 second. Which is big enough for most of devices
under tests. But on a particularly slow machine/VM, 1 second might be
not enough for another thread to be scheduled and attempt to connect().
It is not a problem for tests that expect connect() to succeed as
the timeout value for them (TEST_TIMEOUT_SEC) is intentionally bigger.

One obvious way to solve this would be to increase TEST_RETRANSMIT_SEC.
But as all tests would increase the timeouts, that's going to sum up.

But here is less obvious way that keeps timeouts for expected connect()
failures low: just synchronize the two threads, which will assure that
before counter checks the other thread got a chance to run and timeout
on connect(). The expected increase of the related counter for listen()
socket will yet test the expected failure.

Never happens on my machine, but I suppose the majority of netdev's
connect-deny-* flakes [1] are caused by this.

Prevents the following testing issue:
> # selftests: net/tcp_ao: connect-deny_ipv6
> # 1..21
> # # 462[lib/setup.c:243] rand seed 1720905426
> # TAP version 13
> # ok 1 Non-AO server + AO client
> # not ok 2 Non-AO server + AO client: TCPAOKeyNotFound counter did not increase: 0 <= 0
> # ok 3 AO server + Non-AO client
> # ok 4 AO server + Non-AO client: counter TCPAORequired increased 0 => 1
...

[1]: https://netdev-3.bots.linux.dev/vmksft-tcp-ao/results/681741/6-connect-deny-ipv6/stdout
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20240823-tcp-ao-selftests-upd-6-12-v4-7-05623636fe8c@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/tools/testing/selftests/net/tcp_ao/restore.c b/tools/testing/selftests/net/tcp_ao/restore.c
index 7b91d7fde2bc..f6ea2190f43d 100644
--- a/tools/testing/selftests/net/tcp_ao/restore.c
+++ b/tools/testing/selftests/net/tcp_ao/restore.c
@@ -64,6 +64,7 @@ static void try_server_run(const char *tst_name, unsigned int port,
 		else
 			test_ok("%s: server alive", tst_name);
 	}
+	synchronize_threads(); /* 3: counters checks */
 	if (test_get_tcp_ao_counters(sk, &ao2))
 		test_error("test_get_tcp_ao_counters()");
 	after_cnt = netstat_get_one(cnt_name, NULL);
@@ -82,7 +83,7 @@ static void try_server_run(const char *tst_name, unsigned int port,
 	 * Before close() as that will send FIN and move the peer in TCP_CLOSE
 	 * and that will prevent reading AO counters from the peer's socket.
 	 */
-	synchronize_threads(); /* 3: verified => closed */
+	synchronize_threads(); /* 4: verified => closed */
 out:
 	close(sk);
 }
@@ -176,6 +177,7 @@ static void test_sk_restore(const char *tst_name, unsigned int server_port,
 		else
 			test_ok("%s: post-migrate connection is alive", tst_name);
 	}
+	synchronize_threads(); /* 3: counters checks */
 	if (test_get_tcp_ao_counters(sk, &ao2))
 		test_error("test_get_tcp_ao_counters()");
 	after_cnt = netstat_get_one(cnt_name, NULL);
@@ -189,7 +191,7 @@ static void test_sk_restore(const char *tst_name, unsigned int server_port,
 		test_ok("%s: counter %s increased %" PRIu64 " => %" PRIu64,
 			tst_name, cnt_name, before_cnt, after_cnt);
 	}
-	synchronize_threads(); /* 3: verified => closed */
+	synchronize_threads(); /* 4: verified => closed */
 	close(sk);
 }