[PATCH bpf] selftests/bpf: test_progs: fix client/server race in tcp_rtt

[Stanislav Fomichev <sdf@google.com>]

This is the same problem I found earlier in test_sockopt_inherit:
there is a race between server thread doing accept() and client
thread doing connect(). Let's explicitly synchronize them via
pthread conditional variable.

Fixes: b55873984dab ("selftests/bpf: test BPF_SOCK_OPS_RTT_CB")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 tools/testing/selftests/bpf/prog_tests/tcp_rtt.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/tcp_rtt.c b/tools/testing/selftests/bpf/prog_tests/tcp_rtt.c
index fdc0b3614a9e..e64058906bcd 100644
--- a/tools/testing/selftests/bpf/prog_tests/tcp_rtt.c
+++ b/tools/testing/selftests/bpf/prog_tests/tcp_rtt.c
@@ -203,6 +203,9 @@ static int start_server(void)
 	return fd;
 }
 
+static pthread_mutex_t server_started_mtx = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t server_started = PTHREAD_COND_INITIALIZER;
+
 static void *server_thread(void *arg)
 {
 	struct sockaddr_storage addr;
@@ -215,6 +218,10 @@ static void *server_thread(void *arg)
 		return NULL;
 	}
 
+	pthread_mutex_lock(&server_started_mtx);
+	pthread_cond_signal(&server_started);
+	pthread_mutex_unlock(&server_started_mtx);
+
 	client_fd = accept(fd, (struct sockaddr *)&addr, &len);
 	if (CHECK_FAIL(client_fd < 0)) {
 		perror("Failed to accept client");
@@ -248,7 +255,14 @@ void test_tcp_rtt(void)
 	if (CHECK_FAIL(server_fd < 0))
 		goto close_cgroup_fd;
 
-	pthread_create(&tid, NULL, server_thread, (void *)&server_fd);
+	if (CHECK_FAIL(pthread_create(&tid, NULL, server_thread,
+				      (void *)&server_fd)))
+		goto close_cgroup_fd;
+
+	pthread_mutex_lock(&server_started_mtx);
+	pthread_cond_wait(&server_started, &server_started_mtx);
+	pthread_mutex_unlock(&server_started_mtx);
+
 	CHECK_FAIL(run_test(cgroup_fd, server_fd));
 	close(server_fd);
 close_cgroup_fd:
-- 
2.23.0.351.gc4317032e6-goog