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

From: Stanislav Fomichev <[email-masked]>
Date: Mon, 23 Sep 2019 08:38:19 -0700

On 09/22, Andrii Nakryiko wrote:
> On Sun, Sep 22, 2019 at 12:10 PM Stanislav Fomichev <[email-masked]> wrote:
> >
> > 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 <[email-masked]>
> > ---
> >  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);
> 
> 
> If the server fails to listen, then we'll never get a signal, right?
> Let's use timedwait instead to avoid test getting stuck forever in
> such cases?
Good point. How about I do the same thing I do in sockopt_inherit tests:
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/testing/selftests/bpf/prog_tests/sockopt_inherit.c#n73

	err = listen()
	pthread_cond_signal()
	if (CHECK_FAIL(err)) {
		return;
	}

Should fix the problem of getting stuck forever without any timeouts.
I'll send a v2 later today.

> > +
> >         CHECK_FAIL(run_test(cgroup_fd, server_fd));
> >         close(server_fd);
> >  close_cgroup_fd:
> > --
> > 2.23.0.351.gc4317032e6-goog
> >