selftests/xsk: Make sure single threaded test terminates

For single threaded poll tests call pthread_kill() from main thread so
that we are sure worker thread has finished its job and it is possible
to proceed with next test types from test suite. It was observed that on
some platforms it takes a bit longer for worker thread to exit and next
test case sees device as busy in this case.

Signed-off-by: Maciej Fijalkowski <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Magnus Karlsson <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/xskxceiver.c

···
 	munmap(ifobj->umem->buffer, umem_sz);
 }
 
+static void handler(int signum)
+{
+	pthread_exit(NULL);
+}
+
 static int testapp_validate_traffic_single_thread(struct test_spec *test, struct ifobject *ifobj,
 						  enum test_type type)
 {
···
 	test->ifobj_rx->shared_umem = false;
 	test->ifobj_tx->shared_umem = false;
 
+	signal(SIGUSR1, handler);
 	/* Spawn thread */
 	pthread_create(&t0, NULL, ifobj->func_ptr, test);
 
···
 	if (pthread_barrier_destroy(&barr))
 		exit_with_error(errno);
 
+	pthread_kill(t0, SIGUSR1);
 	pthread_join(t0, NULL);
 
 	if (test->total_steps == test->current_step || test->fail) {