selftests/mm: fix deadlock for fork after pthread_create on ARM

On Android with arm, there is some synchronization needed to avoid a
deadlock when forking after pthread_create.

Link: https://lkml.kernel.org/r/20241003211716.371786-3-edliaw@google.com
Fixes: cff29458 ("selftests/mm: extend and rename uffd pagemap test")
Signed-off-by: Edward Liaw <edliaw@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/uffd-unit-tests.c

@@ -241,6 +241,9 @@ static void *fork_event_consumer(void *data)
 	fork_event_args *args = data;
 	struct uffd_msg msg = { 0 };
 
+	/* Ready for parent thread to fork */
+	pthread_barrier_wait(&ready_for_fork);
+
 	/* Read until a full msg received */
 	while (uffd_read_msg(args->parent_uffd, &msg));
 
@@ -308,8 +311,12 @@ static int pagemap_test_fork(int uffd, bool with_event, bool test_pin)
 
 	/* Prepare a thread to resolve EVENT_FORK */
 	if (with_event) {
+		pthread_barrier_init(&ready_for_fork, NULL, 2);
 		if (pthread_create(&thread, NULL, fork_event_consumer, &args))
 			err("pthread_create()");
+		/* Wait for child thread to start before forking */
+		pthread_barrier_wait(&ready_for_fork);
+		pthread_barrier_destroy(&ready_for_fork);
 	}
 
 	child = fork();