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

commit e142cc8 upstream.

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

Link: https://lkml.kernel.org/r/[email protected]
Fixes: cff2945 ("selftests/mm: extend and rename uffd pagemap test")
Signed-off-by: Edward Liaw <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: edliaw

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

@@ -237,6 +237,9 @@ 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));
 
@@ -304,8 +307,12 @@ 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();