selftests/mm: fix deadlock for fork after pthread_create with atomic_bool

Some additional synchronization is needed on Android ARM64; we see a
deadlock with pthread_create when the parent thread races forward before
the child has a chance to start doing work.

Link: https://lkml.kernel.org/r/20241018171734.2315053-4-edliaw@google.com
Fixes: cff2945 ("selftests/mm: extend and rename uffd pagemap test")
Signed-off-by: Edward Liaw <edliaw@google.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Edward Liaw

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

@@ -241,6 +241,8 @@ static void *fork_event_consumer(void *data)
 	fork_event_args *args = data;
 	struct uffd_msg msg = { 0 };
 
+	ready_for_fork = true;
+
 	/* Read until a full msg received */
 	while (uffd_read_msg(args->parent_uffd, &msg));
 
@@ -308,8 +310,11 @@ static int pagemap_test_fork(int uffd, bool with_event, bool test_pin)
 
 	/* Prepare a thread to resolve EVENT_FORK */
 	if (with_event) {
+		ready_for_fork = false;
 		if (pthread_create(&thread, NULL, fork_event_consumer, &args))
 			err("pthread_create()");
+		while (!ready_for_fork)
+			; /* Wait for the poll_thread to start executing before forking */
 	}
 
 	child = fork();