[PATCH 2/2] selftests/mm: fix deadlock for fork after pthread_create on ARM

[Edward Liaw]

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

Fixes: cff294582798 ("selftests/mm: extend and rename uffd pagemap test")
Signed-off-by: Edward Liaw <edl...@google.com>
---
 tools/testing/selftests/mm/uffd-unit-tests.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c 
b/tools/testing/selftests/mm/uffd-unit-tests.c
index 3db2296ac631..c8a3b1c7edff 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/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();
-- 
2.46.1.824.gd892dcdcdd-goog