Fixes a race between parent and child threads in futex_requeue.

Similar to fbf4dec70277 ("selftests/futex: Order calls to futex_lock_pi"), which fixed a flake in futex_lock_pi due to racing between the parent and child threads.

The same issue can occur in the futex_requeue test, because it expects waiterfn to make progress to futex_wait before the parent starts to requeue. This is mitigated by the parent sleeping for WAKE_WAIT_US, but it still fails occasionally. This can be reproduced by adding a sleep in the waiterfn before futex_wait:

TAP version 13 1..2 not ok 1 futex_requeue simple returned: 0 not ok 2 futex_requeue simple returned: 0 not ok 3 futex_requeue many returned: 0 not ok 4 futex_requeue many returned: 0

Instead, replace the sleep with barriers to make the sequencing explicit.

Fixes: 7cb5dd8e2c8c ("selftests: futex: Add futex compare requeue test") Signed-off-by: Edward Liaw edliaw@google.com Reviewed-by: Muhammad Usama Anjum usama.anjum@collabora.com Reviewed-by: André Almeida andrealmeid@igalia.com --- .../selftests/futex/functional/futex_requeue.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/futex/functional/futex_requeue.c b/tools/testing/selftests/futex/functional/futex_requeue.c index 51485be6eb2f..8f7d3e8bf32a 100644 --- a/tools/testing/selftests/futex/functional/futex_requeue.c +++ b/tools/testing/selftests/futex/functional/futex_requeue.c @@ -12,9 +12,9 @@

#define TEST_NAME "futex-requeue" #define timeout_ns 30000000 -#define WAKE_WAIT_US 10000

volatile futex_t *f1; +static pthread_barrier_t barrier;

void usage(char *prog) { @@ -32,6 +32,8 @@ void *waiterfn(void *arg) to.tv_sec = 0; to.tv_nsec = timeout_ns;

+ pthread_barrier_wait(&barrier); + if (futex_wait(f1, *f1, &to, 0)) printf("waiter failed errno %d\n", errno);

@@ -70,13 +72,15 @@ int main(int argc, char *argv[]) ksft_print_msg("%s: Test futex_requeue\n", basename(argv[0]));

+ pthread_barrier_init(&barrier, NULL, 2); /* * Requeue a waiter from f1 to f2, and wake f2. */ if (pthread_create(&waiter[0], NULL, waiterfn, NULL)) error("pthread_create failed\n", errno);

- usleep(WAKE_WAIT_US); + pthread_barrier_wait(&barrier); + pthread_barrier_destroy(&barrier);

info("Requeuing 1 futex from f1 to f2\n"); res = futex_cmp_requeue(f1, 0, &f2, 0, 1, 0); @@ -99,6 +103,7 @@ int main(int argc, char *argv[]) ksft_test_result_pass("futex_requeue simple succeeds\n"); }

+ pthread_barrier_init(&barrier, NULL, 11);

/* * Create 10 waiters at f1. At futex_requeue, wake 3 and requeue 7. @@ -109,7 +114,8 @@ int main(int argc, char *argv[]) error("pthread_create failed\n", errno); }

- usleep(WAKE_WAIT_US); + pthread_barrier_wait(&barrier); + pthread_barrier_destroy(&barrier);

info("Waking 3 futexes at f1 and requeuing 7 futexes from f1 to f2\n"); res = futex_cmp_requeue(f1, 0, &f2, 3, 7, 0); -- 2.46.0.662.g92d0881bb0-goog