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Peter Anvin" , Andy Lutomirski Cc: linux-kernel@vger.kernel.org, kernel-team@android.com, Edward Liaw , Muhammad Usama Anjum , Davidlohr Bueso , llvm@lists.linux.dev 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 Reviewed-by: Muhammad Usama Anjum Reviewed-by: André Almeida --- .../selftests/futex/functional/futex_requeue.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) -- 2.46.0.662.g92d0881bb0-goog 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);