It is found that the test_cgfreezer_ptrace test of test_freezer can intermittently fail on some architectures like arm64 and ppc64.
After further tracing of the mechanics of the test_cgfreezer_ptrace test, it is found that the ptrace(PTRACE_DETACH) call temporaily unfreezes the cgroup and then freezes it again afterward in the detaching process. The reading of the frozen flag from cgroup.events is done from a different process running maybe on a different CPU. As a result, racing is possible and the intermediate unfrozen state can be read leading to occasional test failures especially on architectures with a weak memory model like arm64. Fix that by adding a short 1 ms delay before reading the frozen state to ensure that the final frozen value will be read. By running test_freezer 100 times in a loop, there were 28 test_cgfreezer_ptrace failures out of 100 on an arm64 test system before the patch. After applying the patch, there was no test failure at all in 100 runs of test_freezer. Signed-off-by: Waiman Long <[email protected]> --- tools/testing/selftests/cgroup/test_freezer.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/tools/testing/selftests/cgroup/test_freezer.c b/tools/testing/selftests/cgroup/test_freezer.c index 0569e93fa6b0..8e2cca74d212 100644 --- a/tools/testing/selftests/cgroup/test_freezer.c +++ b/tools/testing/selftests/cgroup/test_freezer.c @@ -625,6 +625,18 @@ static int test_cgfreezer_ptrace(const char *root) if (ptrace(PTRACE_DETACH, pid, NULL, NULL)) goto cleanup; + /* + * The ptrace(PTRACE_DETACH) call will temporaily unfreeze the cgroup + * and then freeze it again afterward in the detaching process. The + * reading of the frozen flag from cgroup.events is done from a + * different process running maybe on a different CPU. As a result, + * racing is possible and the intermediate unfrozen state can be read + * leading to occasional test failure especially on architectures with + * a weak memory model like arm64. This intermittent test failure can + * be avoided by adding a 1ms short delay before reading the frozen + * state. + */ + usleep(1000); if (cg_check_frozen(cgroup, true)) goto cleanup; -- 2.55.0
