From 1f5fee2cf232f9fac05b65f21107d2cf3c32092c Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli <aarcange@redhat.com> Date: Tue, 22 Sep 2015 14:59:00 -0700 Subject: [PATCH] userfaultfd: selftest: avoid my_bcmp false positives with powerpc Keep a non-zero placeholder after the count, for the my_bcmp comparison of the page against the zeropage. The lockless increment between 255 to 256 against a lockless my_bcmp could otherwise return false positives on ppc32le. Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> Tested-by: Michael Ellerman <mpe@ellerman.id.au> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com> Cc: Shuah Khan <shuahkh@osg.samsung.com> Cc: Thierry Reding <treding@nvidia.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> --- tools/testing/selftests/vm/userfaultfd.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c index 0671ae1d1052e..10897092823da 100644 --- a/tools/testing/selftests/vm/userfaultfd.c +++ b/tools/testing/selftests/vm/userfaultfd.c @@ -465,6 +465,14 @@ static int userfaultfd_stress(void) *area_mutex(area_src, nr) = (pthread_mutex_t) PTHREAD_MUTEX_INITIALIZER; count_verify[nr] = *area_count(area_src, nr) = 1; + /* + * In the transition between 255 to 256, powerpc will + * read out of order in my_bcmp and see both bytes as + * zero, so leave a placeholder below always non-zero + * after the count, to avoid my_bcmp to trigger false + * positives. + */ + *(area_count(area_src, nr) + 1) = 1; } pipefd = malloc(sizeof(int) * nr_cpus * 2); @@ -610,8 +618,8 @@ int main(int argc, char **argv) fprintf(stderr, "Usage: <MiB> <bounces>\n"), exit(1); nr_cpus = sysconf(_SC_NPROCESSORS_ONLN); page_size = sysconf(_SC_PAGE_SIZE); - if ((unsigned long) area_count(NULL, 0) + sizeof(unsigned long long) > - page_size) + if ((unsigned long) area_count(NULL, 0) + sizeof(unsigned long long) * 2 + > page_size) fprintf(stderr, "Impossible to run this test\n"), exit(2); nr_pages_per_cpu = atol(argv[1]) * 1024*1024 / page_size / nr_cpus; -- 2.30.2