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>

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 0671ae1..1089709 100644 (file)
--- 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;