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 <[email protected]>
Tested-by: Michael Ellerman <[email protected]>
Cc: Dr. David Alan Gilbert <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Thierry Reding <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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;