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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>
This commit is contained in:
Andrea Arcangeli 2015-09-22 14:59:00 -07:00 committed by Linus Torvalds
parent 56ed8f169e
commit 1f5fee2cf2
1 changed files with 10 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
tools/testing/selftests/vm/userfaultfd.c
View File

@ -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;