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Commit 1f5fee2c authored by Andrea Arcangeli's avatar Andrea Arcangeli Committed by Linus Torvalds
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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: default avatarAndrea Arcangeli <aarcange@redhat.com>
Tested-by: default avatarMichael 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 56ed8f16

tools/testing/selftests/vm/userfaultfd.c

+10 −2
Original line number Diff line number Diff line
@@ -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;