[PATCH] selftests/cgroup: Fix intermittent test_cgfreezer_ptrace test failures

[Waiman Long <longman@redhat.com>]

It is found that the test_cgfreezer_ptrace test of test_freezer can
intermittently fail on some architectures like arm64 and ppc64.

After further tracing of the mechanics of the test_cgfreezer_ptrace test,
it is found that the ptrace(PTRACE_DETACH) call temporaily unfreezes the
cgroup and then freezes it again afterward in the detaching process. The
reading of the frozen flag from cgroup.events is done from a different
process running maybe on a different CPU. As a result, racing is possible
and the intermediate unfrozen state can be read leading to occasional test
failures especially on architectures with a weak memory model like arm64.

Fix that by adding a short 1 ms delay before reading the frozen state
to ensure that the final frozen value will be read.

By running test_freezer 100 times in a loop, there were 28
test_cgfreezer_ptrace failures out of 100 on an arm64 test system before
the patch. After applying the patch, there was no test failure at all
in 100 runs of test_freezer.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 tools/testing/selftests/cgroup/test_freezer.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_freezer.c b/tools/testing/selftests/cgroup/test_freezer.c
index 0569e93fa6b0..8e2cca74d212 100644
--- a/tools/testing/selftests/cgroup/test_freezer.c
+++ b/tools/testing/selftests/cgroup/test_freezer.c
@@ -625,6 +625,18 @@ static int test_cgfreezer_ptrace(const char *root)
 	if (ptrace(PTRACE_DETACH, pid, NULL, NULL))
 		goto cleanup;
 
+	/*
+	 * The ptrace(PTRACE_DETACH) call will temporaily unfreeze the cgroup
+	 * and then freeze it again afterward in the detaching process. The
+	 * reading of the frozen flag from cgroup.events is done from a
+	 * different process running maybe on a different CPU. As a result,
+	 * racing is possible and the intermediate unfrozen state can be read
+	 * leading to occasional test failure especially on architectures with
+	 * a weak memory model like arm64. This intermittent test failure can
+	 * be avoided by adding a 1ms short delay before reading the frozen
+	 * state.
+	 */
+	usleep(1000);
 	if (cg_check_frozen(cgroup, true))
 		goto cleanup;
 
-- 
2.55.0