[PATCH 6.19 034/311] selftests/cgroup: Dont require synchronous populated update on task exit

Greg Kroah-Hartman
2026-04-08T18:52:25Z

From: Tejun Heo <tj@kernel.org>

[ Upstream commit 6680c162b4850976ee52b57372eddc4450c1d074 ]

test_cgcore_populated (test_core) and test_cgkill_{simple,tree,forkbomb}
(test_kill) check cgroup.events "populated 0" immediately after reaping
child tasks with waitpid(). This used to work because cgroup_task_exit() in
do_exit() unlinked tasks from css_sets before exit_notify() woke up
waitpid().

d245698d727a ("cgroup: Defer task cgroup unlink until after the task is done
switching out") moved the unlink to cgroup_task_dead() in
finish_task_switch(), which runs after exit_notify(). The populated counter
is now decremented after the parent's waitpid() can return, so there is no
longer a synchronous ordering guarantee. On PREEMPT_RT, where
cgroup_task_dead() is further deferred through lazy irq_work, the race
window is even larger.

The synchronous populated transition was never part of the cgroup interface
contract - it was an implementation artifact. Use cg_read_strcmp_wait() which
retries for up to 1 second, matching what these tests actually need to
verify: that the cgroup eventually becomes unpopulated after all tasks exit.

Fixes: d245698d727a ("cgroup: Defer task cgroup unlink until after the task is done switching out")
Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Christian Brauner <brauner@kernel.org>
Cc: cgroups@vger.kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
tools/testing/selftests/cgroup/lib/cgroup_util.c | 15 +++++++++++++++
.../selftests/cgroup/lib/include/cgroup_util.h | 2 ++
tools/testing/selftests/cgroup/test_core.c | 3 ++-
tools/testing/selftests/cgroup/test_kill.c | 7 ++++---
4 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/cgroup/lib/cgroup_util.c b/tools/testing/selftests/cgroup/lib/cgroup_util.c
index 44c52f620fda1..4b0f2c46d4322 100644
--- a/tools/testing/selftests/cgroup/lib/cgroup_util.c
+++ b/tools/testing/selftests/cgroup/lib/cgroup_util.c
@@ -123,6 +123,21 @@ int cg_read_strcmp(const char *cgroup, const char *control,
return ret;
}

+int cg_read_strcmp_wait(const char *cgroup, const char *control,
+ const char *expected)
+{
+ int i, ret;
+
+ for (i = 0; i < 100; i++) {
+ ret = cg_read_strcmp(cgroup, control, expected);
+ if (!ret)
+ return ret;
+ usleep(10000);
+ }
+
+ return ret;
+}
+
int cg_read_strstr(const char *cgroup, const char *control, const char *needle)
{
char buf[PAGE_SIZE];
diff --git a/tools/testing/selftests/cgroup/lib/include/cgroup_util.h b/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
index 7ab2824ed7b54..1cbe3b0ac6f73 100644
--- a/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
+++ b/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
@@ -59,6 +59,8 @@ extern int cg_read(const char *cgroup, const char *control,
char *buf, size_t len);
extern int cg_read_strcmp(const char *cgroup, const char *control,
const char *expected);
+extern int cg_read_strcmp_wait(const char *cgroup, const char *control,
+ const char *expected);
extern int cg_read_strstr(const char *cgroup, const char *control,
const char *needle);
extern long cg_read_long(const char *cgroup, const char *control);
diff --git a/tools/testing/selftests/cgroup/test_core.c b/tools/testing/selftests/cgroup/test_core.c
index 102262555a599..7b83c7e7c9d4f 100644
--- a/tools/testing/selftests/cgroup/test_core.c
+++ b/tools/testing/selftests/cgroup/test_core.c
@@ -233,7 +233,8 @@ static int test_cgcore_populated(const char *root)
if (err)
goto cleanup;

- if (cg_read_strcmp(cg_test_d, "cgroup.events", "populated 0\n"))
+ if (cg_read_strcmp_wait(cg_test_d, "cgroup.events",
+ "populated 0\n"))
goto cleanup;

/* Remove cgroup. */
diff --git a/tools/testing/selftests/cgroup/test_kill.c b/tools/testing/selftests/cgroup/test_kill.c
index c8c9d306925b6..f6cd23a8ecc71 100644
--- a/tools/testing/selftests/cgroup/test_kill.c
+++ b/tools/testing/selftests/cgroup/test_kill.c
@@ -86,7 +86,7 @@ static int test_cgkill_simple(const char *root)
wait_for_pid(pids[i]);

if (ret == KSFT_PASS &&
- cg_read_strcmp(cgroup, "cgroup.events", "populated 0\n"))
+ cg_read_strcmp_wait(cgroup, "cgroup.events", "populated 0\n"))
ret = KSFT_FAIL;

if (cgroup)
@@ -190,7 +190,8 @@ static int test_cgkill_tree(const char *root)
wait_for_pid(pids[i]);

if (ret == KSFT_PASS &&
- cg_read_strcmp(cgroup[0], "cgroup.events", "populated 0\n"))
+ cg_read_strcmp_wait(cgroup[0], "cgroup.events",
+ "populated 0\n"))
ret = KSFT_FAIL;

for (i = 9; i >= 0 && cgroup[i]; i--) {
@@ -251,7 +252,7 @@ static int test_cgkill_forkbomb(const char *root)
wait_for_pid(pid);

if (ret == KSFT_PASS &&
- cg_read_strcmp(cgroup, "cgroup.events", "populated 0\n"))
+ cg_read_strcmp_wait(cgroup, "cgroup.events", "populated 0\n"))
ret = KSFT_FAIL;

if (cgroup)
--
2.53.0