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selftests/cgroup: Don't require synchronous populated update on task exit

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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().

d245698d72 ("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: d245698d72 ("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
This commit is contained in:
Tejun Heo 2026-03-24 10:21:47 -10:00
parent 1b164b876c
commit 6680c162b4
4 changed files with 23 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
tools/testing/selftests/cgroup/lib/cgroup_util.c
View File

@ -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];

2
tools/testing/selftests/cgroup/lib/include/cgroup_util.h
View File

@ -61,6 +61,8 @@ 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);

3
tools/testing/selftests/cgroup/test_core.c
View File

@ -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. */

7
tools/testing/selftests/cgroup/test_kill.c
View File

@ -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)