cgroup: init_tasks shouldn't be linked to the root cgroup
5153faac18d2 ("cgroup: remove cgroup_enable_task_cg_lists()
optimization") removed lazy initialization of css_sets so that new
tasks are always lniked to its css_set. In the process, it incorrectly
ended up adding init_tasks to root css_set. They show up as PID 0's in
root's cgroup.procs triggering warnings in systemd and generally
confusing people.
Fix it by skip css_set linking for init_tasks.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: https://github.com/joanbm
Link: https://github.com/systemd/systemd/issues/14682
Fixes: 5153faac18d2 ("cgroup: remove cgroup_enable_task_cg_lists() optimization")
Cc: stable@vger.kernel.org # v5.5+
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index b3744872..cf8a36b 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5932,11 +5932,14 @@
spin_lock_irq(&css_set_lock);
- WARN_ON_ONCE(!list_empty(&child->cg_list));
- cset = task_css_set(current); /* current is @child's parent */
- get_css_set(cset);
- cset->nr_tasks++;
- css_set_move_task(child, NULL, cset, false);
+ /* init tasks are special, only link regular threads */
+ if (likely(child->pid)) {
+ WARN_ON_ONCE(!list_empty(&child->cg_list));
+ cset = task_css_set(current); /* current is @child's parent */
+ get_css_set(cset);
+ cset->nr_tasks++;
+ css_set_move_task(child, NULL, cset, false);
+ }
/*
* If the cgroup has to be frozen, the new task has too. Let's set
