linux-kernel - [PATCH 5.5 28/80] cgroup: init_tasks shouldnt be linked to the root cgroup

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Message-Id: <20200218190435.137599128@linuxfoundation.org>
Date:   Tue, 18 Feb 2020 20:54:49 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: [PATCH 5.5 28/80] cgroup: init_tasks shouldnt be linked to the root cgroup

From: Tejun Heo <tj@...nel.org>

commit 0cd9d33ace336bc424fc30944aa3defd6786e4fe upstream.

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@...nel.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@...r.kernel.org # v5.5+
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/cgroup/cgroup.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5932,11 +5932,14 @@ void cgroup_post_fork(struct task_struct
 
 	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