[PATCH cgroup/for-4.14] cgroup: add comment to cgroup_enable_threaded()

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

From c705a00d77457b44ba3790fdf0627ecb8593a254 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Date: Tue, 25 Jul 2017 13:20:18 -0400

Explain cgroup_enable_threaded() and note that the function can never
be called on the root cgroup.

Signed-off-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Suggested-by: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
Hello, Waiman.

Good point.  Added comment explaining what's going on.  Applying the
patch to cgroup/for-4.14.

Thanks.

 kernel/cgroup/cgroup.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index e0a558c..85f6a11 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3129,6 +3129,15 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 	return ret ?: nbytes;
 }
 
+/**
+ * cgroup_enable_threaded - make @cgrp threaded
+ * @cgrp: the target cgroup
+ *
+ * Called when "threaded" is written to the cgroup.type interface file and
+ * tries to make @cgrp threaded and join the parent's resource domain.
+ * This function is never called on the root cgroup as cgroup.type doesn't
+ * exist on it.
+ */
 static int cgroup_enable_threaded(struct cgroup *cgrp)
 {
 	struct cgroup *parent = cgroup_parent(cgrp);
-- 
2.9.3