cgroup: add css_parent()

Currently, controllers have to explicitly follow the cgroup hierarchy
to find the parent of a given css.  cgroup is moving towards using
cgroup_subsys_state as the main controller interface construct, so
let's provide a way to climb the hierarchy using just csses.

This patch implements css_parent() which, given a css, returns its
parent.  The function is guarnateed to valid non-NULL parent css as
long as the target css is not at the top of the hierarchy.

freezer, cpuset, cpu, cpuacct, hugetlb, memory, net_cls and devices
are converted to use css_parent() instead of accessing cgroup->parent
directly.

* __parent_ca() is dropped from cpuacct and its usage is replaced with
  parent_ca().  The only difference between the two was NULL test on
  cgroup->parent which is now embedded in css_parent() making the
  distinction moot.  Note that eventually a css->parent field will be
  added to css and the NULL check in css_parent() will go away.

This patch shouldn't cause any behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 1db686e47a2..657a73cd44c 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -62,11 +62,7 @@ static inline struct freezer *task_freezer(struct task_struct *task)
 
 static struct freezer *parent_freezer(struct freezer *freezer)
 {
-	struct cgroup *pcg = freezer->css.cgroup->parent;
-
-	if (pcg)
-		return cgroup_freezer(pcg);
-	return NULL;
+	return css_freezer(css_parent(&freezer->css));
 }
 
 bool cgroup_freezing(struct task_struct *task)
@@ -234,7 +230,7 @@ static void freezer_fork(struct task_struct *task)
 	 * The root cgroup is non-freezable, so we can skip the
 	 * following check.
 	 */
-	if (!freezer->css.cgroup->parent)
+	if (!parent_freezer(freezer))
 		goto out;
 
 	spin_lock_irq(&freezer->lock);