cgroup: add cgroup->dummy_css

cgroup subsystem API is being converted to use css
(cgroup_subsys_state) as the main handle, which makes things a bit
awkward for subsystem agnostic core features - the "cgroup.*"
interface files and various iterations - a bit awkward as they don't
have a css to use.

This patch adds cgroup->dummy_css which has NULL ->ss and whose only
role is pointing back to the cgroup.  This will be used to support
subsystem agnostic features on the coming css based API.

css_parent() is updated to handle dummy_css's.  Note that css will
soon grow its own ->parent field and css_parent() will be made
trivial.

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

1 files changed, 10 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 5db8138a048..b0d5f53ae5e 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -225,6 +225,9 @@ struct cgroup {
 	struct list_head pidlists;
 	struct mutex pidlist_mutex;
 
+	/* dummy css with NULL ->ss, points back to this cgroup */
+	struct cgroup_subsys_state dummy_css;
+
 	/* For css percpu_ref killing and RCU-protected deletion */
 	struct rcu_head rcu_head;
 	struct work_struct destroy_work;
@@ -668,7 +671,13 @@ struct cgroup_subsys_state *css_parent(struct cgroup_subsys_state *css)
 {
 	struct cgroup *parent_cgrp = css->cgroup->parent;
 
-	return parent_cgrp ? parent_cgrp->subsys[css->ss->subsys_id] : NULL;
+	if (!parent_cgrp)
+		return NULL;
+
+	if (css->ss)
+		return parent_cgrp->subsys[css->ss->subsys_id];
+	else
+		return &parent_cgrp->dummy_css;
 }
 
 /**