cgroup: add subsystem pointer to cgroup_subsys_state

Currently, given a cgroup_subsys_state, there's no way to find out
which subsystem the css is for, which we'll need to convert the cgroup
controller API to primarily use @css instead of @cgroup.  This patch
adds cgroup_subsys_state->ss which points to the subsystem the @css
belongs to.

While at it, remove the comment about accessing @css->cgroup to
determine the hierarchy.  cgroup core will provide API to traverse
hierarchy of css'es and we don't want subsystems to directly walk
cgroup hierarchies anymore.

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

1 files changed, 4 insertions, 5 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 552c5feef733..821678aae4db 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -66,13 +66,12 @@ enum cgroup_subsys_id {
 
 /* Per-subsystem/per-cgroup state maintained by the system. */
 struct cgroup_subsys_state {
-        /*
-         * The cgroup that this subsystem is attached to. Useful
-         * for subsystems that want to know about the cgroup
-         * hierarchy structure
-         */
+        /* the cgroup that this css is attached to */
         struct cgroup *cgroup;
 
+        /* the cgroup subsystem that this css is attached to */
+        struct cgroup_subsys *ss;
+
         /* reference count - access via css_[try]get() and css_put() */
         struct percpu_ref refcnt;