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>

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;
 

kernel/cgroup.c

@@ -4186,6 +4186,7 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
 			       struct cgroup *cgrp)
 {
 	css->cgroup = cgrp;
+	css->ss = ss;
 	css->flags = 0;
 	css->id = NULL;
 	if (cgrp == cgroup_dummy_top)