cgroup: change cgroup_from_id() to css_from_id()

Now we want cgroup core to always provide the css to use to the
subsystems, so change this API to css_from_id().

Uninline css_from_id(), because it's getting bigger and cgroup_css()
has been unexported.

While at it, remove the #ifdef, and shuffle the order of the args.

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

include/linux/cgroup.h

@@ -741,27 +741,11 @@ static inline struct cgroup *task_cgroup(struct task_struct *task,
 	return task_css(task, subsys_id)->cgroup;
 }
 
-/**
- * cgroup_from_id - lookup cgroup by id
- * @ss: cgroup subsys to be looked into
- * @id: the cgroup id
- *
- * Returns the cgroup if there's valid one with @id, otherwise returns NULL.
- * Should be called under rcu_read_lock().
- */
-static inline struct cgroup *cgroup_from_id(struct cgroup_subsys *ss, int id)
-{
-#ifdef CONFIG_PROVE_RCU
-	rcu_lockdep_assert(rcu_read_lock_held() ||
-			   lockdep_is_held(&cgroup_mutex),
-			   "cgroup_from_id() needs proper protection");
-#endif
-	return idr_find(&ss->root->cgroup_idr, id);
-}
-
 struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos,
 					   struct cgroup_subsys_state *parent);
 
+struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss);
+
 /**
  * css_for_each_child - iterate through children of a css
  * @pos: the css * to use as the loop cursor

kernel/cgroup.c

@@ -5717,6 +5717,28 @@ struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
 	return css ? css : ERR_PTR(-ENOENT);
 }
 
+/**
+ * css_from_id - lookup css by id
+ * @id: the cgroup id
+ * @ss: cgroup subsys to be looked into
+ *
+ * Returns the css if there's valid one with @id, otherwise returns NULL.
+ * Should be called under rcu_read_lock().
+ */
+struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
+{
+	struct cgroup *cgrp;
+
+	rcu_lockdep_assert(rcu_read_lock_held() ||
+			   lockdep_is_held(&cgroup_mutex),
+			   "css_from_id() needs proper protection");
+
+	cgrp = idr_find(&ss->root->cgroup_idr, id);
+	if (cgrp)
+		return cgroup_css(cgrp, ss->subsys_id);
+	return NULL;
+}
+
 #ifdef CONFIG_CGROUP_DEBUG
 static struct cgroup_subsys_state *
 debug_css_alloc(struct cgroup_subsys_state *parent_css)