cgroup: implement cgroup_from_id()

This will be used as a replacement for css_lookup().

There's a difference with cgroup id and css id. cgroup id starts with 0,
while css id starts with 1.

v4:
- also check if cggroup_mutex is held.
- make it an inline function.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -720,6 +720,24 @@ static inline struct cgroup* task_cgroup(struct task_struct *task,
 	return task_subsys_state(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 *cgroup_next_sibling(struct cgroup *pos);
 
 /**