blkio: Initialize blkg->stats_lock for the root cfqg too

This fixes the lockdep warning reported by Gui Jianfeng.
Signed-off-by: Divyesh Shah <dpshah@google.com>
Reviewed-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-cgroup.c

@@ -86,12 +86,6 @@ struct blkio_cgroup *cgroup_to_blkio_cgroup(struct cgroup *cgroup)
 }
 EXPORT_SYMBOL_GPL(cgroup_to_blkio_cgroup);
 
-void blkio_group_init(struct blkio_group *blkg)
-{
-	spin_lock_init(&blkg->stats_lock);
-}
-EXPORT_SYMBOL_GPL(blkio_group_init);
-
 /*
  * Add to the appropriate stat variable depending on the request type.
  * This should be called with the blkg->stats_lock held.
@@ -349,6 +343,7 @@ void blkiocg_add_blkio_group(struct blkio_cgroup *blkcg,
 	unsigned long flags;
 
 	spin_lock_irqsave(&blkcg->lock, flags);
+	spin_lock_init(&blkg->stats_lock);
 	rcu_assign_pointer(blkg->key, key);
 	blkg->blkcg_id = css_id(&blkcg->css);
 	hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);

block/blk-cgroup.h

@@ -217,7 +217,6 @@ extern void blkiocg_add_blkio_group(struct blkio_cgroup *blkcg,
 extern int blkiocg_del_blkio_group(struct blkio_group *blkg);
 extern struct blkio_group *blkiocg_lookup_group(struct blkio_cgroup *blkcg,
 						void *key);
-void blkio_group_init(struct blkio_group *blkg);
 void blkiocg_update_timeslice_used(struct blkio_group *blkg,
 					unsigned long time);
 void blkiocg_update_dispatch_stats(struct blkio_group *blkg, uint64_t bytes,
@@ -235,7 +234,6 @@ struct cgroup;
 static inline struct blkio_cgroup *
 cgroup_to_blkio_cgroup(struct cgroup *cgroup) { return NULL; }
 
-static inline void blkio_group_init(struct blkio_group *blkg) {}
 static inline void blkiocg_add_blkio_group(struct blkio_cgroup *blkcg,
 			struct blkio_group *blkg, void *key, dev_t dev) {}
 

block/cfq-iosched.c

@@ -961,7 +961,6 @@ cfq_find_alloc_cfqg(struct cfq_data *cfqd, struct cgroup *cgroup, int create)
 	for_each_cfqg_st(cfqg, i, j, st)
 		*st = CFQ_RB_ROOT;
 	RB_CLEAR_NODE(&cfqg->rb_node);
-	blkio_group_init(&cfqg->blkg);
 
 	/*
 	 * Take the initial reference that will be released on destroy