From: Yu Kuai <yukuai3@huawei.com>

Changes to two step:

1) hold rcu lock and do blkg_lookup() from fast path;
2) hold queue_lock directly from slow path, and don't nest it under rcu
   lock;

Prepare to convert protecting blkcg with blkcg_mutex instead of
queue_lock;

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
 block/blk-cgroup.c | 57 +++++++++++++++++++++++++++++-----------------
 1 file changed, 36 insertions(+), 21 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 2d767ae61d2f..9ffc3195f7e4 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -467,22 +467,17 @@ static struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
 {
 	struct request_queue *q = disk->queue;
 	struct blkcg_gq *blkg;
-	unsigned long flags;
-
-	WARN_ON_ONCE(!rcu_read_lock_held());
 
-	blkg = blkg_lookup(blkcg, q);
-	if (blkg)
-		return blkg;
-
-	spin_lock_irqsave(&q->queue_lock, flags);
+	rcu_read_lock();
 	blkg = blkg_lookup(blkcg, q);
 	if (blkg) {
 		if (blkcg != &blkcg_root &&
 		    blkg != rcu_dereference(blkcg->blkg_hint))
 			rcu_assign_pointer(blkcg->blkg_hint, blkg);
-		goto found;
+		rcu_read_unlock();
+		return blkg;
 	}
+	rcu_read_unlock();
 
 	/*
 	 * Create blkgs walking down from blkcg_root to @blkcg, so that all
@@ -514,8 +509,6 @@ static struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
 			break;
 	}
 
-found:
-	spin_unlock_irqrestore(&q->queue_lock, flags);
 	return blkg;
 }
 
@@ -2077,6 +2070,18 @@ void blkcg_add_delay(struct blkcg_gq *blkg, u64 now, u64 delta)
 	atomic64_add(delta, &blkg->delay_nsec);
 }
 
+static inline struct blkcg_gq *blkg_lookup_tryget(struct blkcg_gq *blkg)
+{
+retry:
+	if (blkg_tryget(blkg))
+		return blkg;
+
+	blkg = blkg->parent;
+	if (blkg)
+		goto retry;
+
+	return NULL;
+}
 /**
  * blkg_tryget_closest - try and get a blkg ref on the closet blkg
  * @bio: target bio
@@ -2089,20 +2094,30 @@ void blkcg_add_delay(struct blkcg_gq *blkg, u64 now, u64 delta)
 static inline struct blkcg_gq *blkg_tryget_closest(struct bio *bio,
 		struct cgroup_subsys_state *css)
 {
-	struct blkcg_gq *blkg, *ret_blkg = NULL;
+	struct request_queue *q = bio->bi_bdev->bd_queue;
+	struct blkcg *blkcg = css_to_blkcg(css);
+	struct blkcg_gq *blkg;
 
 	rcu_read_lock();
-	blkg = blkg_lookup_create(css_to_blkcg(css), bio->bi_bdev->bd_disk);
-	while (blkg) {
-		if (blkg_tryget(blkg)) {
-			ret_blkg = blkg;
-			break;
-		}
-		blkg = blkg->parent;
-	}
+	blkg = blkg_lookup(blkcg, q);
+	if (likely(blkg))
+		blkg = blkg_lookup_tryget(blkg);
 	rcu_read_unlock();
 
-	return ret_blkg;
+	if (blkg)
+		return blkg;
+
+	/*
+	 * Fast path failed, we're probably issuing IO in this cgroup the first
+	 * time, hold lock to create new blkg.
+	 */
+	spin_lock_irq(&q->queue_lock);
+	blkg = blkg_lookup_create(blkcg, bio->bi_bdev->bd_disk);
+	if (blkg)
+		blkg = blkg_lookup_tryget(blkg);
+	spin_unlock_irq(&q->queue_lock);
+
+	return blkg;
 }
 
 /**
-- 
2.39.2