blkcg: convert blkg_lookup_create() to find closest blkg

There are several scenarios where blkg_lookup_create() can fail such as
the blkcg dying, request_queue is dying, or simply being OOM. Most
handle this by simply falling back to the q->root_blkg and calling it a
day.

This patch implements the notion of closest blkg. During
blkg_lookup_create(), if it fails to create, return the closest blkg
found or the q->root_blkg. blkg_try_get_closest() is introduced and used
during association so a bio is always attached to a blkg.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 15 insertions, 9 deletions

diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index b3b1a8187d23..c08e96e521ed 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -545,6 +545,20 @@ static inline struct blkcg_gq *blkg_try_get(struct blkcg_gq *blkg)
 	return NULL;
 }
 
+/**
+ * blkg_try_get_closest - try and get a blkg ref on the closet blkg
+ * @blkg: blkg to get
+ *
+ * This walks up the blkg tree to find the closest non-dying blkg and returns
+ * the blkg that it did association with as it may not be the passed in blkg.
+ */
+static inline struct blkcg_gq *blkg_try_get_closest(struct blkcg_gq *blkg)
+{
+	while (!atomic_inc_not_zero(&blkg->refcnt))
+		blkg = blkg->parent;
+
+	return blkg;
+}
 
 void __blkg_release_rcu(struct rcu_head *rcu);
 
@@ -797,15 +811,7 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 	/* associate blkcg if bio hasn't attached one */
 	bio_associate_blkcg(bio, NULL);
 	blkcg = bio_blkcg(bio);
-
-	blkg = blkg_lookup(blkcg, q);
-	if (unlikely(!blkg)) {
-		spin_lock_irq(&q->queue_lock);
-		blkg = __blkg_lookup_create(blkcg, q);
-		if (IS_ERR(blkg))
-			blkg = NULL;
-		spin_unlock_irq(&q->queue_lock);
-	}
+	blkg = blkg_lookup_create(blkcg, q);
 
 	throtl = blk_throtl_bio(q, blkg, bio);