[PATCH 4/7] blkcg: refactor blkcg_gq lookup and creation in blkcg_bio_issue_check()

[Tejun Heo]

blkcg_bio_issue_check() open codes blkcg_gq lookup and creation using
blkg_lookup() and blkg_lookup_create().  Refactor the code so that

* blkg_lookup_create() is renamed to blkcg_lookup_create_locked().

* blkg_lookup_create() is now a new function which encapsulates the
  RCU protected lookup and queue_locked protected creation.

* blkg_lookup_create() is guaranteed to return a non-NULL blkcg_gq.
  The NULL checks are removed from the users.

This is pure refactoring and doesn't introduce any functional changes.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 block/blk-cgroup.c         |  6 +++---
 block/blk-throttle.c       |  2 +-
 include/linux/blk-cgroup.h | 32 +++++++++++++++++++++-----------
 3 files changed, 25 insertions(+), 15 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 60a4486..490b0a6 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -290,7 +290,7 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
 }
 
 /**
- * blkg_lookup_create - lookup blkg, try to create one if not there
+ * blkg_lookup_create_locked - lookup blkg, try to create one if not there
  * @blkcg: blkcg of interest
  * @q: request_queue of interest
  *
@@ -303,8 +303,8 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
  * value on error.  If @q is dead, returns ERR_PTR(-EINVAL).  If @q is not
  * dead and bypassing, returns ERR_PTR(-EBUSY).
  */
-struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
-				    struct request_queue *q)
+struct blkcg_gq *blkg_lookup_create_locked(struct blkcg *blkcg,
+					   struct request_queue *q)
 {
 	struct blkcg_gq *blkg;
 
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 8631763..1e6916b 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -2123,7 +2123,7 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 		    struct bio *bio)
 {
 	struct throtl_qnode *qn = NULL;
-	struct throtl_grp *tg = blkg_to_tg(blkg ?: q->root_blkg);
+	struct throtl_grp *tg = blkg_to_tg(blkg);
 	struct throtl_service_queue *sq;
 	bool rw = bio_data_dir(bio);
 	bool throttled = false;
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index b5721d32..1de5158 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -172,8 +172,8 @@ extern struct cgroup_subsys_state * const blkcg_root_css;
 
 struct blkcg_gq *blkg_lookup_slowpath(struct blkcg *blkcg,
 				      struct request_queue *q, bool update_hint);
-struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
-				    struct request_queue *q);
+struct blkcg_gq *blkg_lookup_create_locked(struct blkcg *blkcg,
+					   struct request_queue *q);
 int blkcg_init_queue(struct request_queue *q);
 void blkcg_drain_queue(struct request_queue *q);
 void blkcg_exit_queue(struct request_queue *q);
@@ -680,6 +680,24 @@ static inline bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg
 				  struct bio *bio) { return false; }
 #endif
 
+static inline struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
+						  struct request_queue *q)
+{
+	struct blkcg_gq *blkg;
+
+	blkg = blkg_lookup(blkcg, q);
+	if (likely(blkg))
+		return blkg;
+
+	spin_lock_irq(q->queue_lock);
+	blkg = blkg_lookup_create_locked(blkcg, q);
+	spin_unlock_irq(q->queue_lock);
+	if (likely(!IS_ERR(blkg)))
+		return blkg;
+	else
+		return q->root_blkg;
+}
+
 static inline bool blkcg_bio_issue_check(struct request_queue *q,
 					 struct bio *bio)
 {
@@ -693,19 +711,11 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 	/* associate blkcg if bio hasn't attached one */
 	bio_associate_blkcg(bio, &blkcg->css);
 
-	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);
 
 	if (!throtl) {
-		blkg = blkg ?: q->root_blkg;
 		blkg_rwstat_add(&blkg->stat_bytes, bio->bi_opf,
 				bio->bi_iter.bi_size);
 		blkg_rwstat_add(&blkg->stat_ios, bio->bi_opf, 1);
-- 
2.9.5