blkcg: update blkg_lookup_create() to do locking

To know when to create a blkg, the general pattern is to do a
blkg_lookup() and if that fails, lock and do the lookup again, and if
that fails finally create. It doesn't make much sense for everyone who
wants to do creation to write this themselves.

This changes blkg_lookup_create() to do locking and implement this
pattern. The old blkg_lookup_create() is renamed to
__blkg_lookup_create().  If a call site wants to do its own error
handling or already owns the queue lock, they can use
__blkg_lookup_create(). This will be used in upcoming patches.
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -249,7 +249,7 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
 }
 
 /**
- * blkg_lookup_create - lookup blkg, try to create one if not there
+ * __blkg_lookup_create - lookup blkg, try to create one if not there
  * @blkcg: blkcg of interest
  * @q: request_queue of interest
  *
@@ -262,8 +262,8 @@ 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(struct blkcg *blkcg,
+				      struct request_queue *q)
 {
 	struct blkcg_gq *blkg;
 
@@ -293,6 +293,28 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
 	}
 }
 
+/**
+ * blkg_lookup_create - find or create a blkg
+ * @blkcg: target block cgroup
+ * @q: target request_queue
+ *
+ * This looks up or creates the blkg representing the unique pair
+ * of the blkcg and the request_queue.
+ */
+struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
+				    struct request_queue *q)
+{
+	struct blkcg_gq *blkg = blkg_lookup(blkcg, q);
+
+	if (unlikely(!blkg)) {
+		spin_lock_irq(&q->queue_lock);
+		blkg = __blkg_lookup_create(blkcg, q);
+		spin_unlock_irq(&q->queue_lock);
+	}
+
+	return blkg;
+}
+
 static void blkg_destroy(struct blkcg_gq *blkg)
 {
 	struct blkcg *blkcg = blkg->blkcg;

block/blk-iolatency.c

@@ -486,7 +486,7 @@ static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio)
 	blkg = blkg_lookup(blkcg, q);
 	if (unlikely(!blkg)) {
 		spin_lock_irq(&q->queue_lock);
-		blkg = blkg_lookup_create(blkcg, q);
+		blkg = __blkg_lookup_create(blkcg, q);
 		if (IS_ERR(blkg))
 			blkg = NULL;
 		spin_unlock_irq(&q->queue_lock);

include/linux/blk-cgroup.h

@@ -181,6 +181,8 @@ 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(struct blkcg *blkcg,
 				    struct request_queue *q);
 int blkcg_init_queue(struct request_queue *q);
@@ -799,7 +801,7 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 	blkg = blkg_lookup(blkcg, q);
 	if (unlikely(!blkg)) {
 		spin_lock_irq(&q->queue_lock);
-		blkg = blkg_lookup_create(blkcg, q);
+		blkg = __blkg_lookup_create(blkcg, q);
 		if (IS_ERR(blkg))
 			blkg = NULL;
 		spin_unlock_irq(&q->queue_lock);