blk-cgroup: move rcu locking from blkcg_bio_issue_check to blk_throtl_bio

The only thing in blkcg_bio_issue_check that needs to be under
rcu_read_lock is blk_throtl_bio, so move the locking there.
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.c

@@ -2168,7 +2168,7 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 	bool throttled = false;
 	struct throtl_data *td = tg->td;
 
-	WARN_ON_ONCE(!rcu_read_lock_held());
+	rcu_read_lock();
 
 	/* see throtl_charge_bio() */
 	if (bio_flagged(bio, BIO_THROTTLED))
@@ -2273,6 +2273,7 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
 	if (throttled || !td->track_bio_latency)
 		bio->bi_issue.value |= BIO_ISSUE_THROTL_SKIP_LATENCY;
 #endif
+	rcu_read_unlock();
 	return throttled;
 }
 

include/linux/blk-cgroup.h

@@ -546,7 +546,6 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 	struct blkcg_gq *blkg = bio->bi_blkg;
 	bool throtl = false;
 
-	rcu_read_lock();
 	throtl = blk_throtl_bio(q, blkg, bio);
 	if (!throtl) {
 		struct blkg_iostat_set *bis;
@@ -582,7 +581,6 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 
 	blkcg_bio_issue_init(bio);
 
-	rcu_read_unlock();
 	return !throtl;
 }