block: factor out a blk_try_enter_queue helper

Factor out the code to try to get q_usage_counter without blocking into
a separate helper.  Both to improve code readability and to prepare for
splitting bio_queue_enter from blk_queue_enter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20210929071241.934472-3-hch@lst.de
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

block/blk-core.c

@@ -416,6 +416,30 @@ void blk_cleanup_queue(struct request_queue *q)
 }
 EXPORT_SYMBOL(blk_cleanup_queue);
 
+static bool blk_try_enter_queue(struct request_queue *q, bool pm)
+{
+	rcu_read_lock();
+	if (!percpu_ref_tryget_live(&q->q_usage_counter))
+		goto fail;
+
+	/*
+	 * The code that increments the pm_only counter must ensure that the
+	 * counter is globally visible before the queue is unfrozen.
+	 */
+	if (blk_queue_pm_only(q) &&
+	    (!pm || queue_rpm_status(q) == RPM_SUSPENDED))
+		goto fail_put;
+
+	rcu_read_unlock();
+	return true;
+
+fail_put:
+	percpu_ref_put(&q->q_usage_counter);
+fail:
+	rcu_read_unlock();
+	return false;
+}
+
 /**
  * blk_queue_enter() - try to increase q->q_usage_counter
  * @q: request queue pointer
@@ -425,47 +449,27 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
 {
 	const bool pm = flags & BLK_MQ_REQ_PM;
 
-	while (true) {
-		bool success = false;
-
-		rcu_read_lock();
-		if (percpu_ref_tryget_live(&q->q_usage_counter)) {
-			/*
-			 * The code that increments the pm_only counter is
-			 * responsible for ensuring that that counter is
-			 * globally visible before the queue is unfrozen.
-			 */
-			if ((pm && queue_rpm_status(q) != RPM_SUSPENDED) ||
-			    !blk_queue_pm_only(q)) {
-				success = true;
-			} else {
-				percpu_ref_put(&q->q_usage_counter);
-			}
-		}
-		rcu_read_unlock();
-
-		if (success)
-			return 0;
-
+	while (!blk_try_enter_queue(q, pm)) {
 		if (flags & BLK_MQ_REQ_NOWAIT)
 			return -EBUSY;
 
 		/*
-		 * read pair of barrier in blk_freeze_queue_start(),
-		 * we need to order reading __PERCPU_REF_DEAD flag of
-		 * .q_usage_counter and reading .mq_freeze_depth or
-		 * queue dying flag, otherwise the following wait may
-		 * never return if the two reads are reordered.
+		 * read pair of barrier in blk_freeze_queue_start(), we need to
+		 * order reading __PERCPU_REF_DEAD flag of .q_usage_counter and
+		 * reading .mq_freeze_depth or queue dying flag, otherwise the
+		 * following wait may never return if the two reads are
+		 * reordered.
 		 */
 		smp_rmb();
-
 		wait_event(q->mq_freeze_wq,
 			   (!q->mq_freeze_depth &&
 			    blk_pm_resume_queue(pm, q)) ||
 			   blk_queue_dying(q));
 		if (blk_queue_dying(q))
 			return -ENODEV;
 	}
+
+	return 0;
 }
 
 static inline int bio_queue_enter(struct bio *bio)