[PATCH V2] scsi: avoid to run synchronize_rcu for each device in scsi_host_block

[Ming Lei <ming.lei@redhat.com>]

scsi_host_block() calls scsi_internal_device_block() for each
scsi_device, and scsi_internal_device_block() calls
blk_mq_quiesce_queue() for each LUN. However synchronize_rcu is
run from blk_mq_quiesce_queue().

This way may become unnecessary slow in case of lots of LUNs.

Use scsi_internal_device_block_nowait() to implement scsi_host_block(),
so it is enough to run synchronize_rcu() once since scsi never
supports blk-mq's BLK_MQ_F_BLOCKING.

Cc: Steffen Maier <maier@linux.ibm.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
V2:
	- fix commit log as pointed by Steffen
	- add comment on BLK_MQ_F_BLOCKING as suggested by Bart

 drivers/scsi/scsi_lib.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 47835c4b4ee0..86007a523145 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -2841,11 +2841,26 @@ scsi_host_block(struct Scsi_Host *shost)
 	struct scsi_device *sdev;
 	int ret = 0;
 
+	/*
+	 * Call scsi_internal_device_block_nowait then we can avoid to
+	 * run synchronize_rcu() for each LUN
+	 */
 	shost_for_each_device(sdev, shost) {
-		ret = scsi_internal_device_block(sdev);
+		mutex_lock(&sdev->state_mutex);
+		ret = scsi_internal_device_block_nowait(sdev);
+		mutex_unlock(&sdev->state_mutex);
 		if (ret)
 			break;
 	}
+
+	/*
+	 * SCSI never enables blk-mq's BLK_MQ_F_BLOCKING, so run
+	 * synchronize_rcu() once is enough
+	 */
+	WARN_ON_ONCE(shost->tag_set.flags & BLK_MQ_F_BLOCKING);
+
+	if (!ret)
+		synchronize_rcu();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(scsi_host_block);
-- 
2.25.2