scsi: core: Avoid calling synchronize_rcu() for each device in scsi_host_block()

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.

Since synchronize_rcu() is called from blk_mq_quiesce_queue(), this can cause
substantial slowdowns on systems with many LUNs.

Use scsi_internal_device_block_nowait() to implement scsi_host_block() so it
is sufficient to run synchronize_rcu() once. This is safe since SCSI does not
set the BLK_MQ_F_BLOCKING flag.

[mkp: commit desc and comment tweaks]

Link: https://lore.kernel.org/r/20200423020713.332743-1-ming.lei@redhat.com
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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_lib.c

@@ -2841,11 +2841,27 @@ scsi_host_block(struct Scsi_Host *shost)
 	struct scsi_device *sdev;
 	int ret = 0;
 
+	/*
+	 * Call scsi_internal_device_block_nowait so we can avoid
+	 * calling 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 flag so
+	 * calling 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);