[SCSI] avoid taking host_lock in scsi_run_queue unless nessecary

If we don't have starved devices we don't need to take the host lock
to iterate over them.  Also split the function up to be more clear.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/scsi_lib.c

@@ -385,29 +385,12 @@ static inline int scsi_host_is_busy(struct Scsi_Host *shost)
 	return 0;
 }
 
-/*
- * Function:	scsi_run_queue()
- *
- * Purpose:	Select a proper request queue to serve next
- *
- * Arguments:	q	- last request's queue
- *
- * Returns:     Nothing
- *
- * Notes:	The previous command was completely finished, start
- *		a new one if possible.
- */
-static void scsi_run_queue(struct request_queue *q)
+static void scsi_starved_list_run(struct Scsi_Host *shost)
 {
-	struct scsi_device *sdev = q->queuedata;
-	struct Scsi_Host *shost;
 	LIST_HEAD(starved_list);
+	struct scsi_device *sdev;
 	unsigned long flags;
 
-	shost = sdev->host;
-	if (scsi_target(sdev)->single_lun)
-		scsi_single_lun_run(sdev);
-
 	spin_lock_irqsave(shost->host_lock, flags);
 	list_splice_init(&shost->starved_list, &starved_list);
 
@@ -459,6 +442,28 @@ static void scsi_run_queue(struct request_queue *q)
 	/* put any unprocessed entries back */
 	list_splice(&starved_list, &shost->starved_list);
 	spin_unlock_irqrestore(shost->host_lock, flags);
+}
+
+/*
+ * Function:   scsi_run_queue()
+ *
+ * Purpose:    Select a proper request queue to serve next
+ *
+ * Arguments:  q       - last request's queue
+ *
+ * Returns:     Nothing
+ *
+ * Notes:      The previous command was completely finished, start
+ *             a new one if possible.
+ */
+static void scsi_run_queue(struct request_queue *q)
+{
+	struct scsi_device *sdev = q->queuedata;
+
+	if (scsi_target(sdev)->single_lun)
+		scsi_single_lun_run(sdev);
+	if (!list_empty(&sdev->host->starved_list))
+		scsi_starved_list_run(sdev->host);
 
 	blk_run_queue(q);
 }