[SCSI] avoid useless free_list lock roundtrips

Avoid hitting the host-wide free_list lock unless we need to put a command
back onto the freelist.

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.c

@@ -320,13 +320,14 @@ void __scsi_put_command(struct Scsi_Host *shost, struct scsi_cmnd *cmd,
 {
 	unsigned long flags;
 
-	/* changing locks here, don't need to restore the irq state */
-	spin_lock_irqsave(&shost->free_list_lock, flags);
 	if (unlikely(list_empty(&shost->free_list))) {
-		list_add(&cmd->list, &shost->free_list);
-		cmd = NULL;
+		spin_lock_irqsave(&shost->free_list_lock, flags);
+		if (list_empty(&shost->free_list)) {
+			list_add(&cmd->list, &shost->free_list);
+			cmd = NULL;
+		}
+		spin_unlock_irqrestore(&shost->free_list_lock, flags);
 	}
-	spin_unlock_irqrestore(&shost->free_list_lock, flags);
 
 	if (likely(cmd != NULL))
 		scsi_pool_free_command(shost->cmd_pool, cmd);