Reattach device handler for multipath devices

The multipath daemon might have specified a different device_handler
than the one a device is attached to by default.
So we should try to re-attach with the user-specified device_handler
and only return an error if that fails.

Signed-off-by: Hannes Reinecke <hare@suse.de

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index 4840733..4e39966 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -592,9 +592,19 @@ static struct pgpath *parse_path(struct arg_set *as, struct path_selector *ps,
 	}
 
 	if (m->hw_handler_name) {
-		r = scsi_dh_attach(bdev_get_queue(p->path.dev->bdev),
-				   m->hw_handler_name);
+		struct request_queue *q = bdev_get_queue(p->path.dev->bdev);
+
+		r = scsi_dh_attach(q, m->hw_handler_name);
+		if (r == -EBUSY) {
+			/*
+			 * Already attached to different hw_handler,
+			 * try to reattach with correct one.
+			 */
+			scsi_dh_detach(q);
+			r = scsi_dh_attach(q, m->hw_handler_name);
+		}
 		if (r < 0) {
+			ti->error = "error attaching hardware handler";
 			dm_put_device(ti, p->path.dev);
 			goto bad;
 		}
diff --git a/drivers/scsi/device_handler/scsi_dh.c b/drivers/scsi/device_handler/scsi_dh.c
index a518f2e..f5d592f 100644
--- a/drivers/scsi/device_handler/scsi_dh.c
+++ b/drivers/scsi/device_handler/scsi_dh.c
@@ -504,12 +504,9 @@ void scsi_dh_detach(struct request_queue *q)
 	if (!sdev)
 		return;
 
-	if (sdev->scsi_dh_data) {
-		/* if sdev is not on internal list, detach */
-		scsi_dh = sdev->scsi_dh_data->scsi_dh;
-		if (!device_handler_match(scsi_dh, sdev))
-			scsi_dh_handler_detach(sdev, scsi_dh);
-	}
+	if (sdev->scsi_dh_data)
+		scsi_dh_handler_detach(sdev, sdev->scsi_dh_data->scsi_dh);
+
 	put_device(&sdev->sdev_gendev);
 }
 EXPORT_SYMBOL_GPL(scsi_dh_detach);