From: Denis Bychkov <manover@gmail.com>

Allows to use register, not register_quiet in udev to avoid "device_busy" error.
The initial patch proposed at https://lkml.org/lkml/2013/8/26/549 by Gabriel de Perthuis 
<g2p.code@gmail.com> does not unlock the mutex and hangs the kernel.

See http://thread.gmane.org/gmane.linux.kernel.bcache.devel/2594 for the discussion.
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1964,6 +1964,8 @@
 			else
 				err = "device busy";
 			mutex_unlock(&bch_register_lock);
+			if (attr == &ksysfs_register_quiet)
+				goto out;
 		}
 		goto err;
 	}
@@ -2002,8 +2004,7 @@
 err_close:
 	blkdev_put(bdev, FMODE_READ|FMODE_WRITE|FMODE_EXCL);
 err:
-	if (attr != &ksysfs_register_quiet)
-		pr_info("error opening %s: %s", path, err);
+	pr_info("error opening %s: %s", path, err);
 	ret = -EINVAL;
 	goto out;
 }