[PATCH] scsi_unregister() oops fix

Some drivers such as aha1542 and aic7xxx_old will call scsi_register() and
then, if some succeeding operations fails they will call scsi_unregister(),
without an intervening scsi_set_host().

This causes an oops in scsi_put_device(), because kobj->parent is NULL.

In other words, scsi_register() immediately followed by scsi_unregister()
is guaranteed to oops.

The patch makes scsi_host_dev_release() more robust against this usage
pattern.

drivers/scsi/hosts.c

@@ -158,7 +158,13 @@ static void scsi_host_dev_release(struct device *dev)
 	scsi_proc_hostdir_rm(shost->hostt);
 	scsi_destroy_command_freelist(shost);
 
-	put_device(parent);
+	/*
+	 * Some drivers (eg aha1542) do scsi_register()/scsi_unregister()
+	 * during probing without performing a scsi_set_device() in between.
+	 * In this case dev->parent is NULL.
+	 */
+	if (parent)
+		put_device(parent);
 	kfree(shost);
 }