I added the checking return value of xenbus_register in usbback_init().
I haven't tested, but probably ok.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@xxxxxxxxxxxxxx>

diff -r ba13757d92ce -r b40bbc96f74c drivers/xen/usbback/usbback.c
--- a/drivers/xen/usbback/usbback.c	Thu Oct 08 15:37:22 2009 +0900
+++ b/drivers/xen/usbback/usbback.c	Fri Oct 16 23:17:17 2009 +0900
@@ -1093,13 +1093,11 @@
 static int __init usbback_init(void)
 {
 	int i, mmap_pages;
+	int err = 0;
 
 	if (!is_running_on_xen())
 		return -ENODEV;
 
-	if (usbstub_init())
-		return -ENODEV;
-
 	mmap_pages = usbif_reqs * USBIF_MAX_SEGMENTS_PER_REQUEST;
 	pending_reqs = kmalloc(sizeof(pending_reqs[0]) *
 			usbif_reqs, GFP_KERNEL);
@@ -1107,8 +1105,10 @@
 			mmap_pages, GFP_KERNEL);
 	pending_pages = alloc_empty_pages_and_pagevec(mmap_pages);
 
-	if (!pending_reqs || !pending_grant_handles || !pending_pages)
-		goto out_of_memory;
+	if (!pending_reqs || !pending_grant_handles || !pending_pages) {
+		err = -ENOMEM;
+		goto out_mem;
+	}
 
 	for (i = 0; i < mmap_pages; i++)
 		pending_grant_handles[i] = USBBACK_INVALID_HANDLE;
@@ -1119,16 +1119,23 @@
 	for (i = 0; i < usbif_reqs; i++)
 		list_add_tail(&pending_reqs[i].free_list, &pending_free);
 
-	usbback_xenbus_init();
+	err = usbstub_init();
+	if (err)
+		goto out_mem;
+
+	err = usbback_xenbus_init();
+	if (err)
+		goto out_xenbus;
 
 	return 0;
 
- out_of_memory:
-	 kfree(pending_reqs);
-	 kfree(pending_grant_handles);
-	 free_empty_pages_and_pagevec(pending_pages, mmap_pages);
-	 printk("%s: out of memory\n", __FUNCTION__);
-	 return -ENOMEM;
+out_xenbus:
+	usbstub_exit();
+out_mem:
+	kfree(pending_reqs);
+	kfree(pending_grant_handles);
+	free_empty_pages_and_pagevec(pending_pages, mmap_pages);
+	return err;
 }
 
 static void __exit usbback_exit(void)
diff -r ba13757d92ce -r b40bbc96f74c drivers/xen/usbback/usbback.h
--- a/drivers/xen/usbback/usbback.h	Thu Oct 08 15:37:22 2009 +0900
+++ b/drivers/xen/usbback/usbback.h	Fri Oct 16 23:17:17 2009 +0900
@@ -145,7 +145,7 @@
 	} while (0)
 
 usbif_t *find_usbif(domid_t domid, unsigned int handle);
-void usbback_xenbus_init(void);
+int usbback_xenbus_init(void);
 void usbback_xenbus_exit(void);
 struct vusb_port_id *find_portid_by_busid(const char *busid);
 struct vusb_port_id *find_portid(const domid_t domid,
diff -r ba13757d92ce -r b40bbc96f74c drivers/xen/usbback/usbstub.c
--- a/drivers/xen/usbback/usbstub.c	Thu Oct 08 15:37:22 2009 +0900
+++ b/drivers/xen/usbback/usbstub.c	Fri Oct 16 23:17:17 2009 +0900
@@ -317,7 +317,7 @@
 	return err;
 }
 
-void __exit usbstub_exit(void)
+void usbstub_exit(void)
 {
 	driver_remove_file(&usbback_usb_driver.driver,
 				&driver_attr_port_ids);
diff -r ba13757d92ce -r b40bbc96f74c drivers/xen/usbback/xenbus.c
--- a/drivers/xen/usbback/xenbus.c	Thu Oct 08 15:37:22 2009 +0900
+++ b/drivers/xen/usbback/xenbus.c	Fri Oct 16 23:17:17 2009 +0900
@@ -327,12 +327,12 @@
 	.remove = usbback_remove,
 };
 
-void usbback_xenbus_init(void)
+int __init usbback_xenbus_init(void)
 {
-	xenbus_register_backend(&usbback_driver);
+	return xenbus_register_backend(&usbback_driver);
 }
 
-void usbback_xenbus_exit(void)
+void __exit usbback_xenbus_exit(void)
 {
 	xenbus_unregister_driver(&usbback_driver);
 }