xs_init() registers xs_reboot_nb before initializing XenStore
communications and starting xenwatch. If either operation fails, the
notifier remains registered and a later initialization attempt can hit a
duplicate registration.

Check the notifier registration result and unregister it on every
subsequent failure path.

Fixes: fd8aa9095a95 ("xen: optimize xenbus driver for multiple concurrent xenstore accesses")
Signed-off-by: Yuho Choi <dbgh9129@gmail.com>
---
 drivers/xen/xenbus/xenbus_xs.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index d1cca4acb6f3..62274d377b3a 100644
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -915,19 +915,27 @@ int xs_init(void)
 	int err;
 	struct task_struct *task;
 
-	register_reboot_notifier(&xs_reboot_nb);
+	err = register_reboot_notifier(&xs_reboot_nb);
+	if (err)
+		return err;
 
 	/* Initialize the shared memory rings to talk to xenstored */
 	err = xb_init_comms();
 	if (err)
-		return err;
+		goto err_unregister_reboot_notifier;
 
 	task = kthread_run(xenwatch_thread, NULL, "xenwatch");
-	if (IS_ERR(task))
-		return PTR_ERR(task);
+	if (IS_ERR(task)) {
+		err = PTR_ERR(task);
+		goto err_unregister_reboot_notifier;
+	}
 
 	/* shutdown watches for kexec boot */
 	xs_reset_watches();
 
 	return 0;
+
+err_unregister_reboot_notifier:
+	unregister_reboot_notifier(&xs_reboot_nb);
+	return err;
 }
-- 
2.43.0