kunit: device: Unregister the kunit_bus on shutdown

If KUnit is built as a module, and it's unloaded, the kunit_bus is not
unregistered. This causes an error if it's then re-loaded later, as we
try to re-register the bus.

Unregister the bus and root_device on shutdown, if it looks valid.

In addition, be more specific about the value of kunit_bus_device. It
is:
- a valid struct device* if the kunit_bus initialised correctly.
- an ERR_PTR if it failed to initialise.
- NULL before initialisation and after shutdown.

Fixes: d03c720e03 ("kunit: Add APIs for managing devices")
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

lib/kunit/device-impl.h

@@ -13,5 +13,7 @@
 
 // For internal use only -- registers the kunit_bus.
 int kunit_bus_init(void);
+// For internal use only -- unregisters the kunit_bus.
+void kunit_bus_shutdown(void);
 
 #endif //_KUNIT_DEVICE_IMPL_H

lib/kunit/device.c

@@ -54,6 +54,20 @@ int kunit_bus_init(void)
 	return error;
 }
 
+/* Unregister the 'kunit_bus' in case the KUnit module is unloaded. */
+void kunit_bus_shutdown(void)
+{
+	/* Make sure the bus exists before we unregister it. */
+	if (IS_ERR_OR_NULL(kunit_bus_device))
+		return;
+
+	bus_unregister(&kunit_bus_type);
+
+	root_device_unregister(kunit_bus_device);
+
+	kunit_bus_device = NULL;
+}
+
 /* Release a 'fake' KUnit device. */
 static void kunit_device_release(struct device *d)
 {

lib/kunit/test.c

@@ -928,6 +928,9 @@ static void __exit kunit_exit(void)
 #ifdef CONFIG_MODULES
 	unregister_module_notifier(&kunit_mod_nb);
 #endif
+
+	kunit_bus_shutdown();
+
 	kunit_debugfs_cleanup();
 }
 module_exit(kunit_exit);