The release function

When the reference count goes to zero...
...the object must be released.

You must define a release function:

void my_release_func(struct kobject *kobj)
{
    struct my_object *object;

    object = container_of(kobj, struct my_object, kobj);
    kfree(object);
}

Any kobject without a release function is broken
No other way to know when it is free