It is very difficult to successfully control access to a shared resource. All users of the shared resource must agree to use the same synchronization mechanism. The wider the scope of visibility of the shared resource, the more difficult it is to control the access. For example, it is quite easy to have thread-safe access to a local all-thread static variable, since only that procedure can access it. It is very difficult to have thread-safe access to an exported all-thread static variable, since any procedure can access it.