It doesn't have to be shared memory specifically, but it has to be concurrent use of something shared even if that something is completely virtual and only exis

It doesn't have to be shared memory specifically, but it has to be concurrent use of something shared even if that something is completely virtual and only exists in your mind. Which then forces you to synchronize access to that something and deal with all those problems. It's not about message ordering though, not unless synchronization is involved.