Heuristics for software-interrupt processing

Fundamentally, humans aren't good about thinking in terms of concurrent modifications - our brains like "first this, then that, then either this or that" sequences, and not "this and this and this in unknown order, then that and that in unknown order". A big global lock is a great way to ensure that there are no concurrent modifications; even better if it's implicit and thus compile-time, rather than something you have to remember to lock before you do your work.