Toilets and race conditions!

A co-worker asked me what issues multi-threading and shared memory can have. So I explained him that stuff with the lock. He wasn't quite sure whether he got it.

Me: imagine you go to the toilet. You check whether there's enough toilet paper in the stall, and it is. BUT now someone else comes in, does business and uses up all paper. CPUs can do shit very fast, can't they? Yeah and now you're sitting on the bowl, and BAMM out of paper. This wouldn't have happened if you had locked the stall, right?

Him: yeah. And with a single thread?

Me: well if you're alone at home in your appartment, there's no reason to lock the door because there's nobody to interfere.

Him: ah, I see. And if I have two threads, but no shared memory, then it is as if my wife and me are at home with each a toilet of our own, then we don't need to lock either.

Me: exactly!