When I slowed down the animation, I saw what was happening: the third thread is waiting on 'addHot', and while (and only while) it is waiting, 'addCold' is bloc

When I slowed down the animation, I saw what was happening: the third thread is waiting on 'addHot', and while (and only while) it is waiting, 'addCold' is blocked - so there is no deadlock here, but is deadlock ruled out in general? More generally, does the claimed ease of modification come at the cost of expanding the amount of reasoning about concurrency that you have to do?