deadlock

deadlock is when mutexes lock in a circular order:

thread 1:

m1.lock();
m2.lock();

thread 2:

m2.lock();
m3.lock();

We prevent this by locking things in the same order. Which maybe hard: because loops.

We need, also, to limit the number of threads competing for a shared resource: imagine all of your threads doing a thing, will it deadlock? If so, limit.