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.