Introduction

I assume we're speaking about synchronization primitives, not atomics

1b. Issues with adding more threads

If we add more even/odd "workers" then conditional_variable::notify_one() will woke up a random thread possibly creating a deadlock if even incrementer awokes another even incrementer.

Also it is worth mentioning that increasing number of threads won't speed things up just because only one increment operation is allowed at time.

1c. How to fix

1b can be fixed in several possible ways but I would prefer to introduce separate condition variable for each group and still use notify_one() to wake up worker from "complementing group".

Simply replacing notify_one() with notify_all() should also do the trick. But it would introduce some wakeups that won't proceed to counter increment which is a waste of resources. But in theory this can speed things up as it allows multiple increments to happen without waiting on conditional variable between them.