The source code for the Process class is here: https://github.com/python/cpython/blob/master/Lib/multiprocessing/process.py

You can see that join and start both modify the global, non thread safe _children set. I'm guessing this is where you're seeing interference between threads.

I'm not sure a lock on start is enough - I think you need to get the lock for the join_process(job, deadline) call as well, because join can modify _children too. (Or, alternatively, manage all processes from a single thread.)