Is there a safe way to create a subprocess from a thread in python?

Question

I work with some existing code that creates several different workflows, which then, in turn, call some shell scripts with subprocess / multiprocessing. From what I saw, no, no, since mutexes can jam in the interpreter of the child process before exec () happens. Is this a real concern? If so, is there some kind of python primitive that "does the right thing" and performs fork / exec without playing with anything unsafe between them?

thanks