Solution threads

Hi guys, I'm reading a deadlock concept and I didn't understand why if we want to solve a problem of race condition of multiple thread over a shared program, then we are assume that there's no threads working inside that program, we do update with the solution and then we check if threads are working over it correctly or not .. my question why we are assuming when we solve deadlock/race condition problems, that the threads/process are off and not working ..and then we are doing solution and then checking back if my solution is working over threads or not ..
what's confused me is, there might be a thread inside my program and I can do update over it although my thread is inside my program.