What every systems programmer should know about concurrency

Seasoned programmers are familiar with tools like mutexes, semaphores, and condition variables. But what makes them work? How do we write concurrent code when we can’t use them, like when we’re working below the operating system in an embedded environment, or when we can’t block due to hard time constraints? And since your system transforms your code into things you didn’t write, running in orders you never asked for, how do multithreaded programs work at all? Concurrency — especially on modern hardware — is a complicated and unintuitive topic, but let’s try to cover some fundamentals.