Memory Ordering and Atomics: When Threads Disagree on Reality

You write `flag = true` on one thread and `if (flag)` on another. The second thread doesn't see the new value -- not because of a missed update, but because the CPU and the compiler are both allowed to reorder your code. Memory ordering is the set of rules that lets concurrent code mean something. The day you ship a bug that 'works on x86 and breaks on ARM' is the day you wish you had this article.