[oconnor663]

> A Data Race is any time when there's concurrent modification of a memory value

I do want to nail down the terminology, so help me with this scenario: Two simultaneous relaxed atomic writes to the same variable from different threads. To my understanding, this is not a data race (since this is allowed, while data races are never allowed), but it is concurrent. Do I have that right?

[tialaramex]

Well spotted. This is arguably a hole, albeit a deliberate one. In practice the main reason people do this is collecting some sort of metric, whose exact value is unimportant and which anyway isn't contemplated by the machine.

If your program tries to actually act on this data then yeah, you have successfully made your own life unnecessarily exciting and debugging your program may be difficult. I think it's fair to say you've only yourself to blame though since you had to explicitly choose this.

[oconnor663]

As an interesting example of doing something meaningful with relaxed arithmetic, Arc uses a relaxed fetch_add to increment the refcount: https://doc.rust-lang.org/src/alloc/sync.rs.html#1331-1343. Decrementing, however, uses acquire-release. Apparently shared_ptr in C++ is similar.

[tialaramex]

Tried to follow this link on my phone and the browser blew up, so I'll look when I'm home. Doubtless in both cases (Rust and C++) people who are much smarter than me have reasoned that it's correct and perhaps if I read the link I'll agree. But if you just asked me off the cuff my guess would have been this wasn't safe and so they should be Acquire-Release.