[dataflow]

Somewhat off topic, but what is a realistic example of where you need atomics with sequential consistency? Like, what useful data structure or pattern requires it? I feel like I've seen every other ordering except that one (and consume) in real world code.

[judofyr]

A mutex would be the most trivial example. I don't believe that is possible to implement, in the general case, with only acquire-release.

Sequential consistency mostly become relevant when you have more than two threads interacting with both reads and writes. However, if you only have single-consumer (i.e. only one thread reading) or single-producer (i.e. only one thread writing) then the acquire-release semantics ends up becoming sequential since the single-consumer/producer implicitly enforces a sequential ordering. I can potentially see some multi-producer multi-consumer queues lock-free queues needing sequential atomics.

I think it's rare to see atomics with sequential consistency in practice since you typically either choose (1) a mutex to simplify the code at the expense of locking or (2) acquire-release (or weaker) to minimize the synchronization.

[dataflow]

> A mutex would be the most trivial example. I don't believe that is possible to implement, in the general case, with only acquire-release.

Wait, what? So you're saying this spinlock is buggy? What's the bug?

https://en.cppreference.com/w/cpp/atomic/atomic_flag.html

[judofyr]

No, sorry. I was just remembering where I've typically seen sequential consistency being used. For instance, Peterson's algorithm was what I had in mind. Spinlock is indeed a good example (although a terrible algorithm which I hope you haven't seen used in practice) of a mutex algorithm which only requires acquire-release.