Moravian Kaleidoscopy Ltd🇪🇺🇺🇦Apr 10

Edit: solved, thanks Paul!

Any bored #RustLang atomics enthusiast around? I'm trying to write a kind of global unique id allocator using fetch_add and compare_exchange but now I'm not even sure if it's possible with the right orderings or if I should just do the simple thing and wrap it in a mutex ...

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=bb48f6352ffff02df49c2e12d98655f1

#rust #atomics

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Show threadMichał FitaApr 10

@algebraicterror `fetch_add()` wraps itself, so you don't need any extra work.

However, if you need to catch the situation, the `compare_exchange()` is ANOTHER atomic operation, and the `fetch_add()` from another thread could happen already, so you're going to compare not what you expect.

Rather something like this would work:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=237b0c34607a226badc4f7f3d4cc2fac

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Show threadMichał Fita
@algebraicterror I've just realised, the second variable in this case is probably redundant, as every thread can load it's prev_value first, worst case you'll get two warnings.
Apr 10 at 11:07amWeb
Show threadMoravian Kaleidoscopy Ltd🇪🇺🇺🇦Apr 10
@michalfita I don't actually need to know when the wrap happens, but I can imagine circumstances when it's needed. Thanks!