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0 pointsvitalyd8y ago0 comments

Note that I didn't mention anything about it being automatic. It's merely a case of being more explicit about what may happen to the value.

Sequential consistency is irrelevant to the example I gave of a single valued struct. I wasn't talking about any ordering with respect other loads/stores.

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dbaupp8y ago

Being explicit isn't relevant: a relaxed store to an atomic followed by publishing a pointer to it won't have the expected happens-before relationship (program order won't be respected).

In any case, the only reason to think about publishing is how loads and stores are ordered, so it isn't irrelevant. The single valued struct can still have the partial initialisation problem, it just appears as the single field being completely uninitialised.

vitalydOP8y ago

That's all true, but I think we're talking past each other a bit. My point isn't about what mechanics would be used or the type of memory order specified (that's the irrelevant part), but merely to highlight that this could use more attention in the docs. The atomic used may be viewed as a "lint" to a reader/user that something interesting may be going on with how this type is used - similar to how *mut isn't automatically Sync; it's an opportunity for people to pause and think things through. If you're looking at a type absent of any atomics or concurrency primitives, it's not apparent that it may be used like that (particularly since Sync is auto-derived and doesn't appear in the source code of the type).

I know one can look at this as being unimportant because only unsafe code that works with Sync types carries the burden of upholding the invariants. But, I still contend that memory ordering ought to be discussed, somewhere, while there's no official memory model documentation (not even sure that's in the plans, although I feel like I may have come across a github issue for that).

Anyway, I don't think I'm saying anything new in this reply over my others. If you feel existing docs are adequate in this regard, that's fine - we can agree to disagree :).