[atomics.order] Clarify whether p11 makes demands towards loads, or just stores

[eisenwave]

The famous [atomics.order] p11 states:

Implementations should make atomic stores visible to atomic loads within a reasonable amount of time.

I've recently argued about whether this paragraph makes demands towards atomic loads, or just stores. For example, we could have:

std::atomic_bool should_wait;
// ...
while (should_wait.load()) { }

Relaxed Reading

We can interpret the paragraph as:

Implementations should make atomic stores, which will be used by atomic loads visible within a reasonable amount of time.

No demand is made towards x.load(), so the following optimization is permissible:

bool b = should_wait.load();
while (b) { } // note: undefined behavior for b == false
              //       because no forward progress is made

Due to this UB, it is also permissible to optimize it to:

[[assume(not should_wait)]];

Strict Reading

We can also interpret it as:

Implementations should make atomic loads use recent atomic stores within a reasonable amount of time.

This would make the above optimization invalid, despite the fact that x.load() isn't synchronized in any way.

Proposed Disambiguation

The strict reading is how users expect atomics to behave, and is also matching existing practice in implementations. The following wording is less ambiguous:

-Implementations should make atomic stores visible to atomic loads within a reasonable amount of time.
+Implementations should make atomic stores visible to atomic loads,
+and atomic loads should observe atomic stores,
+both within a reasonable amount of time.

Note: the both is essential to avoid further ambiguity here.

[JohelEGP]

Unrelated to the actual issue,
this reads to me like a "recommended practice" paragraph
(e.g., see #5905).

[jensmaurer]

Agreed with "recommended practice".

Can we make the phrasing a little more symmetric? for example:
"Atomic stores should be visible to atomic loads, and atomic loads should observe atomic stores, within a reasonable amount of time."

I don't like the suggested "both" placement; what's the ambiguity when leaving it out?

Care to prepare a pull request?

[eisenwave]

"Atomic stores should be visible to atomic loads, and atomic loads should observe atomic stores, within a reasonable amount of time."

I felt like the both is needed, but now that I'm reading it, the comma before within should be disambiguating enough, so that this doesn't just refer to loads.

Care to prepare a pull request?

Will do.