locked reads for atomics

Moving this to a new thread and adding it to the January commitfest.

On Thu, Nov 09, 2023 at 03:27:33PM -0600, Nathan Bossart wrote:
> On Tue, Nov 07, 2023 at 04:58:16PM -0800, Andres Freund wrote:
>> However, even if there's likely some other implied memory barrier that we
>> could piggyback on, the patch much simpler to understand if it doesn't change
>> coherency rules. There's no way the overhead could matter.
>
> I wonder if it's worth providing a set of "locked read" functions. Those
> could just do a compare/exchange with 0 in the generic implementation. For
> patches like this one where the overhead really shouldn't matter, I'd
> encourage folks to use those to make it easy to reason about correctness.

Concretely, like this.

--
Nathan Bossart
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