Re: Atomic ops for unlogged LSN

On Tue, Nov 07, 2023 at 04:58:16PM -0800, Andres Freund wrote:
> On 2023-11-07 11:02:49 -0600, Nathan Bossart wrote:
>> Is there something special about all other backends being shut down that
>> ensures this returns the most up-to-date value and not something from "some
>> point in the past" as the stated contract for this function seems to
>> suggest?
>
> Practically yes - getting to the point of writing the shutdown checkpoint
> implies having gone through a bunch of code that implies memory barriers
> (spinlocks, lwlocks).

Sure.

> 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.

--
Nathan Bossart
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