Re: Atomic ops for unlogged LSN

On Wed, May 24, 2023 at 02:49:58PM -0700, Andres Freund wrote:
> I was a bit confused by Michael's comment as well, due to the section of code
> quoted. But he does have a point: pg_atomic_read_u32() does indeed *not* have
> barrier semantics (it'd be way more expensive), and the patch does contain
> this hunk:

Thanks for the correction. The part about _add was incorrect.

> So we indeed loose some "barrier strength" - but I think that's fine. For one,
> it's a debugging-only value. But more importantly, I don't see what reordering
> the barrier could prevent - a barrier is useful for things like sequencing two
> memory accesses to happen in the intended order - but unloggedLSN doesn't
> interact with another variable that's accessed within the ControlFileLock
> afaict.

This stuff is usually tricky enough that I am never completely sure
whether it is fine without reading again README.barrier, which is
where unloggedLSN is saved in the control file under its LWLock.
Something that I find confusing in the patch is that it does not
document the reason why this is OK.
--
Michael