Re: Atomic ops for unlogged LSN

Hi,

On 2023-11-07 11:02:49 -0600, Nathan Bossart wrote:
> On Tue, Nov 07, 2023 at 11:47:46AM -0500, Stephen Frost wrote:
> > We only care about the value of the unlogged LSN being correct during
> > normal shutdown when we're writing out the shutdown checkpoint, but by
> > that time everything else has been shut down and the value absolutely
> > should not be changing.
>
> I agree that's all true. I'm trying to connect how this scenario ensures
> we see the most up-to-date value in light of this comment above
> pg_atomic_read_u32():
>
> * The read is guaranteed to return a value as it has been written by this or
> * another process at some point in the past. There's however no cache
> * coherency interaction guaranteeing the value hasn't since been written to
> * again.
>
> 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).

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.

Greetings,

Andres Freund