Re: Buffer locking is special (hints, checksums, AIO writes)

On Fri, Aug 22, 2025 at 3:45 PM Andres Freund <andres@anarazel.de> wrote:
> My conclusion from the above is that we ought to:
>
> A) Make Buffer Locks something separate from lwlocks
> B) Merge BufferDesc.state and the content lock
> C) Allow some modifications of BufferDesc.state while holding spinlock

+1 to (A) and (B). No particular opinion on (C) but if it works well, great.

> The order of changes I think makes the most sense is the following:
>
> 1) Allow some modifications while holding the buffer header spinlock
> 2) Reduce buffer pin with just an atomic-sub
> 3) Widen BufferDesc.state to 64 bits
> 4) Implement buffer locking inside BufferDesc.state
> 5) Do IO while holding share-exclusive lock and require all buffer
>    modifications to at least hold share exclusive lock
> 6) Wait for AIO when acquiring an exclusive content lock

No strong objections. I certainly like getting to (5) and (6) and I
think those are in the right order. I'm not sure about the rest. I
thought (1) and (2) were the same change after reading your email; and
it surprises me a little bit that (2) is separate from (4). But I'm
sure you have a much better sense of this than I do.

> DOES ANYBODY HAVE A BETTER NAME THAN SHARE-EXCLUSIVE???!?

AFAIK "share exclusive" or "SX" is standard terminology. While I'm not
wholly hostile to the idea of coming up with something else, I don't
think our tendency to invent our own way to do everything is one of
our better tendencies as a project.

--
Robert Haas
EDB: http://www.enterprisedb.com