Re: WAL Insertion Lock Improvements

On Tue, Jul 25, 2023 at 12:57:37PM -0700, Andres Freund wrote:
> I just rebased my aio tree over the commit and promptly, on the first run, saw
> a hang. I did some debugging on that. Unfortunately repeated runs haven't
> repeated that hang, despite quite a bit of trying.
>
> The symptom I was seeing is that all running backends were stuck in
> LWLockWaitForVar(), even though the value they're waiting for had
> changed. Which obviously "shouldn't be possible".

Hmm. I've also spent a few days looking at this past report that made
the LWLock part what it is today, but I don't quite see immediately
how it would be possible to reach a state where all the backends are
waiting for an update that's not happening:
https://www.postgresql.org/message-id/CAMkU=1zLztROwH3B42OXSB04r9ZMeSk3658qEn4_8+b+K3E7nQ@mail.gmail.com

All the assumptions of this code and its dependencies with
xloginsert.c are hard to come by.

> It's of course possible that this is AIO specific, but I didn't see anything
> in stacks to suggest that.

Or AIO handles the WAL syncs so quickly that it has more chances in
showing a race condition here?

> I do wonder if this possibly exposed an undocumented prior dependency on the
> value update always happening under the list lock.

I would not be surprised by that.
--
Michael