Re: LogwrtResult contended spinlock

Hi,

On 2020-09-03 14:34:52 -0400, Alvaro Herrera wrote:
> Looking at patterns like this
>
> if (XLogCtl->LogwrtRqst.Write < EndPos)
> XLogCtl->LogwrtRqst.Write = EndPos;
>
> It seems possible to implement with
>
> do {
> XLogRecPtr currwrite;
>
> currwrite = pg_atomic_read_u64(LogwrtRqst.Write);
> if (currwrite > EndPos)
> break; // already done by somebody else
> if (pg_atomic_compare_exchange_u64(LogwrtRqst.Write,
> currwrite, EndPos))
> break; // successfully updated
> } while (true);
>
> This assumes that LogwrtRqst.Write never goes backwards, so it doesn't
> seem good material for a general routine.
>
> This *seems* correct to me, though this is muddy territory to me. Also,
> are there better ways to go about this?

Hm, I was thinking that we'd first go for reading it without a spinlock,
but continuing to write it as we currently do.

But yea, I can't see an issue with what you propose here. I personally
find do {} while () weird and avoid it when not explicitly useful, but
that's extremely minor, obviously.

Greetings,

Andres Freund