Re: LogwrtResult contended spinlock

Hi,

On 2020-09-04 10:05:45 -0700, Andres Freund wrote:
> 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.

Re general routine: On second thought, it might actually be worth having
it. Even just for LSNs - there's plenty places where it's useful to
ensure a variable is at least a certain size. I think I would be in
favor of a general helper function.

Greetings,

Andres Freund