Re: pg_atomic_compare_exchange_*() and memory barriers

On Sat, Mar 8, 2025 at 5:20 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2025-03-08 17:06:38 +0200, Alexander Korotkov wrote:
> > On Sat, Mar 8, 2025 at 3:41 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > >
> > > On 2025-03-08 08:02:41 -0500, Andres Freund wrote:
> > > > From the C/C++ standard atomics model it doesn't make sense to say
that a
> > > > failed CAS has release semantics, as there simply isn't a write
that could be
> > > > ordered! What their barriers guarantee is ordering between
multiple memory
> > > > operation, you can't order multiple writes if you don't have
multiple
> > > > writes... The synchronization in the C/C++ model is only
established between
> > > > accesses of the same variable and there's no write in the case of a
failed
> > > > CAS, so there's nothing that could establish a release-acquire
ordering.
> > > >
> > > > Unfortunately that model doesn't mesh well with barriers that
aren't attached
> > > > to read/modify operations. Which is what we ended up with...
> > >
> > > Adding a full barrier to failed CAS would be a rather large overhead,
> > > undesirable in just about any sane algorithm. As a consequence, I
think what
> > > we ought to do is to redefine the barrier semantics to only imply an
acquire
> > > barrier in case CAS fails.
> >
> > Thank you, I'm good with this solution. Can I leave this on you? I'm
> > not feeling myself strong to word this correctly.
>
> Not in the next ~four weeks. If you ping me afterwards, I can give it a
go.

It has been more than a year. Probably not perfect moment again. But
could you, please, revise that comment?

------
Regards,
Alexander Korotkov
Supabase