Re: pg_atomic_compare_exchange_*() and memory barriers

On Sat, Mar 8, 2025 at 3:41 PM Andres Freund <andres@anarazel.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.

------
Regards,
Alexander Korotkov
Supabase