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help / color / mirror / Atom feedFrom: Alexander Korotkov <[email protected]>
To: Andres Freund <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: pg_atomic_compare_exchange_*() and memory barriers
Date: Sat, 8 Mar 2025 17:06:38 +0200
Message-ID: <CAPpHfdtja4qxK5-T+RTdHki+sycbrZaP7==2CD4K+_b+dkUxNA@mail.gmail.com> (raw)
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On Sat, Mar 8, 2025 at 3:41 PM Andres Freund <[email protected]> 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
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To: [email protected]
Cc: [email protected], [email protected]
Subject: Re: pg_atomic_compare_exchange_*() and memory barriers
In-Reply-To: <CAPpHfdtja4qxK5-T+RTdHki+sycbrZaP7==2CD4K+_b+dkUxNA@mail.gmail.com>
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