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help / color / mirror / Atom feedFrom: Andres Freund <[email protected]>
To: Alexander Korotkov <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: pg_atomic_compare_exchange_*() and memory barriers
Date: Sat, 8 Mar 2025 10:20:55 -0500
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Hi,
On 2025-03-08 17:06:38 +0200, Alexander Korotkov wrote:
> 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.
Not in the next ~four weeks. If you ping me afterwards, I can give it a go.
FWIW, I am fairly certain that any non-toy algorithm that requires a full
memory barrier instead of just an acquire in case of a CAS failure is chock
full of concurrency bugs.
Greetings,
Andres Freund
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To: [email protected]
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Subject: Re: pg_atomic_compare_exchange_*() and memory barriers
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