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Date: Sat, 8 Mar 2025 08:41:24 -0500
From: Andres Freund <andres@anarazel.de>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: pg_atomic_compare_exchange_*() and memory barriers
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Hi,

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.

Greetings,

Andres Freund