Date: Thu, 4 Dec 2025 09:51:10 -0600
From: Nathan Bossart <nathandbossart@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: pgsql: Add pg_atomic_unlocked_write_u64
Message-ID: <aTGt7q4Jvn97uGAx@nathan>
References: <E1vQwSk-002fZh-2h@gemulon.postgresql.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <E1vQwSk-002fZh-2h@gemulon.postgresql.org>
Archived-At: <https://www.postgresql.org/message-id/aTGt7q4Jvn97uGAx%40nathan>
Precedence: bulk

On Wed, Dec 03, 2025 at 11:40:47PM +0000, Andres Freund wrote:
> Add pg_atomic_unlocked_write_u64
> 
> The 64bit equivalent of pg_atomic_unlocked_write_u32(), to be used in an
> upcoming patch converting BufferDesc.state into a 64bit atomic.

I noticed that this new function was defined as

	ptr->value = val;

and couldn't figure out why that was safe.  Above
pg_atomic_unlocked_write_u32(), I see this comment:

 * The write is guaranteed to succeed as a whole, i.e. it's not possible to
 * observe a partial write for any reader.  ...

But the new 64-bit version doesn't seem to be surrounded by a check for
PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY, and I found no discussion about it in
the commit message or the linked thread.  Am I missing something here?

-- 
nathan