Feedback-ID: id4a34324:Fastmail
Date: Tue, 31 Mar 2026 20:29:02 -0400
From: Andres Freund <andres@anarazel.de>
To: Yura Sokolov <y.sokolov@postgrespro.ru>
Cc: Melanie Plageman <melanieplageman@gmail.com>,
	Noah Misch <noah@leadboat.com>, Heikki Linnakangas <hlinnaka@iki.fi>,
	Kirill Reshke <reshkekirill@gmail.com>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>,
	pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>,
	Robert Haas <robertmhaas@gmail.com>,
 Michael Paquier <michael.paquier@gmail.com>
Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
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Hi,

On 2026-03-31 18:05:46 -0400, Andres Freund wrote:
> On 2026-03-31 19:02:33 +0300, Yura Sokolov wrote:
> > 27.03.2026 23:00, Andres Freund wrote:
> > > On 2026-03-25 18:35:55 -0400, Andres Freund wrote:
> > >> Running it through valgrind and then will work on reading through one more
> > >> time and pushing them.
> > >
> > > And done.
> > >
> > > Phew, this project took way longer than I'd though it'd take.
> >
> > In addition to bug with BM_IO_ERROR [1] , I found race condition in
> > PinBuffer in this lines of code:
> >
> > 	if (unlikely(skip_if_not_valid && !(old_buf_state & BM_VALID)))
> > 		return false;
> >
> > 	/*
> > 	 * We're not allowed to increase the refcount while the buffer
> > 	 * header spinlock is held. Wait for the lock to be released.
> > 	 */
> > 	if (old_buf_state & BM_LOCKED)
> > 		old_buf_state = WaitBufHdrUnlocked(buf);
> >
> > While we waited for buffer header for being unlocked, it may become
> > invalid, isn't it?
> > Therefore, check related to skip_if_not_valid have to happen after waiting.
>
> Yea, that does seem wrong.  Not sure how it ended up that way.
>
> I think it may be better to add a continue after the WaitBufHdrUnlocked(), so
> that we restart the loop, rather than moving the skip_if_not_valid check.

Done that way. Thanks for finding & reporting this, well spotted!

Greetings,

Andres