public inbox for [email protected]
help / color / mirror / Atom feedFrom: Alexander Korotkov <[email protected]>
To: Andres Freund <[email protected]>
Cc: [email protected]
Subject: Re: Odd code around ginScanToDelete
Date: Wed, 4 Feb 2026 00:32:51 +0200
Message-ID: <CAPpHfdt5BHkpvFnwg_RiMrCdD8W5FhOoahqY2Td-y3kb45UpZw@mail.gmail.com> (raw)
In-Reply-To: <utrlxij43fbguzw4kldte2spc4btoldizutcqyrfakqnbrp3ir@ph3sphpj4asz>
References: <utrlxij43fbguzw4kldte2spc4btoldizutcqyrfakqnbrp3ir@ph3sphpj4asz>
On Tue, Feb 3, 2026 at 6:26 PM Andres Freund <[email protected]> wrote:
>
> While looking at converting more places to UnlockReleaseBuffer(), in the
> course of making UnlockReleaseBuffer() faster than the two separate
> operations, I found this code:
>
> static bool
> ginScanToDelete(GinVacuumState *gvs, BlockNumber blkno, bool isRoot,
> DataPageDeleteStack *parent, OffsetNumber myoff)
> ...
>
> if (!meDelete)
> {
> if (BufferIsValid(me->leftBuffer))
> UnlockReleaseBuffer(me->leftBuffer);
> me->leftBuffer = buffer;
> }
> else
> {
> if (!isRoot)
> LockBuffer(buffer, GIN_UNLOCK);
>
> ReleaseBuffer(buffer);
> }
>
> if (isRoot)
> ReleaseBuffer(buffer);
>
>
> Which sure looks like it'd release buffer twice if isRoot is set? I guess
> that's not reachable, because presumably the root page will always go down the
> !meDelete path. But it sure made me wonder if there's a hard to reach bug.
Yes, it's not possible to have meDelete set for root, because
me->leftBuffer is always InvalidBuffer for the root. So the branch
handling meDelete case should better do Assert(!isRoot).
> This code was introduced in
> commit e14641197a5
> Author: Alexander Korotkov <[email protected]>
> Date: 2019-11-19 23:07:36 +0300
>
> Fix deadlock between ginDeletePage() and ginStepRight()
>
> I didn't trace it further to see if it existed before that in some fashion.
Yes. I think generally this area needs to be reworked to become more
clear, and have vast more comments. It was wrong from my side trying
to fix bugs there without reworking it into something more
appropriate. I'm planning to put work on this during this week.
> There's another oddity here: ginScanToDelete() requires that the root page has
> been locked by the caller already, but will afaict re-read the root page? But
> then have code to avoid locking it again, because that would not have worked?
> Seems odd.
It seems a bit odd for me that caller already have locked buffer, but
passes BlockNumber making us re-read the buffer. But I'm not sure
that's the same as your point. Could you, please, elaborate more on
this?
------
Regards,
Alexander Korotkov
Supabase
view thread (2+ messages)
reply
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: [email protected]
Cc: [email protected], [email protected]
Subject: Re: Odd code around ginScanToDelete
In-Reply-To: <CAPpHfdt5BHkpvFnwg_RiMrCdD8W5FhOoahqY2Td-y3kb45UpZw@mail.gmail.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox