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Subject: Re: pgsql: Avoid spurious deadlocks when upgrading a tuple lock
From: Oleksii Kliukin <alexk@hintbits.com>
In-Reply-To: <20190618222547.GA18769@alvherre.pgsql>
Date: Wed, 19 Jun 2019 15:08:31 +0100
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
 pgsql-hackers@lists.postgresql.org
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Message-Id: <2A0C6E60-661D-47BB-888C-F557D4E7458A@hintbits.com>
References: <20190618222547.GA18769@alvherre.pgsql>
To: =?utf-8?Q?=C3=81lvaro_Herrera?= <alvherre@2ndquadrant.com>
Precedence: bulk

Alvaro Herrera <alvherre@2ndquadrant.com> wrote:

> On 2019-Jun-18, Oleksii Kliukin wrote:
>=20
>> Sorry, I was confused, as I was looking only at
>> =
https://git.postgresql.org/gitweb/?p=3Dpostgresql.git;a=3Dcommitdiff;h=3Dd=
e87a084c0a5ac927017cd0834b33a932651cfc9
>>=20
>> without taking your subsequent commit that silences compiler warnings =
at
>> =
https://git.postgresql.org/gitweb/?p=3Dpostgresql.git;a=3Dcommitdiff;h=3D3=
da73d6839dc47f1f47ca57974bf28e5abd9b572
>> into consideration. With that commit, the danger is indeed in =
resetting the
>> skip mechanism on each jump and potentially causing deadlocks.
>=20
> Yeah, I understand the confusion.
>=20
> Anyway, as bugs go, this one seems pretty benign.  It would result in =
a
> unexplained deadlock, very rarely, and only for people who use a very
> strange locking pattern that includes (row-level) lock upgrades.  I
> think it also requires aborted savepoints too, though I don't rule out
> the possibility that there might be a way to reproduce this without
> that.
>=20
> I pushed the patch again just now, with the new permutation.

Thank you very much for working on it and committing the fix!

Cheers,
Oleksii=