Re: remove spurious CREATE INDEX CONCURRENTLY wait

From: Andres Freund <andres(at)anarazel(dot)de>
To: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Michael Paquier <michael(at)paquier(dot)xyz>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, James Coleman <jtc331(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: remove spurious CREATE INDEX CONCURRENTLY wait
Date: 2021-11-11 02:07:24
Message-ID: 20211111020724.ggsfhcq3krq5r4hb@alap3.anarazel.de
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Hi,

On 2020-11-25 17:03:58 -0300, Alvaro Herrera wrote:
> On 2020-Nov-23, Andres Freund wrote:
>
> > On 2020-11-23 12:30:05 -0300, Alvaro Herrera wrote:
>
> > > In other words, my conclusion is that there definitely is a bug here and
> > > I am going to restore the use of exclusive lock for setting the
> > > statusFlags.
> >
> > Cool.
>
> Here's a patch.
>
> Note it also moves the computation of vacuum's Xmin (per
> GetTransactionSnapshot) to *after* the bit has been set in statusFlags.

> From b813c67a4abe2127b8bd13db7e920f958db15d59 Mon Sep 17 00:00:00 2001
> From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
> Date: Tue, 24 Nov 2020 18:10:42 -0300
> Subject: [PATCH] Restore lock level to update statusFlags
>
> Reverts 27838981be9d (some comments are kept). Per discussion, it does
> not seem safe to relax the lock level used for this; in order for it to
> be safe, there would have to be memory barriers between the point we set
> the flag and the point we set the trasaction Xid, which perhaps would
> not be so bad; but there would also have to be barriers at the readers'
> side, which from a performance perspective might be bad.
>
> Now maybe this analysis is wrong and it *is* safe for some reason, but
> proof of that is not trivial.

I just noticed that this commit (dcfff74fb16) didn't revert the change of lock
level in ReplicationSlotRelease(). Was that intentional?

Greetings,

Andres Freund

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