Date: Mon, 4 Nov 2019 15:43:09 -0800
From: Noah Misch <noah@leadboat.com>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: SimpleLruTruncate() mutual exclusion
Message-ID: <20191104234309.GA375824@rfd.leadboat.com>
References: <20190218073103.GA1434723@rfd.leadboat.com>
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 <29566.1564419497@sss.pgh.pa.us>
 <20190801065117.GA2355684@rfd.leadboat.com>
 <CA+hUKGKoZAZ2CpvYa-ZQ8-Ee4BW9NGcwQv-KRZViDsMGObWtew@mail.gmail.com>
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On Mon, Nov 04, 2019 at 03:26:35PM +1300, Thomas Munro wrote:
> On Thu, Aug 1, 2019 at 6:51 PM Noah Misch <noah@leadboat.com> wrote:
> > vac_truncate_clog() instance 1 starts, considers segment ABCD eligible to unlink
> > vac_truncate_clog() instance 2 starts, considers segment ABCD eligible to unlink
> > vac_truncate_clog() instance 1 unlinks segment ABCD
> > vac_truncate_clog() instance 1 calls SetTransactionIdLimit()
> > vac_truncate_clog() instance 1 finishes
> > some backend calls SimpleLruZeroPage(), creating segment ABCD
> > vac_truncate_clog() instance 2 unlinks segment ABCD
> >
> > Serializing vac_truncate_clog() fixes that.
> 
> I've wondered before (in a -bugs thread[1] about unexplained pg_serial
> wraparound warnings) if we could map 64 bit xids to wide SLRU file
> names that never wrap around and make this class of problem go away.
> Unfortunately multixacts would need 64 bit support too...
> 
> [1] https://www.postgresql.org/message-id/flat/CAEBTBzuS-01t12GGVD6qCezce8EFD8aZ1V%2Bo_3BZ%3DbuVLQBtRg%40mail.gmail.com

That change sounds good to me.