Date: Thu, 3 Sep 2020 14:34:52 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@lists.postgresql.org,
	Jaime Casanova <jaime.casanova@2ndQuadrant.com>
Subject: Re: LogwrtResult contended spinlock
Message-ID: <20200903183452.GA10634@alvherre.pgsql>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <E494450B-9C6B-4716-9902-976DC7D69490@anarazel.de>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

Looking at patterns like this

	if (XLogCtl->LogwrtRqst.Write < EndPos)
		XLogCtl->LogwrtRqst.Write = EndPos;

It seems possible to implement with

    do {
    	XLogRecPtr	currwrite;

        currwrite = pg_atomic_read_u64(LogwrtRqst.Write);
	if (currwrite > EndPos)
            break;  // already done by somebody else
        if (pg_atomic_compare_exchange_u64(LogwrtRqst.Write,
	                                   currwrite, EndPos))
            break;  // successfully updated
    } while (true);

This assumes that LogwrtRqst.Write never goes backwards, so it doesn't
seem good material for a general routine.

This *seems* correct to me, though this is muddy territory to me.  Also,
are there better ways to go about this?

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services