From: Noah Misch <noah@leadboat.com>
To: Andres Freund <andres@2ndquadrant.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Peter Geoghegan <pg@heroku.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Cc: Ants Aasma <ants@cybertec.at>
Subject: Re: better atomics - v0.5
Date: Thu, 26 Jun 2014 10:30:27 -0400
Message-ID: <20140626143027.GA1338311@tornado.leadboat.com> (raw)
In-Reply-To: <20140626102006.GB1926@awork2.anarazel.de>
References: <CA+TgmoYBW+ux5-8Ja=Mcyuy8=VXAnVRHp3Kess6Pn3DMXAPAEA@mail.gmail.com>
<20140625171434.GG24114@awork2.anarazel.de>
<CA+TgmoZShVVqjaKUL6P3kDvZVPibtgkzoti3M+Fvvjg5V+XJ0A@mail.gmail.com>
<20140625203635.GJ24114@awork2.anarazel.de>
<CA+TgmoY4OqKa9=G+5F0s0gPTpOB4cz=Mts34jrdf8399MWGcqw@mail.gmail.com>
<20140626102006.GB1926@awork2.anarazel.de>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
On Thu, Jun 26, 2014 at 12:20:06PM +0200, Andres Freund wrote:
> On 2014-06-25 20:16:08 -0400, Robert Haas wrote:
> > On Wed, Jun 25, 2014 at 4:36 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> > > Since it better be legal to manipulate a atomic variable while holding a
> > > spinlock we cannot simply use an arbitrary spinlock as backing for
> > > atomics. That'd possibly cause us to wait on ourselves or cause
> > > deadlocks.
> >
> > I think that's going to fall afoul of Tom's previously-articulated "no
> > loops inside spinlocks" rule. Most atomics, by nature, are
> > loop-until-it-works.
>
> Well, so is TAS itself :).
>
> More seriously, I think we're not going to have much fun if we're making
> up the rule that you can't do an atomic add/sub while a spinlock is
> held. That just precludes to many use cases and will make the code much
> harder to understand. I don't think we're going to end up having many
> problems if we allow atomic read/add/sub/write in there.
I agree it's valuable to have a design that permits invoking an atomic
operation while holding a spinlock.
> > > I added the x86 inline assembler because a fair number of buildfarm
> > > animals use ancient gcc's and because I could easily test it. It's also
> > > more efficient for gcc < 4.6. I'm not wedded to keeping it.
> >
> > Hmm. gcc 4.6 is only just over a year old, so if pre-4.6
> > implementations aren't that good, that's a pretty good argument for
> > keeping our own implementations around. :-(
GCC 4.6 was released 2011-03-25, though that doesn't change the bottom line.
--
Noah Misch
EnterpriseDB http://www.enterprisedb.com
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
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: pgsql-hackers@postgresql.org
Cc: noah@leadboat.com, andres@2ndquadrant.com, robertmhaas@gmail.com, tgl@sss.pgh.pa.us, pg@heroku.com, ants@cybertec.at
Subject: Re: better atomics - v0.5
In-Reply-To: <20140626143027.GA1338311@tornado.leadboat.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox