Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!gegeweb.org!de-l.enfer-du-nord.net!feeder2.enfer-du-nord.net!weretis.net!feeder1.news.weretis.net!news1.tnib.de!feed.news.tnib.de!news.tnib.de!feed.cnntp.org!news.cnntp.org!not-for-mail
From: Anthony Williams <anthony.ajw@gmail.com>
Newsgroups: comp.programming.threads
Subject: Re: questions about memory_order_seq_cst fence
References: <43eb372b-6517-4914-b647-860f1f7136ba@k15g2000pri.googlegroups.com> <it5sht$i7j$1@node2.news.atman.pl> <b7657dae-d49f-4859-8020-b5d76bfae2cd@p9g2000prh.googlegroups.com> <itbb7k$uhh$1@node2.news.atman.pl>
Date: Thu, 16 Jun 2011 08:35:59 +0100
Message-ID: <87boxyfctc.fsf@justsoftwaresolutions.co.uk>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux)
Cancel-Lock: sha1:rkB0SC7s1pc3uiSL7+OWLBR29p8=
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Lines: 24
Organization: CNNTP
NNTP-Posting-Host: 5dd50c2f.read.cnntp.org
X-Trace: DXC=Ekn8C;Gfk=m[O`b`RHllLhWoT\PAgXa?aGfg=Y\0KjYf31?8ZVJ3gMeGj6PdB418WnjW@o2`Y5L7mFf;8Mio\>Mc
X-Complaints-To: abuse@cnntp.org
Xref: x330-a1.tempe.blueboxinc.net comp.programming.threads:288

Edek <edek.pienkowski@gmail.com> writes:

> As a side note, I compiled all ops on x86_64, and atomic<int>::load
> is mfence/load/mfence even for relaxed. I'd need some time to analyse
> the thing from perspective of other ops implementations in all
> orderings, and this leads me to the not so subtle question:

Ouch. That is a seriously over-cautious implementation. Atomics should
not need any fences on x86_64 unless you actually write
atomic_thread_fence(memory_order_seq_cst)

See my blog entry for the required operations:

http://www.justsoftwaresolutions.co.uk/threading/intel-memory-ordering-and-c++-memory-model.html

The only one that is not a plain MOV is a seq_cst store, which is an
XCHG.

Anthony
-- 
Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
just::thread C++0x thread library             http://www.stdthread.co.uk
Just Software Solutions Ltd       http://www.justsoftwaresolutions.co.uk
15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976