Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937522AbXHPCDi (ORCPT ); Wed, 15 Aug 2007 22:03:38 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756225AbXHPCDY (ORCPT ); Wed, 15 Aug 2007 22:03:24 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:58586 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751363AbXHPCDW (ORCPT ); Wed, 15 Aug 2007 22:03:22 -0400 Date: Thu, 16 Aug 2007 07:45:44 +0530 (IST) From: Satyam Sharma X-X-Sender: satyam@enigma.security.iitk.ac.in To: Christoph Lameter cc: "Paul E. McKenney" , Paul Mackerras , Stefan Richter , Chris Snook , Linux Kernel Mailing List , linux-arch@vger.kernel.org, Linus Torvalds , netdev@vger.kernel.org, Andrew Morton , ak@suse.de, heiko.carstens@de.ibm.com, davem@davemloft.net, schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org, rpjday@mindspring.com, jesper.juhl@gmail.com, segher@kernel.crashing.org, Herbert Xu Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures In-Reply-To: Message-ID: References: <46C2D6F3.3070707@s5r6.in-berlin.de> <18115.35524.56393.347841@cargo.ozlabs.ibm.com> <20070816003948.GY9645@linux.vnet.ibm.com> <20070816005348.GA9645@linux.vnet.ibm.com> <20070816011414.GC9645@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1899 Lines: 44 On Wed, 15 Aug 2007, Christoph Lameter wrote: > On Wed, 15 Aug 2007, Paul E. McKenney wrote: > > > Understood. My point is not that the impact is precisely zero, but > > rather that the impact on optimization is much less hurtful than the > > problems that could arise otherwise, particularly as compilers become > > more aggressive in their optimizations. > > The problems arise because barriers are not used as required. Volatile > has wishy washy semantics and somehow marries memory barriers with data > access. It is clearer to separate the two. Conceptual cleanness usually > translates into better code. If one really wants the volatile then lets > make it explicit and use > > atomic_read_volatile() Completely agreed, again. To summarize again (had done so about ~100 mails earlier in this thread too :-) ... atomic_{read,set}_volatile() -- guarantees volatility also along with atomicity (the two _are_ different concepts after all, irrespective of whether callsites normally want one with the other or not) atomic_{read,set}_nonvolatile() -- only guarantees atomicity, compiler free to elid / coalesce / optimize such accesses, can keep the object in question cached in a local register, leads to smaller text, etc. As to which one should be the default atomic_read() is a question of whether majority of callsites (more weightage to important / hot codepaths, lesser to obscure callsites) want a particular behaviour. Do we have a consensus here? (hoping against hope, probably :-) [ This thread has gotten completely out of hand ... for my mail client alpine as well, it now seems. Reminds of that 1000+ GPLv3 fest :-) ] - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/