Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758561AbZGCTR0 (ORCPT ); Fri, 3 Jul 2009 15:17:26 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756983AbZGCTRT (ORCPT ); Fri, 3 Jul 2009 15:17:19 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:48448 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756904AbZGCTRS (ORCPT ); Fri, 3 Jul 2009 15:17:18 -0400 Date: Fri, 3 Jul 2009 21:17:09 +0200 From: Ingo Molnar To: Linus Torvalds Cc: Eric Dumazet , mingo@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH -tip] x86: atomic64: inline atomic64_read() Message-ID: <20090703191709.GA17057@elte.hu> References: <4A4E1AA2.30002@gmail.com> <20090703180450.GB3405@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1353 Lines: 39 * Linus Torvalds wrote: > On Fri, 3 Jul 2009, Ingo Molnar wrote: > > > + > > > + asm volatile( > > > + "mov %%ebx, %%eax\n\t" > > > + "mov %%ecx, %%edx\n\t" > > > + LOCK_PREFIX "cmpxchg8b %1\n" > > > + : "=&A" (res) > > > + : "m" (*ptr) > > > + ); > > > > That clobbers or affects eax, ebx, ecx, edx. The only registers > > it does not clobber is esi, edi and rbp. > > No, it only clobbers eax/edx. Yes, it touches ebx/ecx, but that's > kind of incidental, and read-only, and doesn't matter. > > > Linus, what's your preference in this case? > > I think it's kind of subtle to do that thing, but it _is_ very > much a special case. It's odd to use a cmpxchg8b to just do a read > in the first place, so the fact that we then play other games with > it is kind of immaterial. > > Adding a comment about the magic "we don't care about the initial > values in eax/edx and ebx/ecx, but they have to match in case we > end up doing a writeback" might be a good idea. Ok - i've applied it and added a comment as well about the reasons an subtleties. Ingo -- 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/