Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758883Ab1FVU7o (ORCPT ); Wed, 22 Jun 2011 16:59:44 -0400 Received: from claw.goop.org ([74.207.240.146]:50188 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758550Ab1FVU7n (ORCPT ); Wed, 22 Jun 2011 16:59:43 -0400 Message-ID: <4E0257BC.9050509@goop.org> Date: Wed, 22 Jun 2011 13:59:40 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110428 Fedora/3.1.10-1.fc15 Lightning/1.0b3pre Thunderbird/3.1.10 MIME-Version: 1.0 To: "H. Peter Anvin" CC: Peter Zijlstra , Ingo Molnar , the arch/x86 maintainers , Linux Kernel Mailing List , Nick Piggin , Jeremy Fitzhardinge , Linus Torvalds Subject: Re: [PATCH RFC 0/7] x86: convert ticketlocks to C and remove duplicate code References: <1308664868.26237.173.camel@twins> <4E0240AC.9060603@goop.org> <4E024E5B.7090103@zytor.com> In-Reply-To: <4E024E5B.7090103@zytor.com> X-Enigmail-Version: 1.1.2 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1588 Lines: 40 On 06/22/2011 01:19 PM, H. Peter Anvin wrote: > On 06/22/2011 12:21 PM, Jeremy Fitzhardinge wrote: >> A friend just pointed out that gcc has a "__sync_fetch_and_add()" >> intrinsic, which compiles to xadd when used in this context. What's the >> general feeling about using these kinds of gcc features? >> > In general they are good, IF: > > a) they cover all versions of gcc we care about (or we have a fallback), What is the supported range for these days? > and > b) they have the right semantics. My main concern was making sure that its a strong enough barrier, but the documentation is pretty explicit about that. > Using gcc intrinsics can generate better code than we can in inline > assembly. It does seem to do a pretty good job; it generates a plain locked add if you never look at the returned value, for example. > However, (b) is a killer since gcc doesn't have a way to generate our > lock prefix annotations, and so it isn't really useful here. Yeah, I thought about that. Its a bit unfortunate we're getting into spinlock code at all on a UP system, but we don't have a mechanism to stomp locking at a higher level. (Ignoring all the insane stuff that happens when the system becomes UP transiently just because all the other CPUs have been unplugged for suspend, etc; we just shouldn't bother in that case.) J -- 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/