Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750726AbWAEWTE (ORCPT ); Thu, 5 Jan 2006 17:19:04 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751075AbWAEWTE (ORCPT ); Thu, 5 Jan 2006 17:19:04 -0500 Received: from smtp.osdl.org ([65.172.181.4]:43979 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S1750726AbWAEWTC (ORCPT ); Thu, 5 Jan 2006 17:19:02 -0500 Date: Thu, 5 Jan 2006 14:17:28 -0800 (PST) From: Linus Torvalds To: Ingo Molnar cc: Nicolas Pitre , Joel Schopp , lkml , Andrew Morton , Arjan van de Ven , Jes Sorensen , Al Viro , Oleg Nesterov , David Howells , Alan Cox , Christoph Hellwig , Andi Kleen , Russell King , Anton Blanchard Subject: Re: [patch 00/21] mutex subsystem, -V14 In-Reply-To: <20060105220305.GA8372@elte.hu> Message-ID: References: <20060104144151.GA27646@elte.hu> <43BC5E15.207@austin.ibm.com> <20060105144016.GB16816@elte.hu> <20060105220305.GA8372@elte.hu> 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: 1369 Lines: 36 On Thu, 5 Jan 2006, Ingo Molnar wrote: > > [ long details removed ] > > to sum it up: atomic_dec/inc_return() alone is not enough to implement > critical sections, on a number of architectures. atomic_xchg() seems to > have similar problems too. Yes. > the patch below adds the smp_mb() barriers to the generic headers, which > should now fulfill all the ordering requirements, on every architecture. > It only relies on one property of the atomic primitives: that they wont > get reordered with respect to themselves, so an atomic_inc_ret() and an > atomic_dec_ret() cannot switch place. > > Can you see any hole in this reasoning? No. The alternative is to just make the ordering requirements for "atomic_dec_return()" and "atomic_xchg()" be absolute. Say that they have to be full memory barriers, and push the problem into the low-level architecture. I _think_ your patch is the right approach, because most architectures are likely to do their own fast-paths for mutexes, and as such the generic ones are more of a template for how to do it, and hopefilly aren't that performance critical. Linus - 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/