From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753753AbaBRRif (ORCPT ); Tue, 18 Feb 2014 12:38:35 -0500 Received: from mail-ve0-f176.google.com ([209.85.128.176]:50880 "EHLO mail-ve0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751005AbaBRRid (ORCPT ); Tue, 18 Feb 2014 12:38:33 -0500 MIME-Version: 1.0 In-Reply-To: References: Date: Tue, 18 Feb 2014 09:38:32 -0800 X-Google-Sender-Auth: qRGfqhgSOI5-D30CRVl8jUlOG_c Message-ID: Subject: Re: [RFC][PATCH 0/5] arch: atomic rework From: Linus Torvalds To: Peter.Sewell@cl.cam.ac.uk Cc: "mark.batty@cl.cam.ac.uk" , Paul McKenney , Peter Zijlstra , Torvald Riegel , Will Deacon , Ramana Radhakrishnan , David Howells , "linux-arch@vger.kernel.org" , Linux Kernel Mailing List , Andrew Morton , Ingo Molnar , "gcc@gcc.gnu.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 18, 2014 at 4:12 AM, Peter Sewell wrote: > > For example, suppose we have, in one compilation unit: > > void f(int ra, int*rb) { > if (ra==42) > *rb=42; > else > *rb=42; > } So this is a great example, and in general I really like your page at: > For more context, this example is taken from a summary of the thin-air > problem by Mark Batty and myself, > , and the problem with > dependencies via other compilation units was AFAIK first pointed out > by Hans Boehm. and the reason I like your page is that it really talks about the problem by pointing to the "unoptimized" code, and what hardware would do. As mentioned, I think that's actually the *correct* way to think about the problem space, because it allows the programmer to take hardware characteristics into account, without having to try to "describe" them at a source level. As to your example of if (ra) atomic_write(rb, A); else atomic_write(rb, B); I really think that it is ok to combine that into atomic_write(rb, ra ? A:B); (by virtue of "exact same behavior on actual hardware"), and then the only remaining question is whether the "ra?A:B" can be optimized to remove the conditional if A==B as in your example where both are "42". Agreed? Now, I would argue that the "naive" translation of that is unambiguous, and since "ra" is not volatile or magic in any way, then "ra?42:42" can obviously be optimized into just 42 - by the exact same rule that says "the compiler can do any transformation that is equivalent in the hardware". The compiler can *locally* decide that that is the right thing to do, and any programmer that complains about that decision is just crazy. So my "local machine behavior equivalency" rule means that that function can be optimized into a single "store 42 atomically into rb". Now, if it's *not* compiled locally, and is instead implemented as a macro (or inline function), there are obviously situations where "ra ? A : B" ends up having to do other things. In particular, X may be volatile or an atomic read that has ordering semantics, and then that expression doesn't become just "42", but that's a separate issue. It's not all that dissimilar to "function calls are sequence points", though, and obviously if the source of "ra" has semantic meaning, you have to honor that semantic meaning. Agreed? Linus