Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755147AbaBRRov (ORCPT ); Tue, 18 Feb 2014 12:44:51 -0500 Received: from mail-ve0-f180.google.com ([209.85.128.180]:37536 "EHLO mail-ve0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754517AbaBRRot (ORCPT ); Tue, 18 Feb 2014 12:44:49 -0500 MIME-Version: 1.0 In-Reply-To: <1392740258.18779.7732.camel@triegel.csb> References: <1392321837.18779.3249.camel@triegel.csb> <20140214020144.GO4250@linux.vnet.ibm.com> <1392352981.18779.3800.camel@triegel.csb> <20140214172920.GQ4250@linux.vnet.ibm.com> <1392486310.18779.6447.camel@triegel.csb> <1392666947.18779.6838.camel@triegel.csb> <20140218030002.GA15857@linux.vnet.ibm.com> <1392740258.18779.7732.camel@triegel.csb> Date: Tue, 18 Feb 2014 09:44:48 -0800 X-Google-Sender-Auth: N80HvWSe2TrmpJOzVUZ7SHnEThY Message-ID: Subject: Re: [RFC][PATCH 0/5] arch: atomic rework From: Linus Torvalds To: Torvald Riegel Cc: Paul McKenney , Will Deacon , Peter Zijlstra , Ramana Radhakrishnan , David Howells , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "akpm@linux-foundation.org" , "mingo@kernel.org" , "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 8:17 AM, Torvald Riegel wrote: >> >> "Consume operation: no reads in the current thread dependent on the >> value currently loaded can be reordered before this load" > > I can't remember seeing that language in the standard (ie, C or C++). > Where is this from? That's just for googling for explanations. I do have some old standard draft, but that doesn't have any concise definitions anywhere that I could find. >> and it could make a compiler writer say that value speculation is >> still valid, if you do it like this (with "ptr" being the atomic >> variable): >> >> value = ptr->val; > > I assume the load from ptr has mo_consume ordering? Yes. >> into >> >> tmp = ptr; >> value = speculated.value; >> if (unlikely(tmp != &speculated)) >> value = tmp->value; >> >> which is still bogus. The load of "ptr" does happen before the load of >> "value = speculated->value" in the instruction stream, but it would >> still result in the CPU possibly moving the value read before the >> pointer read at least on ARM and power. > > And surprise, in the C/C++ model the load from ptr is sequenced-before > the load from speculated, but there's no ordering constraint on the > reads-from relation for the value load if you use mo_consume on the ptr > load. Thus, the transformed code has less ordering constraints than the > original code, and we arrive at the same outcome. Ok, good. > The standard is clear on what's required. I strongly suggest reading > the formalization of the memory model by Batty et al. Can you point to it? Because I can find a draft standard, and it sure as hell does *not* contain any clarity of the model. It has a *lot* of verbiage, but it's pretty much impossible to actually understand, even for somebody who really understands memory ordering. 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/