Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752847AbaBGSoB (ORCPT ); Fri, 7 Feb 2014 13:44:01 -0500 Received: from mx1.redhat.com ([209.132.183.28]:19921 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751403AbaBGSn7 (ORCPT ); Fri, 7 Feb 2014 13:43:59 -0500 Subject: Re: [RFC][PATCH 0/5] arch: atomic rework From: Torvald Riegel To: Peter Zijlstra Cc: Will Deacon , "Paul E. McKenney" , Ramana Radhakrishnan , David Howells , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "torvalds@linux-foundation.org" , "akpm@linux-foundation.org" , "mingo@kernel.org" , "gcc@gcc.gnu.org" In-Reply-To: <20140207170654.GQ5002@laptop.programming.kicks-ass.net> References: <52F3DA85.1060209@arm.com> <20140206185910.GE27276@mudshark.cambridge.arm.com> <20140206192743.GH4250@linux.vnet.ibm.com> <1391721423.23421.3898.camel@triegel.csb> <20140206221117.GJ4250@linux.vnet.ibm.com> <1391730288.23421.4102.camel@triegel.csb> <20140207042051.GL4250@linux.vnet.ibm.com> <20140207074405.GM5002@laptop.programming.kicks-ass.net> <20140207165028.GO4250@linux.vnet.ibm.com> <20140207165548.GR5976@mudshark.cambridge.arm.com> <20140207170654.GQ5002@laptop.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" Date: Fri, 07 Feb 2014 19:43:22 +0100 Message-ID: <1391798602.23421.4802.camel@triegel.csb> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2014-02-07 at 18:06 +0100, Peter Zijlstra wrote: > On Fri, Feb 07, 2014 at 04:55:48PM +0000, Will Deacon wrote: > > Hi Paul, > > > > On Fri, Feb 07, 2014 at 04:50:28PM +0000, Paul E. McKenney wrote: > > > On Fri, Feb 07, 2014 at 08:44:05AM +0100, Peter Zijlstra wrote: > > > > On Thu, Feb 06, 2014 at 08:20:51PM -0800, Paul E. McKenney wrote: > > > > > Hopefully some discussion of out-of-thin-air values as well. > > > > > > > > Yes, absolutely shoot store speculation in the head already. Then drive > > > > a wooden stake through its hart. > > > > > > > > C11/C++11 should not be allowed to claim itself a memory model until that > > > > is sorted. > > > > > > There actually is a proposal being put forward, but it might not make ARM > > > and Power people happy because it involves adding a compare, a branch, > > > and an ISB/isync after every relaxed load... Me, I agree with you, > > > much preferring the no-store-speculation approach. > > > > Can you elaborate a bit on this please? We don't permit speculative stores > > in the ARM architecture, so it seems counter-intuitive that GCC needs to > > emit any additional instructions to prevent that from happening. > > > > Stores can, of course, be observed out-of-order but that's a lot more > > reasonable :) > > This is more about the compiler speculating on stores; imagine: > > if (x) > y = 1; > else > y = 2; > > The compiler is allowed to change that into: > > y = 2; > if (x) > y = 1; If you write the example like that, this is indeed allowed because it's all sequential code (and there's no volatiles in there, at least you didn't show them :). A store to y would happen in either case. You cannot observe the difference between both examples in a data-race-free program. Are there supposed to be atomic/non-sequential accesses in there? If so, please update the example. > Which is of course a big problem when you want to rely on the ordering. > > There's further problems where things like memset() can write outside > the specified address range. Examples are memset() using single > instructions to wipe entire cachelines and then 'restoring' the tail > bit. As Joseph said, this would be a bug IMO. > While valid for single threaded, its a complete disaster for concurrent > code. > > There's more, but it all boils down to doing stores you don't expect in > a 'sane' concurrent environment and/or don't respect the control flow. A few of those got fixed already, because they violated the memory model's requirements. If you have further examples that are valid code in the C11/C++11 model, please report them. -- 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/