From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935312AbdKPTN1 (ORCPT ); Thu, 16 Nov 2017 14:13:27 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:55278 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S933354AbdKPTNS (ORCPT ); Thu, 16 Nov 2017 14:13:18 -0500 Date: Thu, 16 Nov 2017 11:13:07 -0800 From: "Paul E. McKenney" To: Will Deacon Cc: Nick Desaulniers , Peter Zijlstra , Sami Tolvanen , Alex Matveev , Andi Kleen , Ard Biesheuvel , Greg Hackmann , Kees Cook , linux-arm-kernel@lists.infradead.org, Linux Kbuild mailing list , LKML , Mark Rutland , Masahiro Yamada , Maxim Kuvyrkov , Michal Marek , Yury Norov , Matthias Kaehlcke , Alexander Potapenko , Stephen Hines , Pirama Arumuga Nainar , Manoj Gupta Subject: Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO_CLANG Reply-To: paulmck@linux.vnet.ibm.com References: <20171116161731.GA94341@samitolvanen.mtv.corp.google.com> <20171116163054.kcsdsomr7u2mqql2@hirez.programming.kicks-ass.net> <20171116165922.llrojrvomuihabrt@hirez.programming.kicks-ass.net> <20171116173417.nqsh5dpu65uj7b5s@hirez.programming.kicks-ass.net> <20171116174830.GX3624@linux.vnet.ibm.com> <20171116183950.GA3624@linux.vnet.ibm.com> <20171116184508.GC21898@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171116184508.GC21898@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17111619-0040-0000-0000-000003C4361B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008076; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000240; SDB=6.00946820; UDB=6.00477967; IPR=6.00727112; BA=6.00005695; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00018049; XFM=3.00000015; UTC=2017-11-16 19:13:13 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17111619-0041-0000-0000-000007B9580A Message-Id: <20171116191307.GC3624@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-11-16_06:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1711160255 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 16, 2017 at 06:45:08PM +0000, Will Deacon wrote: > On Thu, Nov 16, 2017 at 10:39:51AM -0800, Paul E. McKenney wrote: > > On Thu, Nov 16, 2017 at 10:16:22AM -0800, Nick Desaulniers wrote: > > > > On Thu, Nov 16, 2017 at 06:34:17PM +0100, Peter Zijlstra wrote: > > > >> So the problem is that its very very hard (and painful) to find these > > > >> bugs. Getting the tools people to comment on these specific > > > >> optimizations would really help lots. > > > > > > No doubt; I do not disagree with you. Kernel developers have very > > > important use cases for the language. > > > > > > But the core point I'm trying to make is "do we need to block this > > > patch set until issues with the C standards body in regards to the > > > kernels memory model are resolved?" I would hope the two are > > > orthogonal and that we'd take them and then test them even more > > > extensively than the developer has in order to find out. > > > > Given that I have been working on getting the C and C++ standards to > > correctly handle rcu_dereference() for more than ten years, I recommend > > -against- waiting on standardization in the strongest possible terms. > > And if you think that ten years is bad, the Java standards community has > > been struggling with out-of-thin-air (OOTA) values for almost 20 years. > > And the C and C++ standards haven't solved OOTA, either. > > The problem is, if we go ahead with this change, the compiler *will* break > some address dependencies and something will eventually go wrong. At that > point, what do we do? Turn off some random compiler optimisation? Add a > random barrier()? > > We don't necessarily need standardisation, but we at least need guarantees > from the compiler implementation that LTO/PGO will respect source level > address dependencies. I don't think we have that today. Ah, if "this patch set" meant "adding LTO", I stand corrected and I apologize for my confusion. I agree that we need LTO/PGO to be housebroken from an LKMM viewpoint before it is enabled. Thanx, Paul From mboxrd@z Thu Jan 1 00:00:00 1970 From: paulmck@linux.vnet.ibm.com (Paul E. McKenney) Date: Thu, 16 Nov 2017 11:13:07 -0800 Subject: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO_CLANG In-Reply-To: <20171116184508.GC21898@arm.com> References: <20171116161731.GA94341@samitolvanen.mtv.corp.google.com> <20171116163054.kcsdsomr7u2mqql2@hirez.programming.kicks-ass.net> <20171116165922.llrojrvomuihabrt@hirez.programming.kicks-ass.net> <20171116173417.nqsh5dpu65uj7b5s@hirez.programming.kicks-ass.net> <20171116174830.GX3624@linux.vnet.ibm.com> <20171116183950.GA3624@linux.vnet.ibm.com> <20171116184508.GC21898@arm.com> Message-ID: <20171116191307.GC3624@linux.vnet.ibm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Nov 16, 2017 at 06:45:08PM +0000, Will Deacon wrote: > On Thu, Nov 16, 2017 at 10:39:51AM -0800, Paul E. McKenney wrote: > > On Thu, Nov 16, 2017 at 10:16:22AM -0800, Nick Desaulniers wrote: > > > > On Thu, Nov 16, 2017 at 06:34:17PM +0100, Peter Zijlstra wrote: > > > >> So the problem is that its very very hard (and painful) to find these > > > >> bugs. Getting the tools people to comment on these specific > > > >> optimizations would really help lots. > > > > > > No doubt; I do not disagree with you. Kernel developers have very > > > important use cases for the language. > > > > > > But the core point I'm trying to make is "do we need to block this > > > patch set until issues with the C standards body in regards to the > > > kernels memory model are resolved?" I would hope the two are > > > orthogonal and that we'd take them and then test them even more > > > extensively than the developer has in order to find out. > > > > Given that I have been working on getting the C and C++ standards to > > correctly handle rcu_dereference() for more than ten years, I recommend > > -against- waiting on standardization in the strongest possible terms. > > And if you think that ten years is bad, the Java standards community has > > been struggling with out-of-thin-air (OOTA) values for almost 20 years. > > And the C and C++ standards haven't solved OOTA, either. > > The problem is, if we go ahead with this change, the compiler *will* break > some address dependencies and something will eventually go wrong. At that > point, what do we do? Turn off some random compiler optimisation? Add a > random barrier()? > > We don't necessarily need standardisation, but we at least need guarantees > from the compiler implementation that LTO/PGO will respect source level > address dependencies. I don't think we have that today. Ah, if "this patch set" meant "adding LTO", I stand corrected and I apologize for my confusion. I agree that we need LTO/PGO to be housebroken from an LKMM viewpoint before it is enabled. Thanx, Paul