From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0A490C2D0DB for ; Thu, 23 Jan 2020 17:32:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DD5F821569 for ; Thu, 23 Jan 2020 17:32:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729092AbgAWRcM convert rfc822-to-8bit (ORCPT ); Thu, 23 Jan 2020 12:32:12 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([146.101.78.151]:48710 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727453AbgAWRcL (ORCPT ); Thu, 23 Jan 2020 12:32:11 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-253-NQWIpuFmOiaG-mYGmDywkg-1; Thu, 23 Jan 2020 17:32:07 +0000 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Thu, 23 Jan 2020 17:32:06 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Thu, 23 Jan 2020 17:32:06 +0000 From: David Laight To: 'Will Deacon' CC: "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" , "kernel-team@android.com" , Michael Ellerman , Peter Zijlstra , Linus Torvalds , Segher Boessenkool , Christian Borntraeger , Luc Van Oostenryck , Arnd Bergmann , Peter Oberparleiter , Masahiro Yamada , Nick Desaulniers Subject: RE: [PATCH v2 00/10] Rework READ_ONCE() to improve codegen Thread-Topic: [PATCH v2 00/10] Rework READ_ONCE() to improve codegen Thread-Index: AQHV0gKFFdfdrbkjnU+llP+aeIA9U6f4esgQgAAC9ICAAANF8A== Date: Thu, 23 Jan 2020 17:32:06 +0000 Message-ID: <2bfe2be6da484f15b0d229dd02d16ae6@AcuMS.aculab.com> References: <20200123153341.19947-1-will@kernel.org> <26ad7a8a975c4e06b44a3184d7c86e5f@AcuMS.aculab.com> <20200123171641.GC20126@willie-the-truck> In-Reply-To: <20200123171641.GC20126@willie-the-truck> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-MC-Unique: NQWIpuFmOiaG-mYGmDywkg-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Will Deacon > Sent: 23 January 2020 17:17 > > On Thu, Jan 23, 2020 at 05:07:40PM +0000, David Laight wrote: > > From: Will Deacon > > > Sent: 23 January 2020 15:34 > > ... > > > * Only warn once at build-time if GCC prior to 4.8 is detected... > > > > > > * ... and then raise the minimum GCC version to 4.8, with an error for > > > older versions of the compiler > > > > If the kernel compiled with gcc 4.7 is likely to be buggy, don't these > > need to be in the other order? > > > > Otherwise you need to keep the old versions for use with the old > > compilers. > > I think it depends how much we care about those older compilers. My series > first moves it to "Good luck mate, you're on your own" and then follows up > with a "Let me take that off you it's sharp". Depends on how 'sharp' it is. If the kernel suffers from the code example in the gcc bug itself (where 'volatile' is lost and some code is moved out of a loop) then things will really break somewhere odd. OTOH if it might generate code that reads something twice you'd have to be unlucky as well. Oh - and I need to find a newer compiler :-( David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)