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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=ham 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 C091DC04AB5 for ; Mon, 3 Jun 2019 18:45:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 96DDC27199 for ; Mon, 3 Jun 2019 18:45:06 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=synopsys.com header.i=@synopsys.com header.b="V1wKpwqt" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726761AbfFCSpG (ORCPT ); Mon, 3 Jun 2019 14:45:06 -0400 Received: from smtprelay-out1.synopsys.com ([198.182.47.102]:42428 "EHLO smtprelay-out1.synopsys.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726055AbfFCSpF (ORCPT ); Mon, 3 Jun 2019 14:45:05 -0400 Received: from mailhost.synopsys.com (badc-mailhost2.synopsys.com [10.192.0.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)) (No client certificate requested) by smtprelay-out1.synopsys.com (Postfix) with ESMTPS id 388A9C1E7C; Mon, 3 Jun 2019 18:45:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=synopsys.com; s=mail; t=1559587515; bh=gCcSw5SmMUnuwclvmjbYEVrqMocAkqSgIMbd6lSdpaw=; h=Subject:To:CC:References:From:Date:In-Reply-To:From; b=V1wKpwqtPYP26Le9BZy9giEwn0ggrjNmXmi2v0qFJTKQdt8tyi0et+KKpsGKH+ksJ o2OmjExD1aO3qwdcVpbCpPTngcGO5SSYceAfQXh38VQqxSN+pz+KHB1sG6hoYKssFM QvT9NzOMr2buEu84qBJvEEKRR9Pdkk+1Qemxcq1AmLWZDntJrdcIMTqBUSCrUVOyxM HM8HZabEPDx4LR9mIvASZzsvjq0Le25L0fXno0jnRIo2Cy8H6bwcifYgyjfZ58+F+I ywqJnJQIqgYH76dz32ez+ocQpWTP36jU3Z+vs744Vp3j3j/pJExY4kbiXTFHoP0POw deK0X2OUJrk4w== Received: from US01WEHTC3.internal.synopsys.com (us01wehtc3.internal.synopsys.com [10.15.84.232]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)) (No client certificate requested) by mailhost.synopsys.com (Postfix) with ESMTPS id 62A86A0101; Mon, 3 Jun 2019 18:45:00 +0000 (UTC) Received: from IN01WEHTCB.internal.synopsys.com (10.144.199.106) by US01WEHTC3.internal.synopsys.com (10.15.84.232) with Microsoft SMTP Server (TLS) id 14.3.408.0; Mon, 3 Jun 2019 11:45:00 -0700 Received: from IN01WEHTCA.internal.synopsys.com (10.144.199.103) by IN01WEHTCB.internal.synopsys.com (10.144.199.105) with Microsoft SMTP Server (TLS) id 14.3.408.0; Tue, 4 Jun 2019 00:14:57 +0530 Received: from [10.10.161.35] (10.10.161.35) by IN01WEHTCA.internal.synopsys.com (10.144.199.243) with Microsoft SMTP Server (TLS) id 14.3.408.0; Tue, 4 Jun 2019 00:15:09 +0530 Subject: Re: single copy atomicity for double load/stores on 32-bit systems To: David Laight , Peter Zijlstra , Will Deacon , "Paul E. McKenney" CC: "linux-arch@vger.kernel.org" , arcml , lkml Newsgroups: gmane.linux.kernel.arc,gmane.linux.kernel.cross-arch,gmane.linux.kernel References: <2fd3a455-6267-5d21-c530-41964a4f6ce9@synopsys.com> <895ec12746c246579aed5dd98ace6e38@AcuMS.aculab.com> From: Vineet Gupta Openpgp: preference=signencrypt Autocrypt: addr=vgupta@synopsys.com; keydata= mQINBFEffBMBEADIXSn0fEQcM8GPYFZyvBrY8456hGplRnLLFimPi/BBGFA24IR+B/Vh/EFk B5LAyKuPEEbR3WSVB1x7TovwEErPWKmhHFbyugdCKDv7qWVj7pOB+vqycTG3i16eixB69row lDkZ2RQyy1i/wOtHt8Kr69V9aMOIVIlBNjx5vNOjxfOLux3C0SRl1veA8sdkoSACY3McOqJ8 zR8q1mZDRHCfz+aNxgmVIVFN2JY29zBNOeCzNL1b6ndjU73whH/1hd9YMx2Sp149T8MBpkuQ cFYUPYm8Mn0dQ5PHAide+D3iKCHMupX0ux1Y6g7Ym9jhVtxq3OdUI5I5vsED7NgV9c8++baM 7j7ext5v0l8UeulHfj4LglTaJIvwbUrCGgtyS9haKlUHbmey/af1j0sTrGxZs1ky1cTX7yeF nSYs12GRiVZkh/Pf3nRLkjV+kH++ZtR1GZLqwamiYZhAHjo1Vzyl50JT9EuX07/XTyq/Bx6E dcJWr79ZphJ+mR2HrMdvZo3VSpXEgjROpYlD4GKUApFxW6RrZkvMzuR2bqi48FThXKhFXJBd JiTfiO8tpXaHg/yh/V9vNQqdu7KmZIuZ0EdeZHoXe+8lxoNyQPcPSj7LcmE6gONJR8ZqAzyk F5voeRIy005ZmJJ3VOH3Gw6Gz49LVy7Kz72yo1IPHZJNpSV5xwARAQABtCpWaW5lZXQgR3Vw dGEgKGFsaWFzKSA8dmd1cHRhQHN5bm9wc3lzLmNvbT6JAj4EEwECACgCGwMGCwkIBwMCBhUI AgkKCwQWAgMBAh4BAheABQJbBYpwBQkLx0HcAAoJEGnX8d3iisJeChAQAMR2UVbJyydOv3aV jmqP47gVFq4Qml1weP5z6czl1I8n37bIhdW0/lV2Zll+yU1YGpMgdDTHiDqnGWi4pJeu4+c5 xsI/VqkH6WWXpfruhDsbJ3IJQ46//jb79ogjm6VVeGlOOYxx/G/RUUXZ12+CMPQo7Bv+Jb+t NJnYXYMND2Dlr2TiRahFeeQo8uFbeEdJGDsSIbkOV0jzrYUAPeBwdN8N0eOB19KUgPqPAC4W HCg2LJ/o6/BImN7bhEFDFu7gTT0nqFVZNXlOw4UcGGpM3dq/qu8ZgRE0turY9SsjKsJYKvg4 djAaOh7H9NJK72JOjUhXY/sMBwW5vnNwFyXCB5t4ZcNxStoxrMtyf35synJVinFy6wCzH3eJ XYNfFsv4gjF3l9VYmGEJeI8JG/ljYQVjsQxcrU1lf8lfARuNkleUL8Y3rtxn6eZVtAlJE8q2 hBgu/RUj79BKnWEPFmxfKsaj8of+5wubTkP0I5tXh0akKZlVwQ3lbDdHxznejcVCwyjXBSny d0+qKIXX1eMh0/5sDYM06/B34rQyq9HZVVPRHdvsfwCU0s3G+5Fai02mK68okr8TECOzqZtG cuQmkAeegdY70Bpzfbwxo45WWQq8dSRURA7KDeY5LutMphQPIP2syqgIaiEatHgwetyVCOt6 tf3ClCidHNaGky9KcNSQ Message-ID: Date: Mon, 3 Jun 2019 11:44:52 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <895ec12746c246579aed5dd98ace6e38@AcuMS.aculab.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.10.161.35] Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/31/19 2:41 AM, David Laight wrote: >> While it seems reasonable form hardware pov to not implement such atomicity by >> default it seems there's an additional burden on application writers. They could >> be happily using a lockless algorithm with just a shared flag between 2 threads >> w/o need for any explicit synchronization. But upgrade to a new compiler which >> aggressively "packs" struct rendering long long 32-bit aligned (vs. 64-bit before) >> causing the code to suddenly stop working. Is the onus on them to declare such >> memory as c11 atomic or some such. > A 'new' compiler can't suddenly change the alignment rules for structure elements. > The alignment rules will be part of the ABI. > > More likely is that the structure itself is unexpectedly allocated on > an 8n+4 boundary due to code changes elsewhere. Indeed thats what I meant that the layout changed as is typical of a new compiler.