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=-1.0 required=3.0 tests=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 55C8FC282CE for ; Tue, 4 Jun 2019 07:41:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 361E324C53 for ; Tue, 4 Jun 2019 07:41:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726960AbfFDHlT (ORCPT ); Tue, 4 Jun 2019 03:41:19 -0400 Received: from mail-lj1-f176.google.com ([209.85.208.176]:45419 "EHLO mail-lj1-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726637AbfFDHlS (ORCPT ); Tue, 4 Jun 2019 03:41:18 -0400 Received: by mail-lj1-f176.google.com with SMTP id m23so412355lje.12; Tue, 04 Jun 2019 00:41:17 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=ZAHr1x2OGrSpVVmyEF6SL+BemidY5crrkYYK4WkhFYw=; b=oCBie5I3iCCTadEzaA41MpPoyxl7PNo/ljtORXFf1+LnK6gzh2MOkiPLlFFCIkYhmS N/h7PccGPfVr9b00EB3cAHIMmF5SPIGlU0yXPo9Rc/pNO4IEhUptbe7IVfzcZakU9pBb r4i9bpi8jSbwSOy21fWlgwksa4N77d7TUD4yx3rSN26V5nVlVzr33pm3np4+HWIemKWB 7/heXvEa3kgljnCyP6K7R1qWNNVImH7yxy+jMbP7An2HvduKRihgRGf4DnWd4c6SKeWP bLdnQWMyjWVciwJPibU3z/qPM6UNRtA9BPicxOsZ8C6FyvfToYop2rxrOztQ6ctnRzRB zYVg== X-Gm-Message-State: APjAAAXlnIXcP0gWQB5Zoz1PpkoKzYFmOZgsQMvoOgQApaCGnheKKXOH xO9oCIWm7uCi9JeusoglHhb/c8Q4pkavjklKsvI= X-Google-Smtp-Source: APXvYqzLztAMA/8xqZvEThx/dBoNLHWeEXrEpWdQwYRs3xI2FxEn7V2swIn8FrGUNYkkKJxgDcCVDKMKQoQ+1GalhoQ= X-Received: by 2002:a2e:2b8d:: with SMTP id r13mr3443413ljr.145.1559634076658; Tue, 04 Jun 2019 00:41:16 -0700 (PDT) MIME-Version: 1.0 References: <2fd3a455-6267-5d21-c530-41964a4f6ce9@synopsys.com> <20190531082112.GH2623@hirez.programming.kicks-ass.net> <20190603201324.GN28207@linux.ibm.com> In-Reply-To: <20190603201324.GN28207@linux.ibm.com> From: Geert Uytterhoeven Date: Tue, 4 Jun 2019 09:41:04 +0200 Message-ID: Subject: Re: single copy atomicity for double load/stores on 32-bit systems To: "Paul E. McKenney" Cc: Vineet Gupta , Peter Zijlstra , Will Deacon , arcml , lkml , "linux-arch@vger.kernel.org" Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Paul, On Mon, Jun 3, 2019 at 10:14 PM Paul E. McKenney wrote: > On Mon, Jun 03, 2019 at 06:08:35PM +0000, Vineet Gupta wrote: > > On 5/31/19 1:21 AM, Peter Zijlstra wrote: > > >> I'm not sure how to interpret "natural alignment" for the case of double > > >> load/stores on 32-bit systems where the hardware and ABI allow for 4 byte > > >> alignment (ARCv2 LDD/STD, ARM LDRD/STRD ....) > > > Natural alignment: !((uintptr_t)ptr % sizeof(*ptr)) > > > > > > For any u64 type, that would give 8 byte alignment. the problem > > > otherwise being that your data spans two lines/pages etc.. > > > > Sure, but as Paul said, if the software doesn't expect them to be atomic by > > default, they could span 2 hardware lines to keep the implementation simpler/sane. > > I could imagine 8-byte types being only four-byte aligned on 32-bit systems, > but it would be quite a surprise on 64-bit systems. Or two-byte aligned? M68k started with a 16-bit data bus, and alignment rules were retained when gaining a wider data bus. BTW, do any platforms have issues with atomicity of 4-byte types on 16-bit data buses? I believe some embedded ARM or PowerPC do have such buses. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds