From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f49.google.com (mail-pa0-f49.google.com [209.85.220.49]) by kanga.kvack.org (Postfix) with ESMTP id 60EA66B0255 for ; Sat, 26 Sep 2015 15:52:12 -0400 (EDT) Received: by pacex6 with SMTP id ex6so136200269pac.0 for ; Sat, 26 Sep 2015 12:52:12 -0700 (PDT) Received: from bedivere.hansenpartnership.com (bedivere.hansenpartnership.com. [66.63.167.143]) by mx.google.com with ESMTP id ra1si14753589pbb.202.2015.09.26.12.52.11 for ; Sat, 26 Sep 2015 12:52:11 -0700 (PDT) Message-ID: <1443297128.2181.11.camel@HansenPartnership.com> Subject: Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' From: James Bottomley Date: Sat, 26 Sep 2015 12:52:08 -0700 In-Reply-To: <2524822.pQu4UKMrlb@vostro.rjw.lan> References: <4357538.Wlf88yQie6@vostro.rjw.lan> <2524822.pQu4UKMrlb@vostro.rjw.lan> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Rafael J. Wysocki" Cc: Viresh Kumar , Johannes Berg , Greg Kroah-Hartman , Linaro Kernel Mailman List , QCA ath9k Development , Intel Linux Wireless , "linux-doc@vger.kernel.org" , Linux Kernel Mailing List , "linux-arm-kernel@lists.infradead.org" , Linux ACPI , "open list:BLUETOOTH DRIVERS" , "open list:AMD IOMMU (AMD-VI)" , "netdev@vger.kernel.org" , "open list:NETWORKING DRIVERS (WIRELESS)" , "open list:TARGET SUBSYSTEM" , "open list:ULTRA-WIDEBAND (UWB) SUBSYSTEM:" , "open list:EDAC-CORE" , Linux Memory Management List , "moderated list:SOUND - SOC LAYER / DYNAMIC AUDIO POWER MANAGEM..." On Fri, 2015-09-25 at 22:58 +0200, Rafael J. Wysocki wrote: > On Friday, September 25, 2015 01:25:49 PM Viresh Kumar wrote: > > On 25 September 2015 at 13:33, Rafael J. Wysocki wrote: > > > You're going to change that into bool in the next patch, right? > > > > Yeah. > > > > > So what if bool is a byte and the field is not word-aligned > > > > Its between two 'unsigned long' variables today, and the struct isn't packed. > > So, it will be aligned, isn't it? > > > > > and changing > > > that byte requires a read-modify-write. How do we ensure that things remain > > > consistent in that case? > > > > I didn't understood why a read-modify-write is special here? That's > > what will happen > > to most of the non-word-sized fields anyway? > > > > Probably I didn't understood what you meant.. > > Say you have three adjacent fields in a structure, x, y, z, each one byte long. > Initially, all of them are equal to 0. > > CPU A writes 1 to x and CPU B writes 2 to y at the same time. > > What's the result? I think every CPU's cache architecure guarantees adjacent store integrity, even in the face of SMP, so it's x==1 and y==2. If you're thinking of old alpha SMP system where the lowest store width is 32 bits and thus you have to do RMW to update a byte, this was usually fixed by padding (assuming the structure is not packed). However, it was such a problem that even the later alpha chips had byte extensions. James -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org