From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Bottomley Subject: Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' Date: Sat, 26 Sep 2015 12:52:08 -0700 Message-ID: <1443297128.2181.11.camel@HansenPartnership.com> References: <4357538.Wlf88yQie6@vostro.rjw.lan> <2524822.pQu4UKMrlb@vostro.rjw.lan> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: "moderated list:SOUND - SOC LAYER / DYNAMIC AUDIO POWER MANAGEM..." , Linaro Kernel Mailman List , "open list:TARGET SUBSYSTEM" , "linux-doc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , Viresh Kumar , "open list:ULTRA-WIDEBAND \(UWB\) SUBSYSTEM:" , "open list:NETWORKING DRIVERS \(WIRELESS\)" , QCA ath9k Development , Linux Kernel Mailing List , Intel Linux Wireless , Linux ACPI , "open list:AMD IOMMU \(AMD-VI\)" , "open list:BLUETOOTH DRIVERS" , Linux Memory Management List , Greg Kroah-Hartman Return-path: In-Reply-To: <2524822.pQu4UKMrlb-sKB8Sp2ER+y1GS7QM15AGw@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org List-Id: netdev.vger.kernel.org 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