linux-kernel - Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global

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Message-ID: <2524822.pQu4UKMrlb@vostro.rjw.lan>
Date:	Fri, 25 Sep 2015 22:58:42 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Johannes Berg <johannes@...solutions.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	QCA ath9k Development <ath9k-devel@....qualcomm.com>,
	Intel Linux Wireless <ilw@...ux.intel.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	"open list:BLUETOOTH DRIVERS" <linux-bluetooth@...r.kernel.org>,
	"open list:AMD IOMMU (AMD-VI)" <iommu@...ts.linux-foundation.org>,
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	<linux-wireless@...r.kernel.org>,
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	<alsa-devel@...a-project.org>
Subject: Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock'

On Friday, September 25, 2015 01:25:49 PM Viresh Kumar wrote:
> On 25 September 2015 at 13:33, Rafael J. Wysocki <rjw@...ysocki.net> 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?

Thanks,
Rafael

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