linux-kernel - Re: [PATCH v2] arm: cmpxchg syscall should data abort if page not write

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Date:	Fri, 18 Mar 2011 16:44:44 +0800
From:	Po-Yu Chuang <ratbert.chuang@...il.com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Ashwin Chaugule <ashbertslists@...il.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Po-Yu Chuang <ratbert@...aday-tech.com>, tony@...mide.com,
	lkml <linux-kernel@...r.kernel.org>, joe@...ches.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] arm: cmpxchg syscall should data abort if page not write

Dear Nicolas Pitre,

On Fri, Mar 18, 2011 at 1:57 AM, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> On Thu, 17 Mar 2011, Ashwin Chaugule wrote:
>>
>> Just beginning to understand the subtleties involved, so please
>> correct me if I'm wrong.
>> Wont this patch also fix the problem that was brought up with futexes
>> on ARM SMP ?
>
> Nope.  The code being fixed here was suptly broken so it needs fixing.
> However this code is almost never used, if at all, as it is a fall-back
> solution for when all the better alternatives are not available for some
> reasons (and I'm still wondering what those reasons are for Po-Yu Chuang
> to actually use that code).  In practice this bug should have affected
> no one.

We met this problem while porting an v5 SMP processor because kernel
selects NEEDS_SYSCALL_FOR_CMPXCHG by default if CPU_32v5.

After we added our own implementation of __kuser_cmpxchg, this code
is not needed anymore. But since this is actually a bug, it is still a good
idea to submit a patch. :-)

> If you have SMP then this code is definitively not what you should be
> using.

Best regards,
Po-Yu Chuang
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