linux-kernel - Re: [PATCH] tile: use a more conservative __my_cpu_offset in CONFIG

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Message-ID: <5249BC69.6080506@tilera.com>
Date:	Mon, 30 Sep 2013 14:01:13 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Will Deacon <will.deacon@....com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Nicolas Pitre <nico@...aro.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] tile: use a more conservative __my_cpu_offset in CONFIG_PREEMPT

On 9/30/2013 5:45 AM, Will Deacon wrote:
> On Fri, Sep 27, 2013 at 09:13:15PM +0100, Chris Metcalf wrote:
>> On 9/26/2013 1:57 PM, Will Deacon wrote:
>>> Hi Chris,
>>>
>>> On Thu, Sep 26, 2013 at 06:24:53PM +0100, Chris Metcalf wrote:
>>>> [...]
>>>> +static inline unsigned long __my_cpu_offset(void)
>>>> +{
>>>> +	unsigned long tp;
>>>> +	register unsigned long *sp asm("sp");
>>>> +	asm("move %0, tp" : "=r" (tp) : "m" (*sp));
>>>> +	return tp;
>>>> +}
>>> Hehe, nice to see this hack working out for you too. One thing to check is
>>> whether you have any funky addressing modes (things like writeback or
>>> post-increment), since the "m" constraint can bite you if you don't actually
>>> use it in the asm.
>> Well, we do have post increments, though I don't see why this is a problem here.
>> We define a target specific constraint "U" that excludes post-increments, but
>> again I don't see why "m" would cause trouble here.  What was your experience?
> GCC assumes that each "m" operand is used *exactly once* in the asm, so if
> it decided to generate a post-increment/writeback addressing mode, you can
> end up with pointers off by a word if you didn't make use of the constraint
> in the code. You can try using "o", but GCC sometimes decides that's
> impossible during reload, so we end up combining it with the (undocumented,
> ARM-specific) "Q" constraint which is simply a [rN] addressing mode.

OK, makes sense.  I've changed the code to use a "U" constraint (pushed up to the linux-tile tree).

Thanks!

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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