linux-kernel - Re: [PATCH] [POWERPC] Improve (in|out)

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Date:	Thu, 22 May 2008 15:56:52 -0700 (PDT)
From:	Trent Piepho <tpiepho@...escale.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	David Miller <davem@...emloft.net>, scottwood@...escale.com,
	linuxppc-dev@...abs.org, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

On Fri, 23 May 2008, Benjamin Herrenschmidt wrote:
> On Tue, 2008-05-20 at 15:53 -0700, David Miller wrote:
>> From: Scott Wood <scottwood@...escale.com>
>> Date: Tue, 20 May 2008 17:43:58 -0500
>>> David Miller wrote:
>>>> The __volatile__ in the asm construct disallows movement of the
>>>> inline asm relative to statements surrounding it.
>>>>
>>>> The only reason barrier() in kernel.h needs a memory clobber is
>>>> because of a bug in ancient versions of gcc.  In fact, I think
>>>> that memory clobber might even be removable.
>>>
>>> Current versions of GCC seem quite happy to move non-asm memory accesses
>>> around a volatile asm without a memory clobber; see the test Trent posted.
>>
>> Indeed, and even the GCC manual is clear about this.
>
> So what is the scope of that problem ?
>
> IE. Take an x86 version of that test, writing to memory, doing a writel
> to some MMIO, then another memory write, can those be re-ordered with
> the current x86 version of writel ?

Yes, the same thing can happen on x86.  As far as I could tell, this is
something that all other arches can have happen.  Usually aliasing prevents
it, but it's not hard to constuct a test case where it doesn't.
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