linux-kernel - Re: MMIO and gcc re-ordering issue

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Date:	Mon, 2 Jun 2008 12:36:43 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>, linux-arch@...r.kernel.org,
	scottwood@...escale.com, linuxppc-dev@...abs.org,
	alan@...rguk.ukuu.org.uk, linux-kernel@...r.kernel.org,
	tpiepho@...escale.com
Subject: Re: MMIO and gcc re-ordering issue


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > Here's a UNTESTED patch for x86 that may or may not compile and 
> > work, and which serializes (on a compiler level) the IO accesses 
> > against regular memory accesses.
> 
> Ok, so it at least boots on x86-32. Thus probably on x86-64 too (since 
> the code is now shared). I didn't look at whether it generates much 
> bigger code due to the potential extra serialization, but some of the 
> code generation I looked at looked fine.
> 
> IOW, it doesn't at least create any _obviously_ worse code, and it 
> should be arguably safer than assuming the compiler does volatile 
> accesses the way we want it to.

ok, to pursue this topic of making readl*/writel*() more robust i picked 
up your patch into -tip and created a new topic branch for it: 
tip/x86/mmio.

The patch passed initial light testing in -tip (~30 successful random 
self-builds and bootups on various mixed 32-bit/64-bit boxes) but it's 
still v2.6.27 material IMO.

Failures in this area are subtle so there's no good way to tell whether 
it works as intended - we need wider testing. I've also added the 
tip/x86/mmio topic to tip/auto-x86-next rules as well so these changes 
will be picked up by tomorrow's linux-next tree as well, and by the next 
-mm iteration.

	Ingo
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