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

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Message-ID: <20080531080427.GH5405@ucw.cz>
Date:	Sat, 31 May 2008 10:04:27 +0200
From:	Pavel Machek <pavel@....cz>
To:	Roland Dreier <rdreier@...co.com>
Cc:	benh@...nel.crashing.org, Arjan van de Ven <arjan@...radead.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	tpiepho@...escale.com, linuxppc-dev@...abs.org,
	scottwood@...escale.com, torvalds@...ux-foundation.org,
	David Miller <davem@...emloft.net>, alan@...rguk.ukuu.org.uk
Subject: Re: MMIO and gcc re-ordering issue

Hi!

>  > Though it's my understanding that at least ia64 does require the
>  > explicit barriers anyway, so we are still in a dodgy situation here
>  > where it's not clear what drivers should do and we end up with
>  > possibly excessive barriers on powerpc where I end up with both
>  > the wmb/rmb/mb that were added for ia64 -and- the ones I have in
>  > readl/writel to make them look synchronous... Not nice.
> 
> ia64 is a disaster with a slightly different ordering problem -- the
> mmiowb() issue.  I know Ben knows far too much about this, but for big
> SGI boxes, you sometimes need mmiowb() to avoid problems with driver
> code that does totally sane stuff like
> 
> 	spin_lock(&mmio_lock);
> 	writel(val1, reg1);
> 	writel(val2, reg2);
> 	spin_unlock(&mmio_lock);
> 
> If that snippet is called on two CPUs at the same time, then the device
> might see a sequence like
> 
> 	CPU1 -- write reg1
> 	CPU2 -- write reg1
> 	CPU1 -- write reg2
> 	CPU2 -- write reg2
> 
> in spite of the fact that everything is totally ordered on the CPUs by
> the spin lock.
> 
> The reason this is such a disaster is because the code looks right,
> makes sense, and works fine on 99.99% of all systems out there.  So I
> would bet that 99% of our drivers have missing mmiowb() "bugs" -- no one
> has plugged the hardware into an Altix box and cared enough to stress
> test it.

Yes, ia64 is a disaster, and needs its spinlock implementation fixed
:-).
							Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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