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

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Message-ID: <4843C3D7.7000609@sgi.com>
Date:	Mon, 02 Jun 2008 11:56:39 +0200
From:	Jes Sorensen <jes@....com>
To:	Jeremy Higdon <jeremy@....com>
CC:	Roland Dreier <rdreier@...co.com>, 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

Jeremy Higdon wrote:
> We don't actually have that problem on the Altix.  All writes issued
> by CPU X will be ordered with respect to each other.  But writes by
> CPU X and CPU Y will not be, unless an mmiowb() is done by the
> original CPU before the second CPU writes.  I.e.
> 
> 	CPU X	writel
> 	CPU X	writel
> 	CPU X	mmiowb
> 
> 	CPU Y	writel
> 	...
> 
> Note that this implies some sort of locking.  Also note that if in
> the above, CPU Y did the mmiowb, that would not work.

Hmmm,

Then it's less bad than I thought - my apologies for the confusion.

Would we be able to use Ben's trick of setting a per cpu flag in
writel() then and checking that in spin unlock issuing the mmiowb()
there if needed?


Cheers,
Jes
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