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Date: Tue, 27 May 2008 15:23:51 -0600
From: "Chris Friesen" <cfriesen@nortel.com>
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To: Roland Dreier <rdreier@cisco.com>
Subject: Re: MMIO and gcc re-ordering issue
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Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
	tpiepho@freescale.com,
	James Bottomley <James.Bottomley@HansenPartnership.com>,
	linuxppc-dev@ozlabs.org, scottwood@freescale.com,
	torvalds@linux-foundation.org,
	David Miller <davem@davemloft.net>, alan@lxorguk.ukuu.org.uk,
	Arjan van de Ven <arjan@infradead.org>
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Roland Dreier wrote:

> Writes are posted yes, but not reordered arbitrarily.  If I have code like:
> 
> 	spin_lock(&mmio_lock);
> 	writel(val1, reg1);
> 	writel(val2, reg2);
> 	spin_unlock(&mmio_lock);
> 
> then I have a reasonable expectation that if two CPUs run this at the
> same time, their writes to reg1/reg2 won't be interleaved with each
> other (because the whole section is inside a spinlock).  And Altix
> violates that expectation.

Does that necessarily follow?

If you've got a large system with multiple pci bridges, could you end up 
with posted writes coming from different cpus taking a different amount 
of time to propagate to a device and thus colliding?

Chris