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

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Message-ID: <483C7BE7.10204@nortel.com>
Date:	Tue, 27 May 2008 15:23:51 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Roland Dreier <rdreier@...co.com>
CC:	James Bottomley <James.Bottomley@...senPartnership.com>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>, linuxppc-dev@...abs.org,
	scottwood@...escale.com, torvalds@...ux-foundation.org,
	tpiepho@...escale.com, alan@...rguk.ukuu.org.uk,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: MMIO and gcc re-ordering issue

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
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