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

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Date:	Tue, 27 May 2008 11:07:05 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	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

 > Um, OK, you've said write twice now ... I was assuming you meant read.
 > Even on an x86, writes are posted, so there's no way a spin lock could
 > serialise a write without an intervening read to flush the posting
 > (that's why only reads have a relaxed version on altix).  Or is there
 > something else I'm missing?

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.

 - R.
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