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

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Message-ID: <20080527092830.3535ee7b@bike.lwn.net>
Date:	Tue, 27 May 2008 09:28:30 -0600
From:	Jonathan Corbet <corbet@....net>
To:	benh@...nel.crashing.org
Cc:	Linux Arch list <linux-arch@...r.kernel.org>,
	David Miller <davem@...emloft.net>, scottwood@...escale.com,
	linuxppc-dev@...abs.org, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Trent Piepho <tpiepho@...escale.com>
Subject: Re: MMIO and gcc re-ordering issue

On Tue, 27 May 2008 11:33:46 +1000
Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:

> However, I'm using that as an excuse to bring back my pet subject,
> which is basically, should we instead just finally mandate the use of
> explicit rmb/wmb/mb's (which boils down to barrier() on x86) to
> drivers who want to order memory consistent accesses vs. MMIO ?

To me this seems like asking for a long series of weird bugs in
drivers.  Is the cost of a barrier in an MMIO operation so high that we
can't afford to just put them in?  Alan's suggestion of no-barrier
versions for people who know what they're doing (and who think there's a
reason to care) seems like a better way to go to me.

jon

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