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

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Message-Id: <200806031819.06559.nickpiggin@yahoo.com.au>
Date:	Tue, 3 Jun 2008 18:19:05 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Jeremy Higdon <jeremy@....com>
Cc:	Jes Sorensen <jes@....com>, 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

On Tuesday 03 June 2008 18:15, Jeremy Higdon wrote:
> On Tue, Jun 03, 2008 at 02:33:11PM +1000, Nick Piggin wrote:
> > On Monday 02 June 2008 19:56, Jes Sorensen wrote:

> > > 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?
> >
> > Yes you could, but your writels would still not be strongly ordered
> > within (or outside) spinlock regions, which is what Linus wants (and
> > I kind of agree with).
>
> Yes they would be.  Writes from the same CPU are always ordered.  Writes
> from different CPUs are not, but that's only a concern if you protect

They are not strongly ordered WRT writes to cacheable memory. If they
were, then they would not leak out of spinlocks.

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