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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Jeremy Higdon <jeremy@sgi.com>
Subject: Re: MMIO and gcc re-ordering issue
Date: Tue, 3 Jun 2008 18:19:05 +1000
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Cc: Jes Sorensen <jes@sgi.com>, Roland Dreier <rdreier@cisco.com>,
       benh@kernel.crashing.org, Arjan van de Ven <arjan@infradead.org>,
       linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
       tpiepho@freescale.com, linuxppc-dev@ozlabs.org, scottwood@freescale.com,
       torvalds@linux-foundation.org, David Miller <davem@davemloft.net>,
       alan@lxorguk.ukuu.org.uk
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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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