Subject: Re: MMIO and gcc re-ordering issue
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reply-To: benh@kernel.crashing.org
To: Jes Sorensen <jes@sgi.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>,
       Nick Piggin <nickpiggin@yahoo.com.au>, Jeremy Higdon <jeremy@sgi.com>,
       Roland Dreier <rdreier@cisco.com>,
       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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Date: Thu, 05 Jun 2008 18:43:53 +1000
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On Thu, 2008-06-05 at 10:40 +0200, Jes Sorensen wrote:
> Jesse Barnes wrote:
> > Now, in hindsight, using a PIO write set & test flag approach in 
> > writeX/spin_unlock (ala powerpc) might have been a better approach, but iirc 
> > that never came up in the discussion, probably because we were focused on PCI 
> > posting and not uncached vs. cached ordering.
> 
> Hi Jesse,
> 
> I am going to take a stab at implementing this so we can see how much
> of an impact it will have.

Note that the powerpc implementation currently clears the flag
on spin_lock and tests it on unlock. We are considering changing
that to not touch the flag on spin_lock and just clear it whenever
we do a sync (ie, on unlock, on explicit mmiowb, and possibly even
on readl's where we happen to do sync's).

Ben.


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