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

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Message-Id: <1212655433.9496.109.camel@pasglop>
Date:	Thu, 05 Jun 2008 18:43:53 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Jes Sorensen <jes@....com>
Cc:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Jeremy Higdon <jeremy@....com>,
	Roland Dreier <rdreier@...co.com>,
	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 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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