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

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Message-ID: <20080603081552.GC117059@sgi.com>
Date:	Tue, 3 Jun 2008 01:15:52 -0700
From:	Jeremy Higdon <jeremy@....com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
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 Tue, Jun 03, 2008 at 02:33:11PM +1000, Nick Piggin wrote:
> On Monday 02 June 2008 19:56, Jes Sorensen wrote:
> > Jeremy Higdon wrote:
> > > We don't actually have that problem on the Altix.  All writes issued
> > > by CPU X will be ordered with respect to each other.  But writes by
> > > CPU X and CPU Y will not be, unless an mmiowb() is done by the
> > > original CPU before the second CPU writes.  I.e.
> > >
> > > 	CPU X	writel
> > > 	CPU X	writel
> > > 	CPU X	mmiowb
> > >
> > > 	CPU Y	writel
> > > 	...
> > >
> > > Note that this implies some sort of locking.  Also note that if in
> > > the above, CPU Y did the mmiowb, that would not work.
> >
> > Hmmm,
> >
> > Then it's less bad than I thought - my apologies for the confusion.
> >
> > 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
writing via some sort of lock.  If the lock release forces a barrier,
that should take care of the problem.

jeremy
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