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

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Message-ID: <18492.37741.798622.696166@cargo.ozlabs.ibm.com>
Date:	Wed, 28 May 2008 09:04:13 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	"Chris Friesen" <cfriesen@...tel.com>
Cc:	Roland Dreier <rdreier@...co.com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, tpiepho@...escale.com,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	linuxppc-dev@...abs.org, scottwood@...escale.com,
	torvalds@...ux-foundation.org, David Miller <davem@...emloft.net>,
	alan@...rguk.ukuu.org.uk, Arjan van de Ven <arjan@...radead.org>
Subject: Re: MMIO and gcc re-ordering issue

Chris Friesen writes:

> Roland Dreier wrote:
> 
> > Writes are posted yes, but not reordered arbitrarily.  If I have code like:
> > 
> > 	spin_lock(&mmio_lock);
> > 	writel(val1, reg1);
> > 	writel(val2, reg2);
> > 	spin_unlock(&mmio_lock);
> > 
> > then I have a reasonable expectation that if two CPUs run this at the
> > same time, their writes to reg1/reg2 won't be interleaved with each
> > other (because the whole section is inside a spinlock).  And Altix
> > violates that expectation.
> 
> Does that necessarily follow?
> 
> If you've got a large system with multiple pci bridges, could you end up 
> with posted writes coming from different cpus taking a different amount 
> of time to propagate to a device and thus colliding?

On powerpc we explicitly make sure that can't happen.  That's the "do
a sync in spin_unlock if there were any writels since the last
spin_lock" magic.  The sync instruction makes sure the writes have got
to the host bridge before the spinlock is unlocked.

Paul.
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