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

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Date:	Wed, 11 Jun 2008 13:40:44 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	linux-arch@...r.kernel.org, Roland Dreier <rdreier@...co.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matthew Wilcox <matthew@....cx>,
	Trent Piepho <tpiepho@...escale.com>,
	Russell King <rmk+lkml@....linux.org.uk>,
	David Miller <davem@...emloft.net>, scottwood@...escale.com,
	linuxppc-dev@...abs.org, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: MMIO and gcc re-ordering issue

On Wed, 2008-06-11 at 13:29 +1000, Nick Piggin wrote:
> 
> Exactly, yes. I guess everybody has had good intentions here, but
> as noticed, what is lacking is coordination and documentation.
> 
> You mention strong ordering WRT spin_unlock, which suggests that
> you would prefer to take option #2 (the current powerpc one): io/io
> is ordered and io is contained inside spinlocks, but io/cacheable
> in general is not ordered.

IO/cacheable -is- ordered on powepc in what we believe is the direction
that matter: IO reads are fully ordered vs. anything and IO writes are
ordered vs. previous cacheable stores. The only "relaxed" situation is
IO writes followed by cacheable stores, which I believe shouldn't be
a problem. (except for spinlocks for which we use the flag trick)

> I *would* prefer that io/cacheable actually is strongly ordered with
> the default accessors. Because if you have that, then the driver
> writer never has to care about memory ordering, provided they use
> correct locking for SMP issues. Same as x86. With option 2, there
> are still windows where you could possibly have issues.
> 
> For any high performance drivers that are well maintained (ie. the
> ones where slowdown might be noticed), everyone should have a pretty
> good handle on memory ordering requirements, so it shouldn't take
> long to go through and convert them to relaxed accessors.

Ben.


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