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List:       linux-kernel
Subject:    Re: Memory barriers and spin_unlock safety
From:       Michael Buesch <mbuesch () freenet ! de>
Date:       2006-03-05 2:04:40
Message-ID: 200603050304.41436.mbuesch () freenet ! de
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On Saturday 04 March 2006 11:58, you wrote:
> Linus Torvalds writes:
> 
> > PPC has an absolutely _horrible_ memory ordering implementation, as far as 
> > I can tell. The thing is broken. I think it's just implementation 
> > breakage, not anything really fundamental, but the fact that their write 
> > barriers are expensive is a big sign that they are doing something bad. 
> 
> An smp_wmb() is just an eieio on PPC, which is pretty cheap.  I made
> wmb() be a sync though, because it seemed that there were drivers that
> expected wmb() to provide an ordering between a write to memory and a
> write to an MMIO register.  If that is a bogus assumption then we
> could make wmb() lighter-weight (after auditing all the drivers we're
> interested in, of course, ...).

In the bcm43xx driver there is code which looks like the following:

/* Write some coherent DMA memory */
wmb();
/* Write MMIO, which depends on the DMA memory
 * write to be finished.
 */

Are the assumptions in this code correct? Is wmb() the correct thing
to do here?
I heavily tested this code on PPC UP and did not see any anormaly, yet.

-- 
Greetings Michael.

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