linux-kernel - Re: Uses for memory barriers

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Message-Id: <200609090049.20416.oliver@neukum.org>
Date:	Sat, 9 Sep 2006 00:49:19 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	paulmck@...ibm.com, David Howells <dhowells@...hat.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Uses for memory barriers

Am Samstag, 9. September 2006 00:25 schrieb Alan Stern:
> On Fri, 8 Sep 2006, Oliver Neukum wrote:
> 
> > > Again you have misunderstood.  The original code was _not_ incorrect.  I 
> > > was asking: Given the code as stated, would the assertion ever fail?
> > 
> > I claim the right to call code that fails its own assertions incorrect. :-)
> 
> Touche!
> 
> > > The code _was_ correct for my purposes, namely, to illustrate a technical 
> > > point about the behavior of memory barriers.
> > 
> > I would say that the code may fail the assertion purely based
> > on the formal definition of a memory barrier. And do so in a subtle
> > and inobvious way.
> 
> But what _is_ the formal definition of a memory barrier?  I've never seen 
> one that was complete and correct.

I' d say "mb();" is "rmb();wmb();"

and they work so that:

CPU 0

a = TRUE;
wmb();
b = TRUE;

CPU 1

if (b) {
	rmb();
	assert(a);
}

is correct. Possibly that is not a complete definition though.

	Regards
		Oliver
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