linux-kernel - Re: Uses for memory barriers

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Date:	Fri, 8 Sep 2006 23:46:20 +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 Freitag, 8. September 2006 23:26 schrieb Alan Stern:
> On Fri, 8 Sep 2006, Oliver Neukum wrote:
> 
> > It seems you are correct.
> > Therefore the correct code on CPU 1 would be:
> > 
> > y = -1;
> > b = 1;
> > //mb();
> > //x = a;
> > while (y < 0) relax();
> > 
> > mb();
> > x = a;
> > 
> > assert(x==1 || y==1);   //???
> > 
> > And yes, it is confusing. I've been forced to change my mind twice.
> 
> 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. :-)

> 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.

	Regards
		Oliver
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