Re: Uses for memory barriers

[Oliver Neukum <oliver@neukum.org>]

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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/