Re: [PATCH] checkpatch: Make the memory barrier test noisier

Peter Zijlstra
2013-09-27T15:35:06Z

On Fri, Sep 27, 2013 at 08:17:50AM -0700, Paul E. McKenney wrote:
> > Barriers are fundamentally about order; and order only makes sense if
> > there's more than 1 party to the game.
>
> Oddly enough, there is one exception that proves the rule... On Itanium,
> suppose we have the following code, with x initially equal to zero:
>
> CPU 1: ACCESS_ONCE(x) = 1;
>
> CPU 2: r1 = ACCESS_ONCE(x); r2 = ACCESS_ONCE(x);
>
> Itanium architects have told me that it really is possible for CPU 2 to
> see r1==1 and r2==0. Placing a memory barrier between CPU 2's pair of
> fetches prevents this, but without any other memory barrier to pair with.

Oh man.. its really past time to sink that itanic already.

I suppose it allows the cpu to reorder the reads in its pipeline and the
memory barrier disallows this. Curious.. does our memory-barriers.txt
file mention this 'fun' fact?