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

Paul E. McKenney
2013-09-27T16:04:26Z

On Fri, Sep 27, 2013 at 05:34:34PM +0200, Peter Zijlstra wrote:
> 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?

Probably not. I was recently reminded of it by some people on the C++
standards committee. I had first heard of it about 5 years ago, but
hadn't heard definitively until quite recently.

I defer to the Itanium maintainers to actually make the required changes,
should they choose to do so. I suppose that one way to handle it in the
Linux kernel would be to make ACCESS_ONCE() be architecture specific,
with Itanium placing a memory barrier either before or after --- either
would work. But since Itanium seems to run Linux reliably, I am guessing
that the probability of misordering is quite low. But again, the ball
is firmly in the Itanium maintainers' courts, and I have added them on CC.

Thanx, Paul