Re: Litmus test for question from Al Viro

[Alan Stern]

On Mon, Oct 05, 2020 at 10:12:48AM +0100, Will Deacon wrote:
> On Mon, Oct 05, 2020 at 09:20:03AM +0100, Will Deacon wrote:
> > On Sun, Oct 04, 2020 at 10:38:46PM -0400, Alan Stern wrote:
> > > Considering the bug in herd7 pointed out by Akira, we should rewrite P1 as:
> > > 
> > > P1(int *x, int *y)
> > > {
> > > 	int r2;
> > > 
> > > 	r = READ_ONCE(*y);
> > 
> > (r2?)
> > 
> > > 	WRITE_ONCE(*x, r2);
> > > }
> > > 
> > > Other than that, this is fine.
> > 
> > But yes, module the typo, I agree that this rewrite is much better than the
> > proposal above. The definition of control dependencies on arm64 (per the Arm
> > ARM [1]) isn't entirely clear that it provides order if the WRITE is
> > executed on both paths of the branch, and I believe there are ongoing
> > efforts to try to tighten that up. I'd rather keep _that_ topic separate
> > from the "bug in herd" topic to avoid extra confusion.
> 
> Ah, now I see that you're changing P1 here, not P0. So I'm now nervous
> about claiming that this is a bug in herd without input from Jade or Luc,
> as it does unfortunately tie into the definition of control dependencies
> and it could be a deliberate choice.

I think you misunderstood.  The bug in herd7 affects the way it handles 
P1, not P0.  With

	r2 = READ_ONCE(*y);
	WRITE_ONCE(*x, r2);

herd7 generates a data dependency from the read to the write.  With

	WRITE_ONCE(*x, READ_ONCE(*y));

it doesn't generate any dependency, even though the code does exactly 
the same thing as far as the memory model is concerned.  That's the bug 
I was referring to.

The failure to recognize the dependency in P0 should be considered a 
combined limitation of the memory model and herd7.  It's not a simple 
mistake that can be fixed by a small rewrite of herd7; rather it's a 
deliberate choice we made based on herd7's inherent design.  We 
explicitly said that control dependencies extend only to the code in the 
branches of an "if" statement; anything beyond the end of the statement 
is not considered to be dependent.

Alan