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