On Fri, Mar 05, 2021 at 09:12:30AM +0800, Boqun Feng wrote: > On Thu, Mar 04, 2021 at 11:11:42AM -0500, Alan Stern wrote:
> > Forget about local variables for the time being and just consider > > > > dep ; [Plain] ; rfi > > > > For example: > > > > A: r1 = READ_ONCE(x); > > y = r1; > > B: r2 = READ_ONCE(y); > > > > Should B be ordered after A? I don't see how any CPU could hope to > > excute B before A, but maybe I'm missing something. > > > > Agreed. > > > There's another twist, connected with the fact that herd7 can't detect > > control dependencies caused by unexecuted code. If we have: > > > > A: r1 = READ_ONCE(x); > > if (r1) > > WRITE_ONCE(y, 5); > > r2 = READ_ONCE(y); > > B: WRITE_ONCE(z, r2); > > > > then in executions where x == 0, herd7 doesn't see any control > > dependency. But CPUs do see control dependencies whenever there is a > > conditional branch, whether the branch is taken or not, and so they will > > never reorder B before A. > > > > Right, because B in this example is a write, what if B is a read that > depends on r2, like in my example? Let y be a pointer to a memory > location, and initialized as a valid value (pointing to a valid memory > location) you example changed to: > > A: r1 = READ_ONCE(x); > if (r1) > WRITE_ONCE(y, 5); > C: r2 = READ_ONCE(y); > B: r3 = READ_ONCE(*r2); > > , then A don't have the control dependency to B, because A and B is > read+read. So B can be ordered before A, right? Yes, I think that's right: Both C and B can be executed before A. > > One last thing to think about: My original assessment or Björn's problem > > wasn't right, because the dep in (dep ; rfi) doesn't include control > > dependencies. Only data and address. So I believe that the LKMM > > Ah, right. I was mising that part (ctrl is not in dep). So I guess my > example is pointless for the question we are discussing here ;-( > > > wouldn't consider A to be ordered before B in this example even if x > > was nonzero. > > Yes, and similar to my example (changing B to a read). > > I did try to run my example with herd, and got confused no matter I make > dep; [Plain]; rfi as to-r (I got the same result telling me a reorder > can happen). Now the reason is clear, because this is a ctrl; rfi not a > dep; rfi. > > Thanks so much for walking with me on this ;-) You're welcome. At this point, it looks like the only remaining question is whether to include (dep ; [Plain] ; rfi) in to-r. This doesn't seem to be an urgent question. Alan
