On Fri, Sep 06, 2019 at 04:57:22PM -0400, Alan Stern wrote:
> Currently the Linux Kernel Memory Model gives an incorrect response
> for the following litmus test:
>
> C plain-WWC
>
> {}
>
> P0(int *x)
> {
> WRITE_ONCE(*x, 2);
> }
>
> P1(int *x, int *y)
> {
> int r1;
> int r2;
> int r3;
>
> r1 = READ_ONCE(*x);
> if (r1 == 2) {
> smp_rmb();
> r2 = *x;
> }
> smp_rmb();
> r3 = READ_ONCE(*x);
> WRITE_ONCE(*y, r3 - 1);
> }
>
> P2(int *x, int *y)
> {
> int r4;
>
> r4 = READ_ONCE(*y);
> if (r4 > 0)
> WRITE_ONCE(*x, 1);
> }
>
> exists (x=2 /\ 1:r2=2 /\ 2:r4=1)
>
> The memory model says that the plain read of *x in P1 races with the
> WRITE_ONCE(*x) in P2.
>
> The problem is that we have a write W and a read R related by neither
> fre or rfe, but rather W ->coe W' ->rfe R, where W' is an intermediate
> write (the WRITE_ONCE() in P0). In this situation there is no
> particular ordering between W and R, so either a wr-vis link from W to
> R or an rw-xbstar link from R to W would prove that the accesses
> aren't concurrent.
>
> But the LKMM only looks for a wr-vis link, which is equivalent to
> assuming that W must execute before R. This is not necessarily true
> on non-multicopy-atomic systems, as the WWC pattern demonstrates.
>
> This patch changes the LKMM to accept either a wr-vis or a reverse
> rw-xbstar link as a proof of non-concurrency.
>
> Signed-off-by: Alan Stern <st...@rowland.harvard.edu>
Queued and pushed for review and testing, thank you very much!
Thanx, Paul
> ---
>
> tools/memory-model/linux-kernel.cat | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: usb-devel/tools/memory-model/linux-kernel.cat
> ===================================================================
> --- usb-devel.orig/tools/memory-model/linux-kernel.cat
> +++ usb-devel/tools/memory-model/linux-kernel.cat
> @@ -197,7 +197,7 @@ empty (wr-incoh | rw-incoh | ww-incoh) a
> (* Actual races *)
> let ww-nonrace = ww-vis & ((Marked * W) | rw-xbstar) & ((W * Marked) |
> wr-vis)
> let ww-race = (pre-race & co) \ ww-nonrace
> -let wr-race = (pre-race & (co? ; rf)) \ wr-vis
> +let wr-race = (pre-race & (co? ; rf)) \ wr-vis \ rw-xbstar^-1
> let rw-race = (pre-race & fr) \ rw-xbstar
>
> flag ~empty (ww-race | wr-race | rw-race) as data-race
>
