Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Jonas Oberhauser
2023-01-24T16:41:02Z

On 1/24/2023 5:22 PM, Paul E. McKenney wrote:
> I clearly recall some
> store-based lack of ordering after a grace period from some years back,
> and am thus far failing to reproduce it.
>
> And here is another attempt that herd7 actually does allow.
>
> So what did I mess up this time? ;-)
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> C C-srcu-observed-4
>
> (*
> * Result: Sometimes
> *
> * The Linux-kernel implementation is suspected to forbid this.
> *)
>
> {}
>
> P0(int *x, int *y, int *z, struct srcu_struct *s)
> {
> int r1;
>
> r1 = srcu_read_lock(s);
> WRITE_ONCE(*y, 2);
> WRITE_ONCE(*x, 1);
> srcu_read_unlock(s, r1);
> }
>
> P1(int *x, int *y, int *z, struct srcu_struct *s)
> {
> int r1;
>
> WRITE_ONCE(*y, 1);
> synchronize_srcu(s);
> WRITE_ONCE(*z, 2);
> }
>
> P2(int *x, int *y, int *z, struct srcu_struct *s)
> {
> WRITE_ONCE(*z, 1);
> smp_store_release(x, 2);
> }
>
> exists (x=1 /\ y=1 /\ z=1)

I think even if you implement the unlock as mb() followed by some store
that is read by the gp between mb()s, this would still be allowed.

I have already forgotten the specifics, but I think the power model
allows certain stores never propagating somewhere?
If z=2,z=1,x=2 never propagate to P0, you might start by executing P0,
then P1, and then P2 at which point the memory system decides that x=1
overwrites x=2, and the latter simply doesn't propagate anywhere.

(I'll let anyone who has the model at hand correct me on this, because I
have to take a walk now).

Have fun, jonas