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

Paul E. McKenney
2023-01-24T16:23:10Z

On Tue, Jan 24, 2023 at 04:11:14PM +0100, Jonas Oberhauser wrote:
>
>
> On 1/24/2023 3:54 PM, Paul E. McKenney wrote:
> > On Tue, Jan 24, 2023 at 12:09:48PM +0100, Andrea Parri wrote:
> > > > There is the one below, but I am (1) not sure that I have it right,
> > > > (2) not immediately certain that the Linux-kernel implementation would
> > > > forbid it, (3) not immediately sure that it should be forbidden.
> > > >
> > > > In the meantime, thoughts?
> > > As it stands, P0 to completion, then P1 to completion, then P2 to
> > > completion should meet the "exists" clause; I guess we want "x=1"
> > > in the clause (or the values of the stores to "x" exchanged).
> > OK, so I still don't have it right. ;-)
> >
> > Make that x=1. I think.
> >
>
> If it is x=1, why doesn't LKMM forbid it?
> Because T1:y=1 is read by T1 before the GP, the whole CS is before the GP,
> i.e.,
>
> srcu_read_unlock(s, r1); ->rcu-order synchronize_srcu(s);
>
> The GP is furthermore po;prop;strong-fence;prop;po ordered before the
> unlock, which you can shuffle around to get
>    Wx=2  ->prop;po;rcu-order;po ;  prop;strong-fence  Wx=2
> or
>    Wx=2  ->rb  Wx=2
> which is forbidden because rb is irreflexive.
>
> Right?

Yes according to herd7, hence the "I think". 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)