Re: Interrupts, smp_load_acquire(), smp_store_release(), etc.

Paul E. McKenney
2018-10-20T21:06:56Z

On Sat, Oct 20, 2018 at 10:22:29PM +0200, Andrea Parri wrote:
> [...]
>
> > The second (informal) litmus test has a more interesting Linux-kernel
> > counterpart:
> >
> > void t1_interrupt(void)
> > {
> > r0 = READ_ONCE(y);
> > smp_store_release(&x, 1);
> > }
> >
> > void t1(void)
> > {
> > smp_store_release(&y, 1);
> > }
> >
> > void t2(void)
> > {
> > r1 = smp_load_acquire(&x);
> > r2 = smp_load_acquire(&y);
> > }
> >
> > On store-reordering architectures that implement smp_store_release()
> > as a memory-barrier instruction followed by a store, the interrupt could
> > arrive betweentimes in t1(), so that there would be no ordering between
> > t1_interrupt()'s store to x and t1()'s store to y. This could (again,
> > in paranoid theory) result in the outcome r0==0 && r1==0 && r2==1.
>
> FWIW, I'd rather call "paranoid" the act of excluding such outcome ;-)
> but I admit that I've only run this test in *my mind*: in an SC world,
>
> CPU1 CPU2
>
> t1()
> t1_interrupt()
> r0 = READ_ONCE(y); // =0
> t2()
> r1 = smp_load_acquire(&x); // =0
> smp_store_release(&x, 1);
> smp_store_release(&y, 1);
> r2 = smp_load_acquire(&y); // =1

OK, so did I get the outcome messed up again? :-/

Thanx, Paul

> > So how paranoid should we be with respect to interrupt handlers for
> > smp_store_release(), smp_load_acquire(), and the various RMW atomic
> > operations that are sometimes implemented with separate memory-barrier
> > instructions? ;-)
>
> Good question! ;-)
>
> Andrea
>
>
> >
> > Thanx, Paul
> >
>