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

Andrea Parri
2018-10-20T20:22:41Z

[...]

> 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

> 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
>