Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg

Peter Zijlstra
2022-10-14T15:11:32Z

On Fri, Oct 14, 2022 at 04:31:37PM +0200, Ard Biesheuvel wrote:
> + if (slot != -1) {
> + /*
> + * Use release semantics to ensure that ghes_estatus_cached()
> + * running on another CPU will see the updated cache fields if
> + * it can see the new value of the pointer.
> + */
> + victim = xchg_release(ghes_estatus_caches + slot,
> + RCU_INITIALIZER(new_cache));
> +
> + /*
> + * At this point, victim may point to a cached item different
> + * from the one based on which we selected the slot. Instead of
> + * going to the loop again to pick another slot, let's just
> + * drop the other item anyway: this may cause a false cache
> + * miss later on, but that won't cause any problems.
> + */
> + if (victim) {
> + call_rcu(&rcu_dereference(victim)->rcu,
> + ghes_estatus_cache_rcu_free);
}

I think you can use unrcu_pointer() here instead, there should not be a
data dependency since the ->rcu member itself should be otherwise unused
(and if it were, we wouldn't care about its previous content anyway).

But only Alpha cares about that distinction anyway, so *shrug*.

While I much like the xchg() variant; I still don't really fancy the
verbage the sparse nonsense makes us do.

victim = xchg_release(&ghes_estatus_caches[slot], new_cache);
if (victim)
call_rcu(&victim->rcu, ghes_estatus_cache_rcu_free);

is much nicer code.

Over all; I'd simply ignore sparse (I often do).

> + }
> rcu_read_unlock();
> }