Re: [RFC][PATCH 08/12] s390: Replace cmpxchg_double() with cmpxchg128()

Peter Zijlstra
2023-01-10T08:34:06Z

On Tue, Jan 10, 2023 at 08:23:05AM +0100, Heiko Carstens wrote:

> So, Alexander Gordeev reported that this code was already prior to your
> changes potentially broken with respect to missing READ_ONCE() within the
> cmpxchg_double() loops.

Unless there's an early exit, that shouldn't matter. If you managed to
read garbage the cmpxchg itself will simply fail and the loop retries.

> @@ -1294,12 +1306,16 @@ static void hw_perf_event_update(struct perf_event *event, int flush_all)
> num_sdb++;
>
> /* Reset trailer (using compare-double-and-swap) */
> + /* READ_ONCE() 16 byte header */
> + prev.val = __cdsg(&te->header.val, 0, 0);
> do {
> + old.val = prev.val;
> + new.val = prev.val;
> + new.f = 0;
> + new.a = 1;
> + new.overflow = 0;
> + prev.val = __cdsg(&te->header.val, old.val, new.val);
> + } while (prev.val != old.val);

So this, and

> + /* READ_ONCE() 16 byte header */
> + prev.val = __cdsg(&te->header.val, 0, 0);
> do {
> + old.val = prev.val;
> + new.val = prev.val;
> + orig_overflow = old.overflow;
> + new.f = 0;
> + new.overflow = 0;
> if (idx == aux->alert_mark)
> + new.a = 1;
> else
> + new.a = 0;
> + prev.val = __cdsg(&te->header.val, old.val, new.val);
> + } while (prev.val != old.val);

this case are just silly and expensive. If that initial read is split
and manages to read gibberish the cmpxchg will fail and we retry anyway.

> + /* READ_ONCE() 16 byte header */
> + prev.val = __cdsg(&te->header.val, 0, 0);
> do {
> + old.val = prev.val;
> + new.val = prev.val;
> + *overflow = old.overflow;
> + if (old.f) {
> /*
> * SDB is already set by hardware.
> * Abort and try to set somewhere
> @@ -1490,10 +1509,10 @@ static bool aux_set_alert(struct aux_buffer *aux, unsigned long alert_index,
> */
> return false;
> }
> + new.a = 1;
> + new.overflow = 0;
> + prev.val = __cdsg(&te->header.val, old.val, new.val);
> + } while (prev.val != old.val);

And while this case has an early exit, it only cares about a single bit
(although you made it a full word) and so also shouldn't care. If
aux_reset_buffer() returns false, @overflow isn't consumed.

So I really don't see the point of this patch.