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

[Peter Zijlstra]

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.