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SubjectRe: [PATCH v3 1/3] perf: Allow using AUX data in perf samples
Date
Peter Zijlstra <peterz@infradead.org> writes:

> I have the below delta on top of this patch.
>
> And while I get why we need recursion protection for pmu::snapshot_aux,
> I'm a little puzzled on why it is over the padding, that is, why isn't
> the whole of aux_in_sampling inside (the newly minted)
> perf_pmu_snapshot_aux() ?

No reason. Too long staring at that code by myself.

> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6237,7 +6237,7 @@ perf_output_sample_ustack(struct perf_ou
> }
> }
>
> -static unsigned long perf_aux_sample_size(struct perf_event *event,
> +static unsigned long perf_prepare_sample_aux(struct perf_event *event,
> struct perf_sample_data *data,
> size_t size)
> {
> @@ -6275,9 +6275,9 @@ static unsigned long perf_aux_sample_siz
> return data->aux_size;
> }
>
> -long perf_pmu_aux_sample_output(struct perf_event *event,
> - struct perf_output_handle *handle,
> - unsigned long size)
> +long perf_pmu_snapshot_aux(struct perf_event *event,
> + struct perf_output_handle *handle,
> + unsigned long size)

That makes more sense indeed.

> {
> unsigned long flags;
> long ret;
> @@ -6318,11 +6318,12 @@ static void perf_aux_sample_output(struc
>
> /*
> * Guard against NMI hits inside the critical section;
> - * see also perf_aux_sample_size().
> + * see also perf_prepare_sample_aux().
> */
> WRITE_ONCE(rb->aux_in_sampling, 1);
> + barrier();

Isn't WRITE_ONCE() barrier enough on its own? My thinking was that we
only need a compiler barrier here, hence the WRITE_ONCE.

Thanks,
--
Alex

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