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SubjectRe: [PATCH v3 1/3] perf: Allow using AUX data in perf samples
On Mon, Oct 28, 2019 at 05:27:12PM +0100, Peter Zijlstra wrote:
> 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() ?

That is, given the previous delta, the below.

---
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6292,9 +6292,17 @@ long perf_pmu_snapshot_aux(struct perf_e
* IRQs need to be disabled to prevent IPIs from racing with us.
*/
local_irq_save(flags);
+ /*
+ * Guard against NMI hits inside the critical section;
+ * see also perf_prepare_sample_aux().
+ */
+ WRITE_ONCE(rb->aux_in_sampling, 1);
+ barrier();

ret = event->pmu->snapshot_aux(event, handle, size);

+ barrier();
+ WRITE_ONCE(rb->aux_in_sampling, 0);
local_irq_restore(flags);

return ret;
@@ -6316,13 +6324,6 @@ static void perf_aux_sample_output(struc
if (!rb)
return;

- /*
- * Guard against NMI hits inside the critical section;
- * see also perf_prepare_sample_aux().
- */
- WRITE_ONCE(rb->aux_in_sampling, 1);
- barrier();
-
size = perf_pmu_snapshot_aux(sampler, handle, data->aux_size);

/*
@@ -6348,9 +6349,6 @@ static void perf_aux_sample_output(struc
}

out_clear:
- barrier();
- WRITE_ONCE(rb->aux_in_sampling, 0);
-
ring_buffer_put(rb);
}

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