Re: [PATCH V5 01/12] perf/core: Add aux_pause, aux_resume, aux_start_paused

From: Adrian Hunter
Date: Mon Feb 12 2024 - 01:43:31 EST

On 9/02/24 23:10, Andi Kleen wrote:
>> The writes to rb->aux_in_pause_resume must be done
>> only once. It might be possible to get away without
>> WRITE_ONCE(), but really the compiler should be informed
>> not to make assumptions.
> What stops the NMI from firing here?

That would be fine.

>>>> + if (READ_ONCE(rb->aux_in_pause_resume))
>>>> + /* Guard against NMI, NMI loses here */
>>>> + goto out_restore;
> <----------------------- NMI

That would be fine.

>>>> + WRITE_ONCE(rb->aux_in_pause_resume, 1);

>From here on, the NMI will see rb->aux_in_pause_resume == 1
and do nothing, up to the point when rb->aux_in_pause_resume == 0

This code is about guarding against NMI. Interrupts are
disabled, and the rb and event only operate on 1 cpu at a time.

> Even if it isn't racy it needs a clear comment.

It has a comment, but it could be more explanatory. The code
paradigm is also used in perf_pmu_snapshot_aux().