linux-kernel - Re: [PATCH V5 01/12] perf/core: Add aux_pause, aux_resume, aux_start

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Date: Mon, 12 Feb 2024 08:43:11 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Andi Kleen <ak@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Thomas Richter <tmricht@...ux.ibm.com>,
 Hendrik Brueckner <brueckner@...ux.ibm.com>,
 Suzuki K Poulose <suzuki.poulose@....com>, Mike Leach
 <mike.leach@...aro.org>, James Clark <james.clark@....com>,
 coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
 Yicong Yang <yangyicong@...ilicon.com>,
 Jonathan Cameron <jonathan.cameron@...wei.com>, Will Deacon
 <will@...nel.org>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
 Ian Rogers <irogers@...gle.com>, linux-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org
Subject: Re: [PATCH V5 01/12] perf/core: Add aux_pause, aux_resume,
 aux_start_paused

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
again.

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().