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

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Date: Fri, 9 Feb 2024 01:49:25 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: Adrian Hunter <adrian.hunter@...el.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 Fri, Feb 09, 2024 at 10:14:44AM +0200, Adrian Hunter wrote:
> On 9/02/24 02:13, Andi Kleen wrote:
> >> +static void __perf_event_aux_pause(struct perf_event *event, bool pause)
> >> +{
> >> +	if (pause) {
> >> +		if (!READ_ONCE(event->aux_paused)) {
> >> +			WRITE_ONCE(event->aux_paused, 1);
> >> +			event->pmu->stop(event, PERF_EF_PAUSE);
> >> +		}
> >> +	} else {
> >> +		if (READ_ONCE(event->aux_paused)) {
> >> +			WRITE_ONCE(event->aux_paused, 0);
> >> +			event->pmu->start(event, PERF_EF_RESUME);
> >> +		}
> > 
> > This doesn't look atomic. Either the READ/WRITE once are not needed,
> > or you need an actually atomic construct.
> 
> Yes READ_ONCE / WRITE_ONCE is not really needed here.

Normally you are supposed to add a comment on what the interaction
is (similar to what is done for spinlock declarations)

-Andi