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 10:14:44 +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 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.

> 
>> +
>> +	rb = ring_buffer_get(event);
>> +	if (!rb)
>> +		return;
>> +
>> +	local_irq_save(flags);
>> +	/* Guard against NMI, NMI loses here */
>> +	if (READ_ONCE(rb->aux_in_pause_resume))
>> +		goto out_restore;
>> +	WRITE_ONCE(rb->aux_in_pause_resume, 1);
> 
> 
>> +	barrier();
>> +	__perf_event_aux_pause(event, pause);
>> +	barrier();
>> +	WRITE_ONCE(rb->aux_in_pause_resume, 0);
> 
> Dito.
> 

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.