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 13:10:27 -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

> 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?

> >> +  if (READ_ONCE(rb->aux_in_pause_resume))
> >> +	/* Guard against NMI, NMI loses here */
> >> +          goto out_restore;
<----------------------- NMI
> >> +  WRITE_ONCE(rb->aux_in_pause_resume, 1);


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

-Andi