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Date: Fri, 9 Feb 2024 13:10:27 -0800
From: Andi Kleen <ak@linux.intel.com>
To: Adrian Hunter <adrian.hunter@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Thomas Richter <tmricht@linux.ibm.com>,
	Hendrik Brueckner <brueckner@linux.ibm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Mike Leach <mike.leach@linaro.org>,
	James Clark <james.clark@arm.com>, coresight@lists.linaro.org,
	linux-arm-kernel@lists.infradead.org,
	Yicong Yang <yangyicong@hisilicon.com>,
	Jonathan Cameron <jonathan.cameron@huawei.com>,
	Will Deacon <will@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
	Ian Rogers <irogers@google.com>, linux-kernel@vger.kernel.org,
	linux-perf-users@vger.kernel.org
Subject: Re: [PATCH V5 01/12] perf/core: Add aux_pause, aux_resume,
 aux_start_paused
Message-ID: <ZcaUw4xQT0VcC7IO@tassilo>
References: <20240208113127.22216-1-adrian.hunter@intel.com>
 <20240208113127.22216-2-adrian.hunter@intel.com>
 <ZcVuK1nnxG18ps-a@tassilo>
 <d53ef26d-a7e6-4b3b-b593-be75425c0e77@intel.com>
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> 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