Re: [PATCH V5 01/12] perf/core: Add aux_pause, aux_resume, aux_start_paused

From: Adrian Hunter <adrian.hunter@intel.com>
To: Andi Kleen <ak@linux.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
Date: Mon, 12 Feb 2024 08:43:11 +0200	[thread overview]
Message-ID: <05ac66fc-ddda-46ff-b08d-c15943c16bc1@intel.com> (raw)
In-Reply-To: <ZcaUw4xQT0VcC7IO@tassilo>

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().