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Message-ID: <05ac66fc-ddda-46ff-b08d-c15943c16bc1@intel.com>
Date: Mon, 12 Feb 2024 08:43:11 +0200
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Subject: Re: [PATCH V5 01/12] perf/core: Add aux_pause, aux_resume,
 aux_start_paused
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
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> <ZcaUw4xQT0VcC7IO@tassilo>
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From: Adrian Hunter <adrian.hunter@intel.com>
Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki,
 Business Identity Code: 0357606 - 4, Domiciled in Helsinki
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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().