linux-kernel - Re: [PATCH v2] perf/core: Avoid removing shared pmu

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Date:   Fri, 12 May 2017 14:26:19 -0700
From:   David Carrillo-Cisneros <davidcc@...gle.com>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "# v4 . 11+" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] perf/core: Avoid removing shared pmu_context on unregister

>>
>> Shouldn't be cleaner to keep the check in find_pmu_context, just as it
>> was before commit 1fd7e4169954 ("perf/core: Remove
>> perf_cpu_context::unique_pmu")?
>>
>> (Code below untested)
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 6e75a5c9412d..50d90cbf8418 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -8857,7 +8857,8 @@ static struct perf_cpu_context __percpu
>> *find_pmu_context(int ctxn)
>>  static void free_pmu_context(struct pmu *pmu)
>>  {
>>         mutex_lock(&pmus_lock);
>> -       free_percpu(pmu->pmu_cpu_context);
>> +       if (!find_pmu_context(pmu->task_ctx_nr))
>> +               free_percpu(pmu->pmu_cpu_context);
>>         mutex_unlock(&pmus_lock);
>
> We have the problem that find_pmu_context looks for a matching
> task_ctx_nr, but if a second pmu was registered since our list_del and
> before our search, we would wrongly conclude that it was using our pmu
> context, but it had actually allocated a new one for itself.
>

Makes sense. It'd be nice to have that in a comment. Other than that I
am fine with v2.

David