linux-kernel - Re: [PATCH 2/2] perf report: Don't add to histogram when there is no thread found

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Date:   Wed, 28 Jun 2023 11:34:26 +0100
From:   James Clark <james.clark@....com>
To:     Ian Rogers <irogers@...gle.com>, Namhyung Kim <namhyung@...nel.org>
Cc:     linux-perf-users@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        John Garry <john.g.garry@...cle.com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] perf report: Don't add to histogram when there is no
 thread found



On 27/06/2023 18:19, Ian Rogers wrote:
> On Tue, Jun 27, 2023 at 9:58 AM Namhyung Kim <namhyung@...nel.org> wrote:
>>
>> On Tue, Jun 27, 2023 at 9:43 AM Ian Rogers <irogers@...gle.com> wrote:
>>>
>>> On Mon, Jun 26, 2023 at 5:02 PM Namhyung Kim <namhyung@...nel.org> wrote:
>>>>
>>>> On Mon, Jun 26, 2023 at 05:10:58PM +0100, James Clark wrote:
>>>>> thread__find_map() chooses to exit without assigning a thread to the
>>>>> addr_location in some scenarios, for example when there are samples from
>>>>> a guest and perf_guest == false. This results in a segfault when adding
>>>>> to the histogram because it uses unguarded accesses to the thread member
>>>>> of the addr_location.
>>>>
>>>> Looking at the commit 0dd5041c9a0ea ("perf addr_location: Add
>>>> init/exit/copy functions") that introduced the change, I'm not sure if
>>>> it's the intend behavior.
>>>>
>>>> It might change maps and map, but not thread.  Then I think no reason
>>>> to not set the al->thread at the beginning.
>>>>
>>>> How about this?  Ian?
>>>> (I guess we can get rid of the duplicate 'al->map = NULL' part)
>>>
>>> It seemed strange that we were failing to find a map (the function's
>>> purpose) but then populating the address_location. The change below
>>> brings back that somewhat odd behavior. I'm okay with reverting to the
>>> old behavior, clearly there were users relying on it. We should
>>> probably also copy maps and not just thread, as that was the previous
>>> behavior.
>>
>> Probably.  But it used to support samples without maps and I think
>> that's why it ignores the return value of thread__find_map().  So
>> we can expect al.map is NULL and maybe fine to leave it for now.
>>
>> As machine__resolve() returns -1 if it gets no thread, we should set
>> al.thread when it returns 0.
>>
>> Can I get your Acked-by?
> 
> Yep:
> Acked-by: Ian Rogers <irogers@...gle.com>

Looks good to me too. Should I resend the set with this change instead
of my one?

> 
> Thanks,
> Ian
> 
>> Thanks,
>> Namhyung