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Date:   Fri, 30 Oct 2020 22:56:09 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
To:     Mathieu Poirier <mathieu.poirier@linaro.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Suzuki Poulose <suzuki.poulose@arm.com>,
        Mike Leach <mike.leach@linaro.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>, coresight@lists.linaro.org,
        Stephen Boyd <swboyd@chromium.org>,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCHv2 2/4] coresight: tmc-etf: Fix NULL ptr dereference in
 tmc_enable_etf_sink_perf()
In-Reply-To: <20201030164806.GB1301231@xps15>
References: <174e6461-4d46-cb65-c094-c06ee3b21568@arm.com>
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 <bd8c136d-9dfa-a760-31f9-eb8d6698aced@arm.com>
 <20201023105431.GM2594@hirez.programming.kicks-ass.net>
 <2457de8f-8bc3-b350-fdc7-61276da31ce6@arm.com>
 <20201023131628.GY2628@hirez.programming.kicks-ass.net>
 <728fd89c-78f2-0c5c-0443-c91c62b02f0e@arm.com>
 <20201023134416.GA2628@hirez.programming.kicks-ass.net>
 <20201023203729.GA819775@xps15>
 <70e3a508af119be481c8f0a0acf0a44d@codeaurora.org>
 <20201030164806.GB1301231@xps15>
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Hi Mathieu,

On 2020-10-30 22:18, Mathieu Poirier wrote:
> On Fri, Oct 30, 2020 at 01:29:56PM +0530, Sai Prakash Ranjan wrote:
>> Hello guys,
>> 
>> On 2020-10-24 02:07, Mathieu Poirier wrote:
>> > On Fri, Oct 23, 2020 at 03:44:16PM +0200, Peter Zijlstra wrote:
>> > > On Fri, Oct 23, 2020 at 02:29:54PM +0100, Suzuki Poulose wrote:
>> > > > On 10/23/20 2:16 PM, Peter Zijlstra wrote:
>> > > > > On Fri, Oct 23, 2020 at 01:56:47PM +0100, Suzuki Poulose wrote:
>> > >
>> > > > > > That way another session could use the same sink if it is free. i.e
>> > > > > >
>> > > > > > perf record -e cs_etm/@sink0/u --per-thread app1
>> > > > > >
>> > > > > > and
>> > > > > >
>> > > > > > perf record -e cs_etm/@sink0/u --per-thread app2
>> > > > > >
>> > > > > > both can work as long as the sink is not used by the other session.
>> > > > >
>> > > > > Like said above, if sink is shared between CPUs, that's going to be a
>> > > > > trainwreck :/ Why do you want that?
>> > > >
>> > > > That ship has sailed. That is how the current generation of systems are,
>> > > > unfortunately. But as I said, this is changing and there are guidelines
>> > > > in place to avoid these kind of topologies. With the future
>> > > > technologies, this will be completely gone.
>> > >
>> > > I understand that the hardware is like that, but why do you want to
>> > > support this insanity in software?
>> > >
>> > > If you only allow a single sink user (group) at the same time, your
>> > > problem goes away. Simply disallow the above scenario, do not allow
>> > > concurrent sink users if sinks are shared like this.
>> > >
>> > > Have the perf-record of app2 above fail because the sink is in-user
>> > > already.
>> >
>> > I agree with you that --per-thread scenarios are easy to deal with, but
>> > to
>> > support cpu-wide scenarios events must share a sink (because there is
>> > one event
>> > per CPU).  CPU-wide support can't be removed because it has been around
>> > for close to a couple of years and heavily used. I also think using the
>> > pid of
>> > the process that created the events, i.e perf, is a good idea.  We just
>> > need to
>> > agree on how to gain access to it.
>> >
>> > In Sai's patch you objected to the following:
>> >
>> > > +     struct task_struct *task = READ_ONCE(event->owner);
>> > > +
>> > > +     if (!task || is_kernel_event(event))
>> >
>> > Would it be better to use task_nr_pid(current) instead of event->owner?
>> > The end
>> > result will be exactly the same.  There is also no need to check the
>> > validity of
>> > @current since it is a user process.
>> >
>> 
>> We have devices deployed where these crashes are seen consistently,
>> so for some immediate relief, could we atleast get some fix in this
>> cycle without major design overhaul which would likely take more time.
>> Perhaps my first patch [1] without any check for owner or
>> I can post a new version as Suzuki suggested [2] dropping the export
>> of is_kernel_event(). Then we can always work on top of it based on 
>> the
>> conclusion of this discussion, we will atleast not have the systems
>> crash in the meantime, thoughts?
> 
> For the time being I think [1], exactly the way it is, is a reasonable 
> way
> forward.
> 

Sure, I just checked now and [1] still applies neatly on top of 
coresight
next branch.

[1] https://lore.kernel.org/patchwork/patch/1318098/

Thanks,
Sai

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