Re: [PATCHv2 2/4] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink_perf()

[Sai Prakash Ranjan]

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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