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

[Mathieu Poirier]

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.

Regards,
Mathieu

> 
> [1] https://lore.kernel.org/patchwork/patch/1318098/
> [2]
> https://lore.kernel.org/lkml/fa6cdf34-88a0-1050-b9ea-556d0a9438cb@xxxxxxx/
> 
> Thanks,
> Sai
> 
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