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

[Mathieu Poirier]

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.

Thanks,
Mathieu 

[1]. https://elixir.bootlin.com/linux/latest/source/kernel/events/core.c#L6170

> 
> Only if the hardware has per-CPU sinks can you allow this.