linux-kernel - Re: [PATCHv2 2/4] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink

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Date:   Fri, 23 Oct 2020 15:44:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Suzuki Poulose <suzuki.poulose@....com>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Mike Leach <mike.leach@...aro.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@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, coresight@...ts.linaro.org,
        Stephen Boyd <swboyd@...omium.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv2 2/4] coresight: tmc-etf: Fix NULL ptr dereference in
 tmc_enable_etf_sink_perf()

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/@...k0/u --per-thread app1
> > > 
> > > and
> > > 
> > > perf record -e cs_etm/@...k0/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.

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