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 11:41:15 +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 09:49:53AM +0100, Suzuki Poulose wrote:
> On 10/23/20 8:39 AM, Peter Zijlstra wrote:

> > So then I don't understand the !->owner issue, that only happens when
> > the task dies, which cannot be concurrent with event creation. Are you
> 
> Part of the patch from Sai, fixes this by avoiding the dereferencing
> after event creation (by caching it). But the kernel events needs
> fixing.

I'm fundamentally failing here. Creating a link to the sink is strictly
event-creation time. Why would you ever need it again later? Later you
already have the sink setup.