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Date:   Fri, 12 May 2017 12:42:52 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     linux-kernel@...r.kernel.org
Cc:     David Carrillo-Cisneros <davidcc@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "# v4 . 11+" <stable@...r.kernel.org>
Subject: Re: [PATCH] perf/core: Avoid removing shared pmu_context on
 unregister

On Fri, May 12, 2017 at 11:40:32AM +0100, Chris Wilson wrote:
> In commit 1fd7e4169954 ("perf/core: Remove perf_cpu_context::unique_pmu"),
> the search for another user of the pmu_cpu_context was removed, and so
> we unconditionally free it during perf_pmu_unregister. This leads to
> random corruption later and a BUG at mm/percpu.c:689.
> 
> Fixes: 1fd7e4169954 ("perf/core: Remove perf_cpu_context::unique_pmu")
> Signed-off-by: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: David Carrillo-Cisneros <davidcc@...gle.com>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: <stable@...r.kernel.org> # v4.11+
> ---
>  kernel/events/core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index aaefaa27e1a6..e62b6207925c 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -9005,7 +9005,8 @@ void perf_pmu_unregister(struct pmu *pmu)
>  		device_del(pmu->dev);
>  		put_device(pmu->dev);
>  	}
> -	free_pmu_context(pmu);
> +	if (!find_pmu_context(pmu->task_ctx_nr))

Bleh, that find should be under some guard, such as the mutex we held
ealier.

> +		free_pmu_context(pmu);
>  }
>  EXPORT_SYMBOL_GPL(perf_pmu_unregister);
>  
> -- 
> 2.11.0
> 

-- 
Chris Wilson, Intel Open Source Technology Centre

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