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Date:   Fri, 12 May 2017 13:40:37 -0700
From:   David Carrillo-Cisneros <davidcc@...gle.com>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "# v4 . 11+" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] perf/core: Avoid removing shared pmu_context on unregister

On Fri, May 12, 2017 at 4:45 AM, Chris Wilson <chris@...is-wilson.co.uk> 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.
>
> v2: Check for shared pmu_contexts under the mutex.
>
> 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 | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index aaefaa27e1a6..4f60f66b35ad 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -8983,10 +8983,12 @@ EXPORT_SYMBOL_GPL(perf_pmu_register);
>  void perf_pmu_unregister(struct pmu *pmu)
>  {
>         int remove_device;
> +       int remove_context;
>
>         mutex_lock(&pmus_lock);
>         remove_device = pmu_bus_running;
>         list_del_rcu(&pmu->entry);
> +       remove_context = !find_pmu_context(pmu->task_ctx_nr);
>         mutex_unlock(&pmus_lock);
>
>         /*
> @@ -9005,7 +9007,8 @@ void perf_pmu_unregister(struct pmu *pmu)
>                 device_del(pmu->dev);
>                 put_device(pmu->dev);
>         }
> -       free_pmu_context(pmu);
> +       if (remove_context)
> +               free_pmu_context(pmu);
>  }

Shouldn't be cleaner to keep the check in find_pmu_context, just as it
was before commit 1fd7e4169954 ("perf/core: Remove
perf_cpu_context::unique_pmu")?

(Code below untested)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6e75a5c9412d..50d90cbf8418 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8857,7 +8857,8 @@ static struct perf_cpu_context __percpu
*find_pmu_context(int ctxn)
 static void free_pmu_context(struct pmu *pmu)
 {
        mutex_lock(&pmus_lock);
-       free_percpu(pmu->pmu_cpu_context);
+       if (!find_pmu_context(pmu->task_ctx_nr))
+               free_percpu(pmu->pmu_cpu_context);
        mutex_unlock(&pmus_lock);
 }

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