Re: [PATCH] perf: Fix race in perf_mmap_close function

[Namhyung Kim]

On Fri, Sep 11, 2020 at 4:49 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> ugh, that's right.. how about change below

Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Thanks
Namhyung


>
> jirka
>
>
> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 7ed5248f0445..8ab2400aef55 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5868,11 +5868,11 @@ static void perf_pmu_output_stop(struct perf_event *event);
>  static void perf_mmap_close(struct vm_area_struct *vma)
>  {
>         struct perf_event *event = vma->vm_file->private_data;
> -
>         struct perf_buffer *rb = ring_buffer_get(event);
>         struct user_struct *mmap_user = rb->mmap_user;
>         int mmap_locked = rb->mmap_locked;
>         unsigned long size = perf_data_size(rb);
> +       bool detach_rest = false;
>
>         if (event->pmu->event_unmapped)
>                 event->pmu->event_unmapped(event, vma->vm_mm);
> @@ -5903,7 +5903,8 @@ static void perf_mmap_close(struct vm_area_struct *vma)
>                 mutex_unlock(&event->mmap_mutex);
>         }
>
> -       atomic_dec(&rb->mmap_count);
> +       if (atomic_dec_and_test(&rb->mmap_count))
> +               detach_rest = true;
>
>         if (!atomic_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
>                 goto out_put;
> @@ -5912,7 +5913,7 @@ static void perf_mmap_close(struct vm_area_struct *vma)
>         mutex_unlock(&event->mmap_mutex);
>
>         /* If there's still other mmap()s of this buffer, we're done. */
> -       if (atomic_read(&rb->mmap_count))
> +       if (!detach_rest)
>                 goto out_put;
>
>         /*
>