Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408!

[Shiraz Hashim <shiraz.linux.kernel@gmail.com>]

On Thu, Jan 21, 2016 at 11:08 PM, Christoph Lameter <cl@linux.com> wrote:
> Subject: vmstat: Queue work before clearing cpu_stat_off
>
> There is a race between vmstat_shepherd and quiet_vmstat() because
> the responsibility for checking for counter updates changes depending
> on the state of teh bit in cpu_stat_off. So queue the work before
> changing state of the bit in vmstat_shepherd. That way quiet_vmstat
> is guaranteed to remove the work request when clearing the bit and the
> bug in vmstat_update wont trigger anymore.
>
> Signed-off-by: Christoph Lameter <cl@linux.com>
>
> Index: linux/mm/vmstat.c
> ===================================================================
> --- linux.orig/mm/vmstat.c
> +++ linux/mm/vmstat.c
> @@ -1480,12 +1480,14 @@ static void vmstat_shepherd(struct work_
>         get_online_cpus();
>         /* Check processors whose vmstat worker threads have been disabled */
>         for_each_cpu(cpu, cpu_stat_off)
> -               if (need_update(cpu) &&
> -                       cpumask_test_and_clear_cpu(cpu, cpu_stat_off))
> +               if (need_update(cpu)) {
>
>                         queue_delayed_work_on(cpu, vmstat_wq,
>                                 &per_cpu(vmstat_work, cpu), 0);
>
> +                       cpumask_clear_cpu(smp_processor_id(), cpu_stat_off);
> +               }
> +
>         put_online_cpus();
>
>         schedule_delayed_work(&shepherd,


This can alternatively lead to following where vmstat may not be
scheduled for cpu  when it is back from idle.

CPU0:                                            CPU1:
                                                       vmstat_shepherd
<enter idle>                                    queue_delayed_work_on(CPU0)
quiet_vmstat
  cancel_delayed_work
  cpumask_test_and_set_cpu (0->1)

cpumask_clear_cpu(CPU0) (1->0)

-- 
regards
Shiraz Hashim

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