linux-kernel - Re: [Question] null pointer risk of kernel workqueue

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Message-ID: <20171023140321.GY1302522@devbig577.frc2.facebook.com>
Date:   Mon, 23 Oct 2017 07:03:21 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Li Bin <huawei.libin@...wei.com>
Cc:     tanxiaofei <tanxiaofei@...wei.com>, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org, John Garry <john.garry@...wei.com>
Subject: Re: [Question] null pointer risk of kernel workqueue

Hello,

On Mon, Oct 23, 2017 at 09:34:11AM +0800, Li Bin wrote:
> 
> 
> on 2017/10/21 23:35, Tejun Heo wrote:
> > On Fri, Oct 20, 2017 at 02:57:18PM +0800, tanxiaofei wrote:
> >> Hi Tejun,
> >>
> >> Any comments about this?
> > 
> > I think you're confused, or at least can't understand what you're
> > trying to say.  Can you create a rero?
> > 
> 
> Hi Tejun,
> The case is as following:
> 
> worker_thread()
> |-spin_lock_irq()
> |-process_one_work()
>     |-worker->current_pwq = pwq
>     |-spin_unlock_irq()
>     |-worker->current_func(work)
>     |-spin_lock_irq()
>     |-worker->current_pwq = NULL
> |-spin_unlock_irq()
>                                                                     //interrupt here
>                                                                     |-irq_handler
>                                                                         |-__queue_work()
>                                                                             //assuming that the wq is draining
>                                                                             |-if (unlikely(wq->flags & __WQ_DRAINING) &&WARN_ON_ONCE(!is_chained_work(wq)))
>                                                                                 |-is_chained_work(wq)
>                                                                                     |-current_wq_worker() // Here, 'current' is the interrupted worker!
>                                                                                         |-current->current_pwq is NULL here!
> |-schedule()
> 
> And I think the following patch can solve the bug, right?
> 
> diff --git a/kernel/workqueue_internal.h b/kernel/workqueue_internal.h
> index 8635417..650680c 100644
> --- a/kernel/workqueue_internal.h
> +++ b/kernel/workqueue_internal.h
> @@ -59,7 +59,7 @@ struct worker {
>   */
>  static inline struct worker *current_wq_worker(void)
>  {
> -       if (current->flags & PF_WQ_WORKER)
> +       if (!in_irq() && (current->flags & PF_WQ_WORKER))
>                 return kthread_data(current);
>         return NULL;
>  }

Yeah, that makes sense to me.  Can you please resend the patch with
patch description and SOB?

Thanks.

-- 
tejun