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

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Date:   Mon, 23 Oct 2017 09:34:11 +0800
From:   Li Bin <huawei.libin@...wei.com>
To:     Tejun Heo <tj@...nel.org>, tanxiaofei <tanxiaofei@...wei.com>
CC:     <jiangshanlai@...il.com>, <linux-kernel@...r.kernel.org>,
        John Garry <john.garry@...wei.com>
Subject: Re: [Question] null pointer risk of kernel workqueue



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;
 }


Thanks,
Li Bin

> Thanks.
>