Subject: Re: [Question] null pointer risk of kernel workqueue
To: Tejun Heo <tj@kernel.org>
References: <59C62398.6040101@huawei.com>
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CC: <jiangshanlai@gmail.com>, <linux-kernel@vger.kernel.org>,
        Linuxarm <linuxarm@huawei.com>, John Garry <john.garry@huawei.com>,
        <huawei.libin@huawei.com>
From: tanxiaofei <tanxiaofei@huawei.com>
Message-ID: <59CB6C9C.7000205@huawei.com>
Date: Wed, 27 Sep 2017 17:17:16 +0800
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On 2017/9/25 23:25, Tejun Heo wrote:
> Hello,
> 
> On Sat, Sep 23, 2017 at 05:04:24PM +0800, tanxiaofei wrote:
>> Hi Tejun & Jiangshan,
>>
>> I find an null pointer risk in the code of workqueue. Here is description:
>>
>> If draining, __queue_work() will call the function is_chained_work() to do some checks.
>> In is_chained_work(), worker->current_pwq is used directly. It should be not safe.
>> http://elixir.free-electrons.com/linux/latest/source/kernel/workqueue.c#L1384
>>
>> If you check the thread function of this worker, worker_thread(), you will find worker->current_pwq
>> is null when one work is done or ready to be processed.
>> This issue may happen only if we queue work during executing drain_workqueue().
>> http://elixir.free-electrons.com/linux/latest/source/kernel/workqueue.c#L2173
> 
> Hmmm? I don't get it.  worker->current_pwq is guaranteed to be set
> while a work function is being executed and the chained check can only
> get there iff the the worker is executing a work function.
> 

Hi Tejun,

Thanks for your quick reply.
Hmm, but i think interrupt preemption could make this happen.
For example, the scenario that all following conditions are met.
1.The handler of an interrupt call queue_work() to queue some works, and the workqueue is draining.
2.An worker thread is interrupted by this interrupt.
3.The worker thread is being executed, and at the very moment,worker->current_pwq
is set to null.(This is possible and please check function worker_thread() carefully).

Thanks.