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

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Date:   Mon, 25 Sep 2017 08:25:36 -0700
From:   Tejun Heo <tj@...nel.org>
To:     tanxiaofei <tanxiaofei@...wei.com>
Cc:     jiangshanlai@...il.com, linux-kernel@...r.kernel.org,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [Question] null pointer risk of kernel workqueue

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.

Thanks.

-- 
tejun