Re: [PATCH V2] workqueue: fix possible race condition when rescuer VS pwq-release

[Lai Jiangshan]

On Wed, Apr 16, 2014 at 11:23 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> Hello, Lai.
>
> On Wed, Apr 16, 2014 at 09:25:16AM +0800, Lai Jiangshan wrote:
>> 1) Our aim is to protect unbound pwq, not percpu pwq which can't be be protected by get_pwq().
>> 2) get_pwq() will make reviewers confused/surprised, destroy_workqueue() may destroy percpu pwqs
>>    with ref > 1. At least we need to add more comments explain this behavior. Origin comments:
>>               /*
>>                * The base ref is never dropped on per-cpu pwqs.  Directly
>>                * free the pwqs and wq.
>>                */

Hi, Tejun

OK. It is better to use get_pwq(). I will also change the above comments to:

  The base ref and the possible ref from rerscuer(stopped) are never
dropped on per-cpu pwqs.
  Directly free the pwqs and wq.

The reason I quickly dropped V1 and wrote the V2 is that I saw this comment.
"The base ref" is precise after I used get_pwq() in V1.

Or to avoid to change to this comments.
I can also move the following code down to the bottom of the rescuer_thread().

if (kthread_should_stop()) {
__set_current_state(TASK_RUNNING);
rescuer->task->flags &= ~PF_WQ_WORKER;
return 0;
}

(I reply this email on browser, never mind the syntax).
Maybe the second choice are better.

Any think?

Thanks,
Lai.

>
> You can just comment "pwqs might go away at any time, pin it until
> rescuer is done with it" and that's actually the better way to do it.
> percpu wq's not supporting attribute changes may change in the future.
> What the code path wants is pinning down the pwq no matter where it
> came from.  There's no point in distinguishing unbound and per-cpu
> here.
>
>> 3) get_unbound_pwq() self document.
>
> Not really.  If the name is get_pwq_if_unbound(), maybe.  Functions
> which take args and become noop depending on the argument aren't
> generally good ideas.  There are specific cases that they are suitable
> but this is just gratuituous.
>
> Thanks.
>
> --
> tejun
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