linux-kernel - Re: [PATCH][USBATM]: convert heavy init dances to kthread API

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Date:	Thu, 07 Feb 2008 12:23:55 +0300
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	Duncan Sands <baldrick.bulk@...e.fr>
CC:	linux-usb@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][USBATM]: convert heavy init dances to kthread API

Duncan Sands wrote:
> Hi,
> 
>> The problem is that I couldn't find the maintainer for the code 
>> in drivers/usb/atm/.
> 
> that would be me (though since I haven't used this modem in years I would
> be more than happy to hand it off to someone else).
> 
>> Besides, I don't have a proper hardware to test this.
> 
> I will try to find where I put my old modem and test your patch this weekend.

Oh, that would be great :)

>> @@ -1014,11 +1015,7 @@ static int usbatm_do_heavy_init(void *arg)
>>  	struct usbatm_data *instance = arg;
>>  	int ret;
>>  
>> -	daemonize(instance->driver->driver_name);
>>  	allow_signal(SIGTERM);
>> -	instance->thread_pid = current->pid;
>> -
>> -	complete(&instance->thread_started);
> 
> One reason the completion existed to make sure that the thread was not
> sent SIGTERM before the above call to allow_signal(SIGTERM).  So I think
> you have opened up a (tiny) race by deleting it.

Nope. See my answer below :)

>>  static int usbatm_heavy_init(struct usbatm_data *instance)
>>  {
>> -	int ret = kernel_thread(usbatm_do_heavy_init, instance, CLONE_FS | CLONE_FILES);
>> -
>> -	if (ret < 0) {
>> -		usb_err(instance, "%s: failed to create kernel_thread (%d)!\n", __func__, ret);
> 
> Please don't delete this message.
> 
>> -		return ret;
>> -	}
>> +	struct task_struct *t;
>>  
>> -	wait_for_completion(&instance->thread_started);
>> +	t = kthread_create(usbatm_do_heavy_init, instance,
>> +			instance->driver->driver_name);
>> +	if (IS_ERR(t))
>> +		return PTR_ERR(t);
>>  
>> +	instance->thread = t;
>> +	wake_up_process(t);
> 
> Does the kthread API guarantee that the kthread is not running until you call

It does. That's why the race, you mentioned above is impossible.

> wake_up_process?  Because if not then what is to stop the kthread finishing before
> this thread does "instance->thread = t", resulting in an attempt to send a signal
> to a dead process later on in disconnect?
> 
> Otherwise it looks fine - thanks!
> 
> By the way, the right thing to do is (I think) to replace the thread with a workqueue
> and have users of usbatm register a "shut_down" callback rather than using signals: the
> disconnect method would call shut_down rathering than trying to kill the thread.  That
> way all this mucking around with pids etc wouldn't be needed.  All users of usbatm would
> need to be modified.  I managed to convince myself once that they could all be fixed up
> in a fairly simple manner thanks to a few tricks and a completion or two, but I don't
> recall the details...

Well, that would be also great, since kill_proc will be gone - that's what
I'm trying to achieve.

> Best wishes,
> 
> Duncan.
> 

Thanks,
Pavel
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