Message-Id: <200802071057.41201.baldrick.bulk@free.fr>
Date:	Thu, 7 Feb 2008 10:57:40 +0100
From:	Duncan Sands <baldrick.bulk@...e.fr>
To:	Pavel Emelyanov <xemul@...nvz.org>
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

Hi Pavel,

> >> @@ -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.

I don't see why it helps.  The race I mentioned occurs when the kthread creating thread
runs too fast compared to the kthread.  Let C (creator) be the thread running
usbatm_heavy_init, and K (kthread) be the created kthread.  When C calls wake_up_process,
thread K starts running, however on an SMP system C may also be running.  Now suppose
that for some reason K takes a long time to execute the command "allow_signal(SIGTERM);",
but that C runs very fast and immediately executes the disconnect callback, and sends the
signal to K before K manages to execute allow_signal.  This is the race, and it can only
be fixed by making C run slower (thus the completion).  Of course this is fantastically
unlikely which is why I described it as tiny, but as far as I can see it is a theoretical
possibility.  I don't see that wake_up_process fixes it, it just makes it even less likely.

> > 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.

I think your patch should go in, since I'm not likely to ever implement the
scheme I suggested - I don't use this hardware anymore and have lost interest
in the driver.

Best wishes,

Duncan.
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