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Date: Mon, 22 Mar 2010 12:38:40 +0100
From: Bruno =?UTF-8?B?UHLDqW1vbnQ=?= <bonbons@linux-vserver.org>
To: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Jiri Kosina <jkosina@suse.cz>, linux-input@vger.kernel.org,
       linux-usb@vger.kernel.org, linux-fbdev@vger.kernel.org,
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       Nicu Pavel <npavel@ituner.com>, Oliver Neukum <oliver@neukum.org>,
       Jaya Kumar <jayakumar.lkml@gmail.com>
Subject: Re: [PATCH v2 1/6] hid: new driver for PicoLCD device
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On Sun, 21 March 2010 Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:
> On Sun, Mar 21, 2010 at 05:37:37PM +0100, Bruno Prémont wrote:
> > On Sat, 20 March 2010 Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:
> > > > +	usbhid_submit_report(data->hdev, report, USB_DIR_OUT);
> > > > +	complete_all(&data->ready);
> > > > +	INIT_COMPLETION(data->ready);
> > > 
> > > Umm, what does this do, exactly?
> > 
> > It wakes up anyone waiting on the completion and then resets the completion
> > as otherwise any future attempt to wait on it would succeed immediately.
> > 
> 
> You realize that if you re-initialize the completion right after
> signalling it there is a big chance the waiters will miss it (they do
> check completion->done flags that you reset right away.
> 
> In general completions are suited for something that happens once (a
> single request - allocated - processed - signalled) but not for
> repeating use.

Would below approach be more correct?

- move the completion to struct picolcd_pending
  so it meets the "happens once" requirement

- protect data->pending with a mutex
  (though also use spinlock to prevent race between event which
   signals the completion and picolcd_send_and_wait() around timeout)

- use the data->lock spinlock to protect multi-report requests from
  interleaving

In speudo-code this would be something like:

picolcd_send_and_wait(...)
{
	struct picolcd_pending pending;
	...
	init_completion(&pending->ready);
	aquire_mutex(data->mutex);
	spin_lock_irqsave(&data->lock, flags);
	... // prepare report
	data->pending = &pending;
	usbhid_submit_report(data->hdev, report, USB_DIR_OUT);
	spin_unlock_irqrestore(&data->lock, flags);
	wait_for_completion_interruptible_timeout(&pending->ready, HZ*2);
	spin_lock_irqsave(&data->lock, flags);
	if (data->pending == &pending)
		data->pending = NULL;
	spin_unlock_irqrestore(&data->lock, flags);
	release_mutext(data->mutex);
	...
}

picolcd_raw_event(...)
{
	...
	spin_lock_irqsave(&data->lock, flags);
	if (data->pending) {
		// copy event data to pending
		...
		complete(&pending->ready);
	}
	spin_unlock_irqrestore(&data->lock, flags);
	...
}

In picolcd_remove() and picolcd_reset() I could then do something
similar to picolcd_raw_event() to trigger completion with no
data/error in order to skip timeout on the requester side.

Though how should I prevent races on hot-unplug?

Thanks,
Bruno