Re: Duplicated code in hiddev_open()

[Oliver Neukum <oneukum@suse.com>]

Am Freitag, den 16.08.2019, 13:10 -0400 schrieb Alan Stern:
> Oliver and Jiri:
> 
> Why is there duplicated code in
> drivers/hid/usbhid/hiddev.c:hiddev_open()?
> 
> Line 267:
> 	/*
> 	 * no need for locking because the USB major number
> 	 * is shared which usbcore guards against disconnect
> 	 */
> 	if (list->hiddev->exist) {
> 		if (!list->hiddev->open++) {
> 			res = hid_hw_open(hiddev->hid);
> 			if (res < 0)
> 				goto bail;
> 		}
> 	} else {
> 		res = -ENODEV;
> 		goto bail;
> 	}
> 
> Line 286:
> 	mutex_lock(&hiddev->existancelock);
> 	if (!list->hiddev->open++)
> 		if (list->hiddev->exist) {
> 			struct hid_device *hid = hiddev->hid;
> 			res = hid_hw_power(hid, PM_HINT_FULLON);
> 			if (res < 0)
> 				goto bail_unlock;
> 			res = hid_hw_open(hid);
> 			if (res < 0)
> 				goto bail_normal_power;
> 		}
> 	mutex_unlock(&hiddev->existancelock);
> 
> The second part can never execute, because the first part ensures that 
> list->hiddev->open > 0 by the time the second part runs.
> 
> Even more disturbing, why is one of these code sections protected by a 
> mutex and the other not?

I suppose the comment I made back then:

079034073faf9 drivers/hid/usbhid/hiddev.c (Oliver Neukum               2008-12-16 10:55:15 +0100 268)    * no need for locking because the USB major number
079034073faf9 drivers/hid/usbhid/hiddev.c (Oliver Neukum               2008-12-16 10:55:15 +0100 269)    * is shared which usbcore guards against disconnect

has ceased to be true, but the section was not removed, as the check
for existance was duplicated.

> Note: The second section was added in commit 0361a28d3f9a ("HID: 
> autosuspend support for USB HID") over ten years ago!

Yes and I remember how frustrating keyboards were in testing, but
no further details.

	Regards
		Oliver