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
