linux-kernel - Re: uvcvideo logging kernel warnings on device disconnect

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Date:   Fri, 9 Dec 2016 08:25:52 +0100
From:   Greg KH <greg@...ah.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Dave Stevenson <linux-media@...tevenson.freeserve.co.uk>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: uvcvideo logging kernel warnings on device disconnect

On Fri, Dec 09, 2016 at 01:09:21AM +0200, Laurent Pinchart wrote:
> Hi Dave,
> 
> (CC'ing LKML and Greg KH)
> 
> On Thursday 08 Dec 2016 12:31:55 Dave Stevenson wrote:
> > Hi All.
> > 
> > I'm working with a USB webcam which has been seen to spontaneously
> > disconnect when in use. That's a separate issue, but when it does it
> > throws a load of warnings into the kernel log if there is a file handle
> > on the device open at the time, even if not streaming.
> > 
> > I've reproduced this with a generic Logitech C270 webcam on:
> > - Ubuntu 16.04 (kernel 4.4.0-51) vanilla, and with the latest media tree
> > from linuxtv.org
> > - Ubuntu 14.04 (kernel 4.4.0-42) vanilla
> > - an old 3.10.x tree on an embedded device.
> > 
> > To reproduce:
> > - connect USB webcam.
> > - run a simple app that opens /dev/videoX, sleeps for a while, and then
> > closes the handle.
> > - disconnect the webcam whilst the app is running.
> > - read kernel logs - observe warnings. We get the disconnect logged as
> > it occurs, but the warnings all occur when the file descriptor is
> > closed. (A copy of the logs from my Ubuntu 14.04 machine are below).
> > 
> > I can fully appreciate that the open file descriptor is holding
> > references to a now invalid device, but is there a way to avoid them? Or
> > do we really not care and have to put up with the log noise when doing
> > such silly things?
> 
> This is a known problem, caused by the driver core trying to remove the same 
> sysfs attributes group twice.

Ick, not good.

> The group is first removed when the USB device is disconnected. The input 
> device and media device created by the uvcvideo driver are children of the USB 
> interface device, which is deleted from the system when the camera is 
> unplugged. Due to the parent-child relationship, all sysfs attribute groups of 
> the children are removed.

Wait, why is the USB device being removed from sysfs at this point,
didn't the input and media subsystems grab a reference to it so that it
does not disappear just yet?

> Then, when the device node is closed, the media device and input device are 
> unregistered, causing the corresponding devices to be deleted too. The driver 
> core tries to remove the sysfs attributes groups related to those devices, and 
> issues a warning as they have been removed already.
> 
> I'm not sure how to fix that, any hint from LKML would be appreciated.

Properly grab a reference to the USB device?  :)

If that's already happening, please let me know and I'll see what needs
to be done, but I think that should solve the issue for you.

thanks,

greg k-h