Hi Eugene,
On Wednesday 20 May 2015 17:48:41 Eugene Shatokhin wrote:
> Hi,
>
> There is a race in uvcvideo module between uvc_disconnect() and
> uvc_v4l2_open() on dev->state. Checked and reproduced that with kernel
> 4.1-rc1.
>
> drivers/media/usb/uvc/uvc_driver.c, uvc_disconnect():
>
> dev->state |= UVC_DEV_DISCONNECTED;
>
> drivers/media/usb/uvc/uvc_v4l2.c, uvc_v4l2_open():
>
> if (stream->dev->state & UVC_DEV_DISCONNECTED)
> return -ENODEV;
>
> I checked that the race does happen by introducing a delay in
> uvc_disconnect() right before that assignment and armed a hardware
> breakpoint to detect the access to stream->dev->state from
> uvc_v4l2_open(). When I disconnected the webcam while Google Hangout was
> running, the hardware breakpoint triggered several times for that read
> in uvc_v4l2_open (uvc_v4l2.c:484). uvc_v4l2_open() was called in the
> context of GoogleTalkPlugin processes.
>
> Not sure if the race is intentional but I guess, better to report it
> anyway. Nothing has crashed during my (brief) testing yet, but still.
The race condition between disconnect and open is unavoidable. What is
avoidable, though, is the crashes and other ill side-effects that could result
from it. The following commit handles that.
commit ca9afe6f87b569cdf8e797395381f18ae23a2905
Author: Hans Verkuil <hverk...@xs4all.nl>
Date: Fri Nov 26 06:54:53 2010 -0300
[media] v4l2-dev: fix race condition
The unregister function had a race condition with the v4l2_open
function. Ensure that both functions test and clear the REGISTER
flag from within a critical section.
Thanks to Laurent Pinchart for finding this race.
Signed-off-by: Hans Verkuil <hverk...@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mche...@redhat.com>
The race was previously handled by the uvcvideo driver, and the code got
removed in the following commit after the above commit got merged.
commit 716fdee110ceb816cca8c46c0890d08c5a1addb9
Author: Laurent Pinchart <laurent.pinch...@ideasonboard.com>
Date: Tue Sep 29 21:07:19 2009 -0300
V4L/DVB (13152): uvcvideo: Rely on videodev to reference-count the device
The uvcvideo driver has a driver-wide lock and a reference count to
protect
against a disconnect/open race. Now that videodev handles the race itself,
reference-counting in the driver can be removed.
Signed-off-by: Laurent Pinchart <laurent.pinch...@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mche...@redhat.com>
Setting the UVC_DEV_DISCONNECTED flag seems unneeded nowadays. I'll have to
carefully think about it though, and it's too late right now to do so :-)
--
Regards,
Laurent Pinchart
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