Re: [PATCH] videodev: fix sysfs kobj ref count

["David Ellingsworth" <david@xxxxxxxxxxxxxxxxx>]

OK, here's my analysis of the locking situation:

video_open and video_close are both called with the BKL held and are
therefore non-preemptable. This means that video_open and video_close
will always run from beginning to end without interruption by any of
the other functions. Keeping this in mind, lets evaluate the following
sequences:

video_register_device (preempted)
 video_open

video_register_device (preempted)
 video_close

video_unregister_device (preempted)
 video_open

video_unregister_device (preempted)
 video_close

In all of the above sequences, videodev_lock is obtained by
video_register_device or video_unregister_device. video_open and
video_close will then deadlock while waiting to obtain the lock. Since
video_open and video_close can not be preempted, the lock will never
be obtained. The first and third above currently exist without this
patch. The second and fourth are a result of this patch.

Due to the use of the BKL, I cannot currently think of any viable
solutions to these issues. These issues might be solvable once the BKL
is removed, but it will also present a whole range of other
challenges.

Regards,

David Ellingsworth

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