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Re: [ 3960.758784] 1 lock held by motion/7776: [ 3960.758788] #0: (&queue->mutex){......}, at: [<ffffffff815c62d2>] uvc_queue_enable+0x32/0xc0



Hi,

On 05/07/2012 01:44 PM, Hans Verkuil wrote:
On Monday 07 May 2012 13:06:01 Laurent Pinchart wrote:
Hi Sanser,

On Sunday 06 May 2012 16:54:40 Sander Eikelenboom wrote:
Hello Laurent / Mauro,

I have updated to latest 3.4-rc5-tip, running multiple video grabbers.
I don't see anything specific to uvcvideo anymore, but i do get the possible
circular locking dependency below.

Thanks for the report.

We indeed have a serious issue there (CC'ing Hans Verkuil).

Hans, serializing both ioctl handling an mmap with a single device lock as we
currently do in V4L2 is prone to AB-BA deadlocks (uvcvideo shouldn't be
affected as it has no device-wide lock).

If we want to keep a device-wide lock we need to take it after the mm-
mmap_sem lock in all code paths, as there's no way we can change the lock
ordering for mmap(). The copy_from_user/copy_to_user issue could be solved by
moving locking from v4l2_ioctl to __video_do_ioctl (device-wide locks would
then require using video_ioctl2), but I'm not sure whether that will play
nicely with the QBUF implementation in videobuf2 (which already includes a
workaround for this particular AB-BA deadlock issue).

I've seen the same thing. It was on my TODO list of things to look into. I think
mmap shouldn't take the device wide lock at all. But it will mean reviewing
affected drivers before I can remove it.

To be honest, I wasn't sure whether or not to take the device lock for mmap when
I first wrote that code.

If you look at irc I had a discussion today with HdG about adding flags to
selectively disable locks for fops. It may be an idea to implement this soon so
we can start updating drivers one-by-one.

I've a patch almost ready for this, when I'm happy with it I'll send of a new
version of the (ever growing) gspca use control framework patchset both me and
the other Hans have been working on, which will include this patch.

Regards,

Hans
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