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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 lockordering 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 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html