On Wednesday 25 June 2014 12:58:03 Sasha Levin wrote: > Ping? Hans, I've replied to your previous e-mail with I'm of course fine with a different way to solve the race condition, if we can find a good one. Do you already know how you would like to solve this ? > On 05/23/2014 09:54 AM, Hans Verkuil wrote: > > Hi Laurent, > > > > This patch caused a circular locking dependency as reported by Sasha > > Levin: > > > > https://lkml.org/lkml/2014/5/5/366 > > > > The reason is that copy_to/from_user is called in video_usercopy() with > > the > > core lock held. The copy functions can fault which takes the mmap_sem. If > > it was just video_usercopy() then it would be fairly easy to solve this, > > but the copy_to_/from_user functions are also called from read and write > > and they can be used in other unexpected places. > > > > I'm not sure if vb2_fop_get_unmapped_area() is a problem. I suspect (but > > I'm not sure) that when that one is called the mmap_sem isn't taken, in > > which case taking the lock is fine. > > > > But taking the lock in vb2_fop_mmap() does cause lockdep problems. > > > > Ideally I would like to drop taking that lock in vb2_fop_mmap and resolve > > the race condition that it intended to fix in a different way. > > > > Regards, > > > > Hans > > > > On 08/06/2013 10:10 PM, Laurent Pinchart wrote: > >> The vb2_fop_mmap() and vb2_fop_get_unmapped_area() functions are plug-in > >> implementation of the mmap() and get_unmapped_area() file operations > >> that calls vb2_mmap() and vb2_get_unmapped_area() on the queue > >> associated with the video device. Neither the > >> vb2_fop_mmap/vb2_fop_get_unmapped_area nor the > >> v4l2_mmap/vb2_get_unmapped_area functions in the V4L2 core take any > >> lock, leading to race conditions between mmap/get_unmapped_area and > >> other buffer-related ioctls such as VIDIOC_REQBUFS. > >> > >> Fix it by taking the queue or device lock around the vb2_mmap() and > >> vb2_get_unmapped_area() calls. > >> > >> Signed-off-by: Laurent Pinchart > >> <laurent.pinchart+renesas@xxxxxxxxxxxxxxxx> > >> --- > >> > >> drivers/media/v4l2-core/videobuf2-core.c | 18 ++++++++++++++++-- > >> 1 file changed, 16 insertions(+), 2 deletions(-) > >> > >> diff --git a/drivers/media/v4l2-core/videobuf2-core.c > >> b/drivers/media/v4l2-core/videobuf2-core.c index 9fc4bab..c9b50c7 100644 > >> --- a/drivers/media/v4l2-core/videobuf2-core.c > >> +++ b/drivers/media/v4l2-core/videobuf2-core.c > >> @@ -2578,8 +2578,15 @@ EXPORT_SYMBOL_GPL(vb2_ioctl_expbuf); > >> int vb2_fop_mmap(struct file *file, struct vm_area_struct *vma) > >> { > >> struct video_device *vdev = video_devdata(file); > >> + struct mutex *lock = vdev->queue->lock ? vdev->queue->lock : > >> vdev->lock; > >> + int err; > >> > >> - return vb2_mmap(vdev->queue, vma); > >> + if (lock && mutex_lock_interruptible(lock)) > >> + return -ERESTARTSYS; > >> + err = vb2_mmap(vdev->queue, vma); > >> + if (lock) > >> + mutex_unlock(lock); > >> + return err; > >> } > >> EXPORT_SYMBOL_GPL(vb2_fop_mmap); > >> > >> @@ -2685,8 +2692,15 @@ unsigned long vb2_fop_get_unmapped_area(struct > >> file *file, unsigned long addr,>> > >> unsigned long len, unsigned long pgoff, unsigned long flags) > >> { > >> struct video_device *vdev = video_devdata(file); > >> + struct mutex *lock = vdev->queue->lock ? vdev->queue->lock : > >> vdev->lock; > >> + int ret; > >> > >> - return vb2_get_unmapped_area(vdev->queue, addr, len, pgoff, flags); > >> + if (lock && mutex_lock_interruptible(lock)) > >> + return -ERESTARTSYS; > >> + ret = vb2_get_unmapped_area(vdev->queue, addr, len, pgoff, flags); > >> + if (lock) > >> + mutex_unlock(lock); > >> + return ret; > >> > >> } > >> EXPORT_SYMBOL_GPL(vb2_fop_get_unmapped_area); > >> #endif -- Regards, Laurent Pinchart -- 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
