Re: [PATCH v3 20/24] media: imx: Add Camera Interface subdev driver

[Steve Longerbeam]

On 02/02/2017 02:35 PM, Russell King - ARM Linux wrote:
> <snip>
> However, "*vfd" contains a struct device, and you _correctly_ set the
> release function for "*vfd" to video_device_release via camif_videodev.
>
> However, if you try to rmmod imx-media, then you end up with a kernel
> warning that you're freeing memory containing a held lock, and later
> chaos ensues because kmalloc has been corrupted.
>
> The root cause of this is embedding the device structure within the
> video_device into the driver's private data.  *Any* structure what so
> ever that contains a kref is reference counted, and that includes
> struct device, and therefore also includes struct video_device.  What
> that means is that its lifetime is _not_ under _your_ control, and
> you may not free it except through its release function (which is
> video_device_release().)  However, that also tries to kfree (with an
> offset of 4) your private data, which results in the warning and the
> corrupted kmalloc free lists.
>
> The solution is simple, make "vfd" a pointer in your private data
> structure and kmalloc() it separately, letting video_device_release()
> kfree() that data when it needs to.

Thanks Russell for tracking this down. I remember doing this
when I was reviewing the code for opportunities to "optimize" :-/,
and carelessly caused this bug by not reviewing how video_device
is freed.

Fixed.

Steve