Re: [RFC PATCH v2 0/5] Media Device Allocator API

[Takashi Iwai]

On Tue, 05 Apr 2016 05:35:55 +0200,
Shuah Khan wrote:
> 
> There are known problems with media device life time management. When media
> device is released while an media ioctl is in progress, ioctls fail with
> use-after-free errors and kernel hangs in some cases.
> 
> Media Device can be in any the following states:
> 
> - Allocated
> - Registered (could be tied to more than one driver)
> - Unregistered, not in use (media device file is not open)
> - Unregistered, in use (media device file is not open)
> - Released
> 
> When media device belongs to  more than one driver, registrations should be
> tracked to avoid unregistering when one of the drivers does unregister. A new
> num_drivers field in the struct media_device covers this case. The media device
> should be unregistered only when the last unregister occurs with num_drivers
> count zero.
> 
> When a media device is in use when it is unregistered, it should not be
> released until the application exits when it detects the unregistered
> status. Media device that is in use when it is unregistered is moved to
> to_delete_list. When the last unregister occurs, media device is unregistered
> and becomes an unregistered, still allocated device. Unregister marks the
> device to be deleted.
> 
> When media device belongs to more than one driver, as both drivers could be
> unbound/bound, driver should not end up getting stale media device that is
> on its way out. Moving the unregistered media device to to_delete_list helps
> this case as well.
> 
> I ran bind/unbind loop tests on uvcvideo, au0828, and snd-usb-audio while
> running application that does ioctls. Didn't see any use-after-free errors
> on media device. A couple of known issues seen:
> 
> 1. When application exits, cdev_put() gets called after media device is
>    released. This is a known issue to resolve and Media Device Allocator
>    can't solve this one.
> 2. When au0828 module is removed and then ioctls fail when cdev_get() looks
>    for the owning module as au0828 is very often the module that owns the
>    media devnode. This is a cdev related issue that needs to be resolved and
>    Media Device Allocator can't solve this one.
> 
> Shuah Khan (5):
>   media: Add Media Device Allocator API
>   media: Add driver count to keep track of media device registrations
>   media: uvcvideo change to use Media Device Allocator API
>   media: au0828 change to use Media Device Allocator API
>   sound/usb: Use Media Controller API to share media resources

I don't think we need to include usb-audio patch at this stage yet.
The most important thing for now is to improve / stabilize the API
itself so that other drivers can use it as is.  Once when the API is
really stabilized, we create a solid git branch that may be based for
multiple subsystems, and I'll merge usb-audio stuff through sound git
tree.

Also, the previous usb-audio MC implementation had a few serious bugs,
including quirk NULL dereference.  See the bugzilla below for some fix
patches to 4.6-rc1:
  https://bugzilla.kernel.org/show_bug.cgi?id=115561
Feel free to fold them in, if they are still valid.


thanks,

Takashi