On 04/05/2016 12:10 AM, Takashi Iwai wrote:
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:
- 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.
- 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.
Agreed. I included snd-usb-audio as it provides a good test case for multiple driver use-case. Yes it is a good idea to have a git branch for wider testing.
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.
I folded them in. It is unfortunate that these bugs were introduced towards the end when I was making changes to address review comments and I didn't catch them, especially the quirk NULL dereference.
thanks, -- Shuah