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 refcounted to avoid unregistering when one of the drivers does unregister. A refcount field in the struct media_device covers this case. Unregister on a Media Allocator media device is a kref_put() call. The media device should be unregistered only when the last unregister occurs.
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 am sending this RFC series out for review. I tested to verify media ioctls don't fail when media device is unregistered and it gets released when the last reference goes. I still need to do more testing. I didb't fully test if the media device gets deleted in all cases. So please consider this as work in progress. I decided to send this out to get early review to see if this solution is viable.
Shuah Khan (4): media: Add Media Device Allocator API media: Add Media Device Allocator API documentation media: Add refcount to keep track of media device registrations drivers: change au0828, uvcvideo, snd-usb-audio to use Media Device Allocator
drivers/media/Makefile | 3 +- drivers/media/media-dev-allocator.c | 153 +++++++++++++++++++++++++++++++++ drivers/media/media-device.c | 53 ++++++++++++ drivers/media/media-devnode.c | 3 + drivers/media/usb/au0828/au0828-core.c | 7 +- drivers/media/usb/au0828/au0828.h | 1 + drivers/media/usb/uvc/uvc_driver.c | 32 ++++--- drivers/media/usb/uvc/uvcvideo.h | 3 +- include/media/media-dev-allocator.h | 113 ++++++++++++++++++++++++ include/media/media-device.h | 32 +++++++ sound/usb/media.c | 10 ++- sound/usb/media.h | 1 + 12 files changed, 390 insertions(+), 21 deletions(-) create mode 100644 drivers/media/media-dev-allocator.c create mode 100644 include/media/media-dev-allocator.h