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: >> >> 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. 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 -- 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
