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

[Mauro Carvalho Chehab]

Em Tue, 5 Apr 2016 08:10:11 +0200
Takashi Iwai <tiwai@xxxxxxx> escreveu:

> 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.

I agree. Let's first fix MC races first, then address the multi-driver
issues. Only after having those fixed, we should look at the sound/usb
patch.

Ok, we could keep it on some testing tree, but, IMHO,  it doesn't make any
sense to submit it to review, while the core is not fixed.

> 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.

Works for me. After we have this properly fixed and stabilized on media,
I'll pass you a stable topic branch for you. This way, you can test a
new version of the sound/usb patch and apply on your tree when it fits
well for you.

> 
> 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


-- 

Cheers,
Mauro