Hi Sakari, On 12/07/2016 03:52 AM, Sakari Ailus wrote: > Hi Shuah, > > On Mon, Dec 05, 2016 at 05:38:23PM -0700, Shuah Khan wrote: >> On 12/05/2016 04:21 PM, Laurent Pinchart wrote: >>> Hi Shuah, >>> >>> On Monday 05 Dec 2016 15:44:30 Shuah Khan wrote: >>>> On 11/30/2016 03:01 PM, Shuah Khan wrote: >>>>> Change ALSA driver to use Media Controller API to share media resources >>>>> with DVB, and V4L2 drivers on a AU0828 media device. >>>>> >>>>> Media Controller specific initialization is done after sound card is >>>>> registered. ALSA creates Media interface and entity function graph >>>>> nodes for Control, Mixer, PCM Playback, and PCM Capture devices. >>>>> >>>>> snd_usb_hw_params() will call Media Controller enable source handler >>>>> interface to request the media resource. If resource request is granted, >>>>> it will release it from snd_usb_hw_free(). If resource is busy, -EBUSY is >>>>> returned. >>>>> >>>>> Media specific cleanup is done in usb_audio_disconnect(). >>>>> >>>>> Signed-off-by: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx> >>>> >>>> Hi Takashi, >>>> >>>> If you are good with this patch, could you please Ack it, so Mauro >>>> can pull it into media tree with the other two patches in this series, >>>> when he is ready to do so. >>> >>> I *really* want to address the concerns raised by Sakari before pulling more >>> code that makes fixing the race conditions more difficult. Please, let's all >>> work on fixing the core code to build a stable base on which we can build >>> additional features. V4L2 and MC need teamwork, it's time to give the >>> subsystem the love it deserves. >>> >> >> Hi Laurent, >> >> The issue Sakari brought up is specific to using devm for video_device in >> omap3 and vsp1. I tried reproducing the problem on two different drivers >> and couldn't on Linux 4.9-rc7. >> >> After sharing that with Sakari, I suggested to Sakari to pull up his patch >> that removes the devm usage and see if he still needs all the patches in his >> patch series. He didn't back to me on that. I also requested him to rebase on > > Just to see what remains, I made a small hack to test this with omap3isp by > just replacing the devm_() functions by their plain counterparts. The memory > is thus never released, for there is no really a proper moment to release it > --- something which the patchset resolves. The result is here: > > <URL:http://www.retiisi.org.uk/v4l2/tmp/media-ref-dmesg.txt> Did you test this on 4.9-rc7 without any of your other patches? If you haven't could you please run this test with just the removing devm usage from omap3isp? It would be good to get a baseline on the current with just the not using devm first and then see what needs fixing. Also, could you please send me the complete dmesg. thanks, -- Shuah > > What happens there is that as part of the device unbinding, the graph > objects are unregistered (by media_device_unregister()) while streaming is > ongoing. Their parent media device pointers are set to NULL. > > Then, when the user process exists, the streaming media pipeline is stopped. > This requires parsing of the media graph, a job which will obviously fail > miserably and immediately: the media device is obtained from the entity > where graph traversal is expected to begin. > > Two mutexes are also acquired but they've been already destroyed at that > point (and the memory of which would also be released now without the hack > to test this "without devm"). There's just a single warning on this though. > >> top of media dev allocator because the allocator routines he has don't address >> the shared media device need. > > I do strongly prefer fixing the existing object lifetime issues in the > framework before extending it and thus making the problem domain more > complex than it is already. > > What I mentioned as an example of this are the other callbacks to the media > device: presumably they do suffer from the exactly same problems as the > enable_source() and disable_source() ones. > > As drivers do refer to other entities and controls they do expose also to > the user space, we can't really even remove entities safely at the moment. > We should have a solution to this as well, my patchset at the moment does > not address this. > > What we do need a sound basis for the framework rather than hastily written > improvements to support a particular device. Also, testing device removal > with a particular device in a particular use case does not guarantee that no > further object lifetime issues related to device removal exist, even with > that same driver, let alone other devices. > > Instead, we must consider how the frameworks and drivers manage the memory > allocated for the various objects, what are relations of those objects and > how they're exposed to the user space either directly or indirectly. As long > as objects (such as the media graph objects) with different lifetimes are > referred to from other objects without taking a reference or alternatively > serialising code paths that may access those objects and those that remove > them, we do have a problem. > > The media graph with all the subsystem specific device nodes that are > exported to the user space is a rather complex data structure. I don't think > that acquiring the media graph lock in order to fix all the serialisation > problems is the right approach here. > >> >> He also didn't respond to my response regarding the reasons for choosing >> graph_mutex to protect enable_source and disable_source handlers. > > I did. The point is that this is a partial fix and that does not properly > address the problem of device removal. It probably does decrease of the > probability of hitting the bug but it does not fix it. > >> >> So I am not sure how to move forward at the moment without a concrete plan >> for Sakari's RFC series. Sakari's patch series is still RFC and doesn't address >> shared media_device and requires all drivers to change. > > I'm all for extending the functionality of the Media controller framework, > but existing known problems touching the same parts of the framework should > be fixed first. > > Reviewing patches, pointing out problems and proposing improvements would > certainly help to achieve that goal. > -- 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
