On Sun, Aug 13, 2023 at 1:08 AM Takashi Iwai <tiwai@xxxxxxx> wrote: > > On Wed, 09 Aug 2023 23:11:45 +0200, > Curtis Malainey wrote: > > > > > > > > And now looking back at kobj code and device code, they do refcount > > > parent objects. Maybe the problem is in our side -- the all devices > > > are created with the original real device as the parent, including the > > > card_dev, while there are some dependencies among children. So, if we > > > build up a proper tree, pci_dev -> card_dev -> ctl_dev, pcm_dev, etc, > > > one of the problems could be solved. It's more or less similar as > > > what I suggested initially (referring card_dev at pcm), while changing > > > the parent would make it implicitly. > > > > Yes I think this would be the long term proper way to go, that way > > parents just put children and remove their reference, then they > > cleanup on their own time making everyone happy. My first patch was a > > very lazy attempt that, if we wanted to do the right thing we would > > obviously have to split the structs and free functions to operate in > > their own release. If you have time feel free to take another swing at > > the patches, otherwise I won't be able to start until next week. > > Now looking back at the problem again, I noticed that actually my > previous comment was wrong: as default, the device dependencies aren't > kept at the release time, but it's already cleared at device_del() > call. The device_del() calls kobject_del() and put_device(parent). > So after this moment, both device releases become independent, and > it'll hit a problem if the released object has still some dependency > (such as the case of card vs ctl_dev in our case). > > An extra dependency to card_dev as I put in my early patch would "fix" > it. But, there is yet another problem: the call of dev_free call for > snd_device object with SNDRV_DEV_LOWLEVEL can happen before releasing > PCM and other devices when the delayed kobj release is enabled. And, > usually this callback does release the top-level resources, which > might be still accessed during the other releases. I was doing some testing late last week that confirms these fears actually. Basically the rig was opening a non-blocking PCM and just never sending data, removing the hw sound device (in this case a USB peripheral) but not closing the userspace app. As a result kobjects were released out of order with the top level "sound" being released at disconnect of the USB device, but the actual card not being released until the app closed. See annotated logs below [ 690.528577] kobject: 'sound' (00000000266cd308): kobject_add_internal: parent: '3-9:1.0', set: '(null)' <----- plug in device [ 690.528607] kobject: 'card1' (00000000f7fa0903): kobject_add_internal: parent: 'sound', set: 'devices' [ 690.528732] kobject: 'card1' (00000000f7fa0903): kobject_uevent_env [ 690.528756] kobject: 'card1' (00000000f7fa0903): fill_kobj_path: path = '/devices/pci0000:00/0000:00:14.0/usb3/3-9/3-9:1.0/sound/card1' [ 690.528988] kobject: 'pcmC1D0p' (0000000095ff4473): kobject_add_internal: parent: 'card1', set: 'devices' [ 690.529640] kobject: 'pcmC1D0p' (0000000095ff4473): fill_kobj_path: path = '/devices/pci0000:00/0000:00:14.0/usb3/3-9/3-9:1.0/sound/card1/pcmC1D0p' [ 690.529778] kobject: 'pcmC1D0c' (00000000c4879d24): kobject_add_internal: parent: 'card1', set: 'devices' [ 690.530006] kobject: 'pcmC1D0c' (00000000c4879d24): fill_kobj_path: path = '/devices/pci0000:00/0000:00:14.0/usb3/3-9/3-9:1.0/sound/card1/pcmC1D0c' [ 690.530108] kobject: 'controlC1' (00000000a0a25449): kobject_add_internal: parent: 'card1', set: 'devices' [ 690.530373] kobject: 'controlC1' (00000000a0a25449): fill_kobj_path: path = '/devices/pci0000:00/0000:00:14.0/usb3/3-9/3-9:1.0/sound/card1/controlC1' [ 690.671889] kobject: 'card1' (00000000f7fa0903): kobject_uevent_env [ 690.671906] kobject: 'card1' (00000000f7fa0903): fill_kobj_path: path = '/devices/pci0000:00/0000:00:14.0/usb3/3-9/3-9:1.0/sound/card1' [ 700.009244] kobject: 'controlC1' (00000000a0a25449): fill_kobj_path: path = '/devices/pci0000:00/0000:00:14.0/usb3/3-9/3-9:1.0/sound/card1/controlC1' [ 700.010131] kobject: 'pcmC1D0p' (0000000095ff4473): fill_kobj_path: path = '/devices/pci0000:00/0000:00:14.0/usb3/3-9/3-9:1.0/sound/card1/pcmC1D0p' [ 700.011344] kobject: 'pcmC1D0c' (00000000c4879d24): fill_kobj_path: path = '/devices/pci0000:00/0000:00:14.0/usb3/3-9/3-9:1.0/sound/card1/pcmC1D0c' [ 700.012916] kobject: 'card1' (00000000f7fa0903): kobject_uevent_env [ 700.012951] kobject: 'card1' (00000000f7fa0903): fill_kobj_path: path = '/devices/pci0000:00/0000:00:14.0/usb3/3-9/3-9:1.0/sound/card1' <---- start blocked playback here [ 700.013057] kobject: 'sound' (00000000266cd308): kobject_release, parent 0000000000000000 (delayed 1000) <--- unplug usb device [ 701.054221] kobject: 'sound' (00000000266cd308): kobject_cleanup, parent 0000000000000000 [ 701.054236] kobject: 'sound' (00000000266cd308): calling ktype release [ 701.054257] kobject: 'sound': free name [ 713.639843] kobject: 'card1' (00000000f7fa0903): kobject_release, parent 0000000000000000 (delayed 3000) <--- Send EOF to playback stream [ 716.669776] kobject: 'card1' (00000000f7fa0903): kobject_cleanup, parent 0000000000000000 [ 716.669810] kobject: 'card1' (00000000f7fa0903): calling ktype release [ 716.670834] kobject: 'card1': free name > > > So, if we tie the object resource with each struct device release, we > have a lot of works: > 1. Add extra dependencies among device hierarchy > 2. Don't use card_dev refcount for managing the sync to device closes, > introduce another kref instead; otherwise card_dev refcount would > never reach to zero > 3. Fix race of devres vs card_dev release > 4. Move the second half part of snd_card_do_free() to the release > callback of card_dev itself to sync with the top-level release > 5. Rewrite all SNDRV_DEV_LOWLEVEL usages to be called via > card->private_free or such; > maybe the only problem is hda_intel.c and hda_tegra.c that need > some work at the disconnection time, and we may introduce another > hook in the card object to replace that > > And, at this moment, I feel that it'd be easier to go back to the > early way of device management, i.e. it'll be just like your patch, > managing the device object independently from the rest resources. > (This means also that the way freeing the resource for hwdep and > rawmidi will go back again without the embedded device, too; they > also suffer from the same problem of .) I agree, I think as a simple stopgap, my earlier patch would at least appease the test until we can figure out the best way to do some heavier work on the kobj. I think the proxy pointer for devres would also be the best short term for 3. > > The change 2 and 3 above can be still applied with your change, which > will fix the remaining devres-vs-card_dev problem. I am not sure I follow the need for 2. If we broke ctl_dev out into its own memory region and structured everything as a proper tree we would have a proper cleanup and be able to use the refcounting properly. > > Once after fixing the current problem, we may work further on other > stuff (e.g. item 5), so that we can switch again to the device-release > model eventually later, too. Agreed, I don't have any experience with SNDRV_DEV_LOWLEVEL but I am happy to help out here where I can. I am going to see if I can split the release card as mentioned but also have refcount work as expected and have the release calls roll up the tree. Curtis > > > Takashi
