On Fri, Apr 03, 2020 at 04:51:33PM +0200, Daniel Vetter wrote: > On Fri, Apr 3, 2020 at 4:47 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > > > > On Fri, Apr 3, 2020 at 4:17 PM Greg Kroah-Hartman > > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > > > > > On Fri, Apr 03, 2020 at 03:57:45PM +0200, Daniel Vetter wrote: > > > > In drm we've added nice drm_device (the main gpu driver thing, which > > > > also represents the userspace interfaces and has everything else > > > > dangling off it) init functions using devres, devm_drm_dev_init and > > > > soon devm_drm_dev_alloc (this patch series adds that). > > > > > > > > A slight trouble is that drm_device itself holds a reference on the > > > > struct device it's sitting on top (for sysfs links and dmesg debug and > > > > lots of other things), so there's a reference loop. For real drivers > > > > this is broken at remove/unplug time, where all devres resources are > > > > released device_release_driver(), before the final device reference is > > > > dropped. So far so good. > > > > > > > > There's 2 exceptions: > > > > - drm/vkms|vgem: Virtual drivers for which we create a fake/virtual > > > > platform device to make them look more like normal devices to > > > > userspace. These aren't drivers in the driver model sense, we simple > > > > create a platform_device and register it. > > > > > > That's a horrid abuse of platform devices, just use a "virtual" device > > > please, create/remove it when you need it, and all should be fine. > > > > > > > - drm/i915/selftests, where we create minimal mock devices, and again > > > > the selftests aren't proper drivers in the driver model sense. > > > > > > Again, virtual devices are best to use for this. > > > > Hm yeah, I guess we should fix that. i915 selftests do use raw struct > > device though, and it's not really the problem. > > > > > > For these two cases the reference loop isn't broken, because devres is > > > > only cleaned up when the last device reference is dropped. But that's > > > > not happening, because the drm_device holds that last struct device > > > > reference. > > > > > > > > Thus far this wasn't a problem since the above cases simply > > > > hand-rolled their cleanup code. But I want to convert all drivers over > > > > to the devm_ versions, hence it would be really nice if these > > > > virtual/fake/mock uses-cases could also be managed with devres > > > > cleanup. > > > > > > > > I see three possible approaches: > > > > > > > > - Clean up devres from device_del (or platform_device_unregister) even > > > > when no driver is bound. This seems like the simplest solution, but > > > > also the one with the widest impact, and what this patch implements. > > > > We might want to include more of the cleanup than just > > > > devres_release_all, but this is all I need to get my use case going. > > > > > > After device_del, you should never be using that structure again anyway. > > > So why is there any "resource leak"? You can't recycle the structure, > > > and you can't assign it to anything else, so eventually you have to do > > > a final put on the thing, which will free up the resources. > > > > I guess I should have spent more time explaining this. There's two > > references involved: > > > > - drm_device->dev points at the underlying struct device. The > > drm_device holds a reference until it's fully cleaned up, so that we > > can do nice stuff like use dev_ versions of printk functions, and you > > always know that there's not going to be a use-after free. > > > > - now the other dependency is that as long as the device exists (not > > just in memory, but in the device model, i.e. between device_add() and > > device_del()) the drm_device should exist. So what we do in the > > bus-specific remove/disconnect callback is that we call > > drm_dev_unregister(). This drops the drm_device refcount that the drm > > chardev was holding, to make sure that an open() on the chardev can > > actually get at the memory without going boom. Then after the > > drm_dev_unregister, again in the remove/disconnect callback of th > > driver, there's a drm_dev_put(). Which might or might not be the final > > drm_dev_put(), since if there's currently some open fd we keep the > > refcount elevated, to avoid oopses and fun stuff like that. And > > drm_device pointers get shared very widely, thanks to fun stuff like > > dma_buf buffer sharing and dma_fence hw syncpt sharing across > > processes and drivers. > > > > Once the final drm_dev_put() is called we also end up calling > > put_device() and everything is happy. > > > > So far so good. > > > > Now the problem is that refcount is hard, and most drm drivers get it > > wrong in some fashion or another, so I'm trying to solve all this with > > more magic. > > > > Since all drivers need to have a drm_dev_put() at the end of their > > driver's remove/disconnect callback we've added a devm_drm_dev_init > > function which registers a devres action to do that drm_dev_put() at > > device_del time (which might or might not be the final drm_dev_put()). > > Nothing has changed thus far. > > > > Now this works really well because when you have a real driver model > > driver attached, then device_del ends up calling devres_release_all(), > > which ends up triggering the multi-stage cleanup of drm_devices. But > > if you do _not_ have a real driver attached, then device_del does > > nothing wrt devres cleanup. Instead this is delayed until the final > > put_device(). > > > > Unfortunately that final put_device() will never happen, because > > drm_device is still holding a reference to the struct device. And the > > final drm_dev_put of that drm_device will never happen, because that > > drm_dev_put call is in a devres actions, which wont ever get called. > > > > This is the only case where this reference loop happens and doesn't > > get broken. By calling devres_release_all at device_del time, > > irrespective of whether a driver is bound or not, we make both cases > > work the same. And at both cases the devres cleanup happens device_del > > time, and not when the final put_device is called. > > > > Aside: The final put_device has another devres_release_all() call, > > which given your explanation sounds very wrong - at that point the > > physical device is long gone, and cleaning up devres actions at that > > point is way too late. I think a good cleanup patch on top of this > > would be to remove that call, and replace it with an assert that no > > one managed to sneak in a devres_add_action between device_del and the > > final put_device(). > > I've done a bit more digging, and found this commit: > > commit a525a3ddeaca69f405d98442ab3c0746e53168dc > Author: Ming Lei <tom.leiming@xxxxxxxxx> > Date: Wed Jul 25 01:42:29 2012 +0800 > > driver core: free devres in device_release > > Before this devres_release_all was called at device_del() time, > unconditionally whether a driver was bound or not. Which seems to have > been the intention at least back then. So in a way my patch simply > restores that behaviour for the case where no driver has been bound to > a device structure, but we still have devres resources hanging off it. Hey, I was right, I guessed well :) I'll respond to the parent email, but it looks like you can't do this, sorry. greg k-h _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
