[Intel-gfx] [PATCH 01/44] drivers/base: Always release devres on device_del

[Greg Kroah-Hartman]

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 at ffwll.ch> wrote:
> >
> > On Fri, Apr 3, 2020 at 4:17 PM Greg Kroah-Hartman
> > <gregkh at linuxfoundation.org> 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 at gmail.com>
> 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