Re: [PATCH v5 1/8] drivers: kunit: Generic helpers for test device creation

[Maxime Ripard <maxime@cerno.tech>]

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On Sat, Mar 25, 2023 at 05:50:44PM +0000, Jonathan Cameron wrote:
> On Fri, 24 Mar 2023 13:31:57 +0100
> Maxime Ripard <maxime@cerno.tech> wrote:
> 
> > On Fri, Mar 24, 2023 at 08:11:52AM +0200, Matti Vaittinen wrote:
> > > On 3/23/23 18:36, Maxime Ripard wrote:  
> > > > On Thu, Mar 23, 2023 at 03:02:03PM +0200, Matti Vaittinen wrote:  
> > > > > On 3/23/23 14:29, Maxime Ripard wrote:  
> > > > > > On Thu, Mar 23, 2023 at 02:16:52PM +0200, Matti Vaittinen wrote:
> > > > > > 
> > > > > > This is the description of what was happening:
> > > > > > https://lore.kernel.org/dri-devel/20221117165311.vovrc7usy4efiytl@houat/  
> > > > > 
> > > > > Thanks Maxime. Do I read this correcty. The devm_ unwinding not being done
> > > > > when root_device_register() is used is not because root_device_unregister()
> > > > > would not trigger the unwinding - but rather because DRM code on top of this
> > > > > device keeps the refcount increased?  
> > > > 
> > > > There's a difference of behaviour between a root_device and any device
> > > > with a bus: the root_device will only release the devm resources when
> > > > it's freed (in device_release), but a bus device will also do it in
> > > > device_del (through bus_remove_device() -> device_release_driver() ->
> > > > device_release_driver_internal() -> __device_release_driver() ->
> > > > device_unbind_cleanup(), which are skipped (in multiple places) if
> > > > there's no bus and no driver attached to the device).
> > > > 
> > > > It does affect DRM, but I'm pretty sure it will affect any framework
> > > > that deals with device hotplugging by deferring the framework structure
> > > > until the last (userspace) user closes its file descriptor. So I'd
> > > > assume that v4l2 and cec at least are also affected, and most likely
> > > > others.  
> > > 
> > > Thanks for the explanation and patience :)
> > >   
> > > >   
> > > > > If this is the case, then it sounds like a DRM specific issue to me.  
> > > > 
> > > > I mean, I guess. One could also argue that it's because IIO doesn't
> > > > properly deal with hotplugging.  
> > > 
> > > I must say I haven't been testing the IIO registration API. I've only tested
> > > the helper API which is not backed up by any "IIO device". (This is fine for
> > > the helper because it must by design be cleaned-up only after the
> > > IIO-deregistration).
> > > 
> > > After your explanation here, I am not convinced IIO wouldn't see the same
> > > issue if I was testing the devm_iio_device_alloc() & co.  
> > 
> > It depends really. The issue DRM is trying to solve is that, when a
> > device is gone, some application might still have an open FD and could
> > still poke into the kernel, while all the resources would have been
> > free'd if it was using devm.
> > 
> > So everything is kept around until the last fd is closed, so you still
> > have a reference to the device (even though it's been removed from its
> > bus) until that time.
> > 
> > It could be possible that IIO just doesn't handle that case at all. I
> > guess most of the devices aren't hotpluggable, and there's not much to
> > interact with from a userspace PoV iirc, so it might be why.
> 
> Lars-Peter Clausen (IIRC) fixed up the IIO handling of the similar cases a
> long time ago now. It's simpler that for some other subsystems as we don't
> have as many interdependencies as occur in DRM etc.
> 
> I 'think' we are fine in general with the IIO approach to this (I think we
> did have one report of a theoretical race condition in the remove path that
> was never fully addressed).
> 
> For IIO we also have fds that can be open but all accesses to them are proxied
> through the IIO core and one of the things iio_device_unregister() or the devm
> equivalent does is to set indio_dev->info = NULL  (+ wake up anyone waiting on
> data etc). Alongside removing the callbacks, that is also used as a flag
> to indicate the device has gone.

Sorry if it came as trying to put IIO under a bad light, it certainly
wasn't my intention. I was trying to come up with possible explanations
as to why IIO's design was simpler than DRM is :)

> Note that we keep a reference to the struct indio_dev->dev (rather that the
> underlying device) so that is not freed until the last fd is closed.
> Thus, although devm unwinding has occurred that doesn't mean all the data
> that was allocated with devm_xx calls is cleared up immediately.

I'm not sure I get that part though. devm unwinding can happen even if the refcount is > 1

Maxime

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