[PATCH 0/3] drm/i915: Fix harmfull driver register/unregister assymetry

[Krzysztof Niemiec]

Hi Andi,

On 2025-02-10 at 14:01:19 GMT, Andi Shyti wrote:
> Hi Janusz,
> 
> On Thu, Feb 06, 2025 at 07:07:38PM +0100, Janusz Krzysztofik wrote:
> > We return immediately from i915_driver_register() if drm_dev_register()
> > fails, skipping remaining registration steps.  However, the _unregister()
> > counterpart called at device remove knows nothing about that skip and
> > executes reverts for all those steps.  For that to work correctly, those
> > revert functions must be resistant to being called even on uninitialized
> > objects, or we must not skip their initialization.
> > 
> > Three cases have been identified and fixes proposed.  Call traces are
> > taken from CI results of igt at i915_driver_load@reload-with-fault-injection
> > execution, reported to several separate Gitlab issues (links provided).
> > 
> > Immediate return was introduced to i915_driver_register() by commit
> > ec3e00b4ee27 ("drm/i915: stop registering if drm_dev_register() fails"),
> > however, quite a few things have changed since then.  That's why I haven't
> > mentioned it in a Fixes: tag to avoid it being picked up by stable, which
> > I haven't tested.
> 
> I'm not fully convinced about this series as I think that you are
> fixing a subset of what needs to be handled properly. What about
> hwmon? What about gt? what about debugfs?
> 
> In my opinion we need to check in _unregister whether the
> drm_dev_register has succeded and one way would be, e.g., to
> check for the drm minor value, or even set the drm device tu NULL
> (first things that come to my mind, maybe there are smarter ways
> of doing it). This way we could skip some of the _unregister()
> steps.
> 

Is there any situation in which having the driver loaded after failing
drm_dev_register() is of any use? Because if not, instead of recording
the fact of registration failure, we can just stop the driver from
loading altogether by checking drm_dev_register()'s return value,
then calling _only_ the required _unregister steps as cleanup in an
error path, and propagating the result up to driver probe. This way we
don't need to store any additional information at all.

> Andi

Thanks
Krzysztof