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

[Andi Shyti <andi.shyti@linux.intel.com>]

Hi Janusz,

On Tue, Feb 11, 2025 at 01:12:37PM +0100, Janusz Krzysztofik wrote:
> On Monday, 10 February 2025 14:01:19 CET Andi Shyti wrote:
> > 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@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?
> 
> For all of those, their _unregister() functions seem to be designed to be safe 
> to call even if not registered.  Like e.g. kfree() -- you can call it safely 
> even with NULL argument, you don't need to check for NULL and call it 
> conditionally.  However, ...
> 
> > 
> > In my opinion we need to check in _unregister whether the
> > drm_dev_register has succeded 
> 
> I agree with you that it would be more clear if we skipped not only 
> _register() but also _unregister() steps symmetrically, based on result of 
> drm_dev_register().
> 
> > 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). 
> 
> As long as drm doesn't provide explicit support for checking if registration 
> succeeded other than examining the return value of drm_dev_register(), I would 
> rather store that value somewhere in our drm_i915_private structure instead of 
> depending on drm internals.  What do you think?

yes, I think we could have a local flag.

Andi

> Thanks,
> Janusz
> 
> 
> > This way we could skip some of the _unregister()
> > steps.
> > 
> > Andi
> > 
> 
> 
>