Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Thierry Reding <thierry.reding@gmail.com>]

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On Fri, Nov 21, 2014 at 09:36:33PM +0100, Daniel Vetter wrote:
> On Fri, Nov 21, 2014 at 09:27:04PM +0100, Thierry Reding wrote:
> > On Sat, Nov 22, 2014 at 03:10:01AM +0800, Yao Cheng wrote:
> > > on vlv, if ipvr is installed, it need be manually unloaded before
> > > i915, otherwise user might run into use-after-free issue.
> > 
> > Huh? That doesn't sound right. What exactly is it that's going wrong?
> > You should never have to do this. If you do you're almost certainly
> > doing something wrong in the kernel module.
> 
> It's the hilarity called platform devices. Removing them is somewhat racy,
> so doing that upfront makes the entire thing a bit safer. The use after
> free is on the text, since grabbing a module refcount for the platform
> device doesn't work (it would pin the module forever).

I don't understand what the issue is here. I've used platform devices
quite extensively on ARM and I've never encountered a situation where
they were insufficient (or racy for that matter).

If I understand correctly what this commit tries to achieve, then it
unloads one module before another module that it depends on so that the
dependency can be removed subsequently without causing a crash. That
sounds really brittle to me. How are you going to document this for
users so that they don't accidentally go and unload the i915 module and
crash their system?

Thierry

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