Re: [PATCH v6 3/3] drm/i915: Make i915_modparams members const

[Jani Nikula <jani.nikula@intel.com>]

On Wed, 20 Sep 2017, Joonas Lahtinen <joonas.lahtinen@linux.intel.com> wrote:
> On Tue, 2017-09-19 at 19:38 +0000, Michal Wajdeczko wrote:
>> We should discourage developers from modifying modparams.
>> Introduce special macro for easier tracking of changes done
>> in modparams and enforce its use by defining existing modparams
>> members as const. Note that defining whole modparams struct
>> as const makes checkpatch unhappy.
>> 
>> v2: rebased
>> 
>> Credits-to: Coccinelle
>> 
>> @@
>> identifier n;
>> expression e;
>> @@
>> (
>> -	i915_modparams.n = e;
>> +	i915_modparams_set(n, e);
>
> Not cool with such a brief name, it really needs to be something more
> standing out to make the developer think they've failed design if
> they're calling the function.
>
> 'i915_modparams_force_write' is my current favourite.
>
> And we need huge kerneldoc comment for the function about the concerns
> expressed by Jani, me and Ville. There must be no potential readers for
> the variables while they're being changed, compiler optimizations need
> to be watched for etc.
>
> Because really, if we change a module parameter variable while somebody
> is for example running a loop based on it, we're in deep problems.
>
> Might be worthwhile having a i915_modparams_lock to be taken when
> sanitization of options begins, and asserting that lock is held when
> _force_write() is being called. rw_semaphore sounds like the right
> choice here. Many can read but only one can write.
>
> Any opinions on that?

It can't protect against users changing the parameters via sysfs, and I
think fixing that at the moment would have an air of overengineering.

I'm thinking review and merge patch 1 to fix the i915 name collision,
and forget about the rest for now. Too much controversy, no real rush or
pressure to do anything right now beyond patch 1. Don't just do
something, stand there.

BR,
Jani.

-- 
Jani Nikula, Intel Open Source Technology Center
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