On Wed, Sep 20, 2017 at 11:54:03AM +0200, Michal Wajdeczko wrote:
> On Wed, 20 Sep 2017 10:34:43 +0200, Jani Nikula <jani.nikula@xxxxxxxxx>
> wrote:
>
> > On Tue, 19 Sep 2017, Michal Wajdeczko <michal.wajdeczko@xxxxxxxxx> 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.
> >
> > Checkpatch is the least of all reasons to not make the modparams struct
> > const.
> >
> > We can get away with having some fields (such as device info within
> > dev_priv) const, even if that's dubious.
> >
> > IIUC modifying const data is undefined behaviour at best, could cause
> > subtle bugs through compiler optimizing reads of the data away because
> > it assumes no modifications, and the data gets placed in rodata at
> > worst.
>
> Note that we're forcing modparams to be stored in dedicated section:
>
> #define __read_mostly __attribute__((__section__(".data..read_mostly")))
>
> >
> > I kinda like the union trick in this patch, but IMO we need to double
> > check what the standard says about it. Making the fellow developers
> > check the standard is always a bad sign, even if it turns out to be fine
> > after all.
>
> ISO/IEC 9899: 6.5.2.3
>
> If the member used to access the contents of a union object is not the same
> as the member last used to store a value in the object, the appropriate
> part
> of the object representation of the value is reinterpreted as an object
> representation in the new type as described in 6.2.6 (a process sometimes
> called "type punning").
There's a bunch of text in 6.7.3 which may or may not make that part
irrelevant. Very hard to say since there's no explicit example of
this kind in the spec.
--
Ville Syrjälä
Intel OTC
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