Re: [PATCH v2] jump_label: reduce the size of struct static_key

[Jason Baron]

On 01/20/2017 02:19 AM, Ingo Molnar wrote:
> * Jason Baron <jbaron@xxxxxxxxxx> wrote:
>
> >  struct static_key {
> >  	atomic_t enabled;
> > +/*
> > + * bit 0 => 1 if key is initially true
> > + *	    0 if initially false
> > + * bit 1 => 1 if points to struct static_key_mod
> > + *	    0 if points to struct jump_entry
> > + */
> > +	union {
> > +		unsigned long type;
> > +		struct jump_entry *entries;
> > +		struct static_key_mod *next;
> > +	};
>
>
> > +			key->type = (unsigned long)jlm2 | static_key_type(key);
>
> > +		key->type = (unsigned long)jlm | static_key_type(key);
>
> > +		*prev = (struct static_key_mod *)((unsigned long)jlm->next |
> > +				((unsigned long)*prev & JUMP_TYPE_MASK));
>
> > +			key->type = (unsigned long)jlm->entries |
> > +					static_key_type(key);
>
> I really hate these very ugly type conversions. Is there no cleaner way?
>
> For example the last line could sure be written as:
>
> 			key->entries = jlm->entries;
> 			key->type |= static_key_type(key);
>
> right?

Hi,

So that is going to over-write the static_key_type(key) in the first 
assignment. If the order is reversed we can't just |= in the pointer type.

How about:

static void jump_key_set_entries(struct static_key *key, struct 
jump_entry *entries)
{
       unsigned long type;

       type = static_key_type(key);
       key->entries = entries;
       key->type |= type;
}

and then we can also add:

void jump_key_set_mod(struct static_key *key, struct static_key_mod *mod)

doing basically the same thing. That will avoid the casts that you 
called out.

better?

Thanks,

-Jason