linux-kernel - Re: [PATCH v2] jump_label: reduce the size of struct static

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Date:   Fri, 20 Jan 2017 15:02:01 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     peterz@...radead.org, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] jump_label: reduce the size of struct static_key

On 01/20/2017 02:19 AM, Ingo Molnar wrote:
>
> * Jason Baron <jbaron@...mai.com> 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