Re: [PATCH 5/5] null_blk: Zero-initialize read buffers in non-memory-backed mode

[Bart Van Assche <bvanassche@acm.org>]

On 2020-05-17 20:12, Damien Le Moal wrote:
> On 2020/05/18 11:56, Bart Van Assche wrote:
>> On 2020-05-17 19:10, Damien Le Moal wrote:
>>> On 2020/05/18 10:32, Bart Van Assche wrote:
>>>> On 2020-05-17 18:12, Damien Le Moal wrote:
>>>>> On 2020/05/16 9:19, Bart Van Assche wrote:
>>>>>> +static void nullb_zero_rq_data_buffer(const struct request *rq)
>>>>>> +{
>>>>>> +	struct req_iterator iter;
>>>>>> +	struct bio_vec bvec;
>>>>>> +
>>>>>> +	rq_for_each_bvec(bvec, rq, iter)
>>>>>> +		zero_fill_bvec(&bvec);
>>>>>> +}
>>>>>> +
>>>>>> +static void nullb_zero_read_cmd_buffer(struct nullb_cmd *cmd)
>>>>>> +{
>>>>>> +	struct nullb_device *dev = cmd->nq->dev;
>>>>>> +
>>>>>> +	if (dev->queue_mode == NULL_Q_BIO && bio_op(cmd->bio) == REQ_OP_READ)
>>>>>> +		zero_fill_bio(cmd->bio);
>>>>>> +	else if (req_op(cmd->rq) == REQ_OP_READ)
>>>>>> +		nullb_zero_rq_data_buffer(cmd->rq);
>>>>>> +}
>>>>>
>>>>> Shouldn't the definition of these two functions be under a "#ifdef CONFIG_KMSAN" ?
>>>>
>>>> It is on purpose that I used IS_ENABLED(CONFIG_KMSAN) below instead of
>>>> #ifdef CONFIG_KMSAN. CONFIG_KMSAN is not yet upstream and I want to
>>>> expose the above code to the build robot.
>>>
>>> But then you will get a "defined but unused" build warning, no ?
>>
>> Not when using IS_ENABLED(...).
> 
> I do not understand: the "if (IS_ENABLED(CONFIG_KMSAN))" will be compiled out if
> CONFIG_KMSAN is not enabled/defined, but the function definitions will still
> remain, won't they ? That will lead to "defined but unused" warning, no ? What
> am I missing here ?

"if (IS_ENABLED(CONFIG_KMSAN))" won't be removed by the preprocessor.
The preprocessor will convert it into if (0).

This is what I found in the gcc documentation about -Wunused-function:
"-Wunused-function
Warn whenever a static function is declared but not defined or a
non-inline static function is unused. This warning is enabled by -Wall."
I think that "if (0) func(...)" counts as using func().

Bart.