Re: [PATCH 1/2] null_blk: remove hardcoded alloc_cmd() parameter

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 2/16/22 18:30, Chaitanya Kulkarni wrote:
> Only caller of alloc_cmd() is null_submit_bio() unconditionally sets
> second parameter to true & that is statically hard-coded in null_blk.

Nit: s/&/and (let's write in proper English :))

> There is no point in having statically hardcoded function parameter.
> 
> Remove the unnecessary parameter can_wait and adjust the code so it
> can retain existing behavior of waiting when we don't get valid
> nullb_cmd from __alloc_cmd() in alloc_cmd().
> 
> The restructured code avoids multiple return statements, multiple
> calls to __alloc_cmd() and resulting a fast path call to
> prepare_to_wait() due to removal of first alloc_cmd() call.
> 
> Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
> ---
>  drivers/block/null_blk/main.c | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
> index 13004beb48ca..d78fc3edb22e 100644
> --- a/drivers/block/null_blk/main.c
> +++ b/drivers/block/null_blk/main.c
> @@ -719,26 +719,23 @@ static struct nullb_cmd *__alloc_cmd(struct nullb_queue *nq)
>  	return NULL;
>  }
>  
> -static struct nullb_cmd *alloc_cmd(struct nullb_queue *nq, int can_wait)
> +static struct nullb_cmd *alloc_cmd(struct nullb_queue *nq)
>  {
>  	struct nullb_cmd *cmd;
>  	DEFINE_WAIT(wait);
>  
> -	cmd = __alloc_cmd(nq);
> -	if (cmd || !can_wait)
> -		return cmd;
> -
>  	do {
> -		prepare_to_wait(&nq->wait, &wait, TASK_UNINTERRUPTIBLE);
> +		/*
> +		 * This avoids multiple return statements, multiple calls to
> +		 * __alloc_cmd() and a fast path call to prepare_to_wait().
> +		 */
>  		cmd = __alloc_cmd(nq);
>  		if (cmd)
> -			break;
> -
> +			return cmd;
> +		prepare_to_wait(&nq->wait, &wait, TASK_UNINTERRUPTIBLE);
>  		io_schedule();
> +		finish_wait(&nq->wait, &wait);
>  	} while (1);
> -
> -	finish_wait(&nq->wait, &wait);
> -	return cmd;
>  }

Personally, I would simplify further like this:


	while (!(cmd = __alloc_cmd(nq)) {
		prepare_to_wait(&nq->wait, &wait, TASK_UNINTERRUPTIBLE);
		io_schedule();
		finish_wait(&nq->wait, &wait);
  	}

	return cmd;

But that is a matter of taste I guess.

>  
>  static void end_cmd(struct nullb_cmd *cmd)
> @@ -1478,7 +1475,7 @@ static void null_submit_bio(struct bio *bio)
>  	struct nullb_queue *nq = nullb_to_queue(nullb);
>  	struct nullb_cmd *cmd;
>  
> -	cmd = alloc_cmd(nq, 1);
> +	cmd = alloc_cmd(nq);
>  	cmd->bio = bio;
>  
>  	null_handle_cmd(cmd, sector, nr_sectors, bio_op(bio));


-- 
Damien Le Moal
Western Digital Research