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Re: [PATCH 36/51] writeback: implement bdi_for_each_wb()

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  Re: [PATCH 36/51] writeback: implement bdi_for_each_wb() Jan Kara <jack@suse.cz> - 2015-07-01 09:30 +0200
    Re: [PATCH 36/51] writeback: implement bdi_for_each_wb() Tejun Heo <tj@kernel.org> - 2015-07-03 19:10 +0200

#1175033 — Re: [PATCH 36/51] writeback: implement bdi_for_each_wb()

On Fri 22-05-15 17:13:50, Tejun Heo wrote:
> This will be used to implement bdi-wide operations which should be
> distributed across all its cgroup bdi_writebacks.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Jan Kara <jack@suse.cz>

One comment below.

> @@ -445,6 +500,14 @@ static inline void wb_blkcg_offline(struct blkcg *blkcg)
>  {
>  }
>  
> +struct wb_iter {
> +	int		next_id;
> +};
> +
> +#define bdi_for_each_wb(wb_cur, bdi, iter, start_blkcg_id)		\
> +	for ((iter)->next_id = (start_blkcg_id);			\
> +	     ({	(wb_cur) = !(iter)->next_id++ ? &(bdi)->wb : NULL; }); )
> +

This looks quite confusing. Won't it be easier to understand as:

struct wb_iter {
} __attribute__ ((unused));

#define bdi_for_each_wb(wb_cur, bdi, iter, start_blkcg_id) \
  if (((wb_cur) = (!start_blkcg_id ? &(bdi)->wb : NULL)))

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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#1176727

On Fri, Jul 03, 2015 at 02:26:27PM +0200, Jan Kara wrote:
> That's a good point. Thanks for explanation. Maybe add a comment like:
> /*
>  * We use use this seemingly complicated 'for' loop so that 'break' and
>  * 'continue' continue to work as expected.
>  */

This kinda feel superflous for me.  This is something true for all
iteration wrappers which falls within the area of well-established
convention, I think.  If it's doing something weird like combining
if-else clause to do post-conditional processing, sure, but this is
really kinda standard.

Thanks.

-- 
tejun
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