linux-kernel - Re: [PATCH 36/51] writeback: implement bdi_for_each

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Date:	Wed, 1 Jul 2015 22:22:26 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jan Kara <jack@...e.cz>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org, hch@...radead.org,
	hannes@...xchg.org, linux-fsdevel@...r.kernel.org,
	vgoyal@...hat.com, lizefan@...wei.com, cgroups@...r.kernel.org,
	linux-mm@...ck.org, mhocko@...e.cz, clm@...com,
	fengguang.wu@...el.com, david@...morbit.com, gthelen@...gle.com,
	khlebnikov@...dex-team.ru
Subject: Re: [PATCH 36/51] writeback: implement bdi_for_each_wb()

On Wed, Jul 01, 2015 at 09:27:57AM +0200, Jan Kara wrote:
> > +#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)))

But then break or continue wouldn't work as expected.  It can get
really confusing when it's wrapped by an outer loop.

Thanks.

-- 
tejun
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