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

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Date:	Fri, 3 Jul 2015 14:26:27 +0200
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jan Kara <jack@...e.cz>, 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 01-07-15 22:22:26, Tejun Heo wrote:
> 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.

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.
 */

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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