linux-kernel - Re: [PATCH 44/51] writeback: implement bdi_wait_for

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Message-ID: <20150702030624.GM26440@mtj.duckdns.org>
Date:	Wed, 1 Jul 2015 23:06:24 -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 44/51] writeback: implement bdi_wait_for_completion()

Hello, Jan.

On Wed, Jul 01, 2015 at 06:04:37PM +0200, Jan Kara wrote:
> I'd find it better to extend completions to allow doing what you need. It
> isn't that special. It seems it would be enough to implement
> 
> void wait_for_completions(struct completion *x, int n);
> 
> where @n is the number of completions to wait for. And the implementation
> can stay as is, only in do_wait_for_common() we change checks for x->done ==
> 0 to "x->done < n". That's about it...

I don't know.  While I agree that it'd be nice to have a generic event
count & trigger mechanism in the kernel, I don't think extending
completion is a good idea - the count then works both ways as the
event counter && listener counter and effectively becomes a semaphore
which usually doesn't end well.  There are very few cases where we
want the counter works both ways and I personally think we'd be far
better served if those rare cases implement something custom rather
than generic mechanism becoming cryptic trying to cover everything.

Thanks.

-- 
tejun
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