linux-kernel - Re: [dm-devel] [PATCH 02/13] dm: kill dm_rq_bio

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Message-ID: <20120518185027.GA9673@google.com>
Date:	Fri, 18 May 2012 11:50:27 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Tejun Heo <tj@...nel.org>, axboe@...nel.dk, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, agk@...hat.com,
	Junichi Nomura <j-nomura@...jp.nec.com>,
	Mikulas Patocka <mpatocka@...hat.com>
Subject: Re: [dm-devel] [PATCH 02/13] dm: kill dm_rq_bio_destructor

On Fri, May 18, 2012 at 05:43:19PM +0100, Alasdair G Kergon wrote:
> On Fri, May 18, 2012 at 08:57:29AM -0700, Tejun Heo wrote:
> > Please explain why this is done and how it's safe.  Alasdair / dm
> > folks, can you please ack this?
>  
> I think it's relying on there being never more than one reference on those 
> bios so that that endio fn, called exactly once, always frees it and there
> are no dm_puts elsewhere.

Is that a safe assumption? From my perusal of the code it certainly
looks like it should be, but I don't know dm all that well.

Seems like it might be better to use bio set's front_pad to put it in
the same allocation as the bio, but I don't really want to get into that
myself.
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