linux-kernel - Re: Distributed storage.

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Message-ID: <20070813091348.GJ23758@kernel.dk>
Date:	Mon, 13 Aug 2007 11:13:49 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	Evgeniy Polyakov <johnpol@....mipt.ru>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Distributed storage.

On Mon, Aug 13 2007, Daniel Phillips wrote:
> On Monday 13 August 2007 00:45, Jens Axboe wrote:
> > On Mon, Aug 13 2007, Jens Axboe wrote:
> > > > You did not comment on the one about putting the bio destructor
> > > > in the ->endio handler, which looks dead simple.  The majority of
> > > > cases just use the default endio handler and the default
> > > > destructor.  Of the remaining cases, where a specialized
> > > > destructor is needed, typically a specialized endio handler is
> > > > too, so combining is free.  There are few if any cases where a
> > > > new specialized endio handler would need to be written.
> > >
> > > We could do that without too much work, I agree.
> >
> > But that idea fails as well, since reference counts and IO completion
> > are two completely seperate entities. So unless end IO just happens
> > to be the last user holding a reference to the bio, you cannot free
> > it.
> 
> That is not a problem.  When bio_put hits zero it calls ->endio instead 
> of the destructor.  The ->endio sees that the count is zero and 
> destroys the bio.

You can't be serious? You'd stall end io completion notification because
someone holds a reference to a bio. Surely you jest.

Needless to say, that will never go in.

-- 
Jens Axboe

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