linux-kernel - Re: 2.6.35-rc2 module reference counting broken

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Message-ID: <20100608234814.GA19826@ZenIV.linux.org.uk>
Date:	Wed, 9 Jun 2010 00:48:14 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jari Ruusu <jariruusu@...rs.sourceforge.net>
Subject: Re: 2.6.35-rc2 module reference counting broken

On Mon, Jun 07, 2010 at 07:44:12AM +0100, Al Viro wrote:
> On Mon, Jun 07, 2010 at 08:20:30AM +0300, Jari Ruusu wrote:
> > Someone broke block device module reference counting. Problem occours when a
> > modular block device is mounted and unmounted. Not when it is directly read.
> > 2.6.34 kernel works OK, but 2.6.35-rc2 kernel seems to increase usage count
> > by one for each mount + umount pair.
> 
> Very interesting...  Looks like mount() bumps refcount by 2.  umount() after
> that drops refcount by 1, so it's not leaking superblocks.
> 
> Which probably means that open_bdev_exclusive() is fscked.  Interesting...
> FWIW, quick look through the history seems to point to this:
> commit 6b4517a7913a09d3259bb1d21c9cb300f12294bd
> Author: Tejun Heo <tj@...nel.org>
> Date:   Wed Apr 7 18:53:59 2010 +0900
> 
>     block: implement bd_claiming and claiming block
> 
> I'm far too sleepy right now, but I'd start with reviewing what that
> thing is doing to module refcounting...

Yeah...  bd_start_claiming() grabs a reference to gendisk and we never
let it go.  There's your leak...
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