linux-kernel - Re: [PATCH 2/4] chardev: Increment cdev reference count when i

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Message-ID: <20141104155543.GD21902@quack.suse.cz>
Date:	Tue, 4 Nov 2014 16:55:43 +0100
From:	Jan Kara <jack@...e.cz>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Jan Kara <jack@...e.cz>, hch@....de, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] chardev: Increment cdev reference count when i_cdev
 references it

On Tue 04-11-14 15:40:30, Al Viro wrote:
> On Tue, Nov 04, 2014 at 04:37:52PM +0100, Jan Kara wrote:
> > On Tue 04-11-14 14:46:11, Al Viro wrote:
> > > On Wed, Oct 22, 2014 at 10:14:11PM +0200, Jan Kara wrote:
> > > > Currently i_cdev reference to a character device isn't accounted in the
> > > > reference count of the character device. This then requires us to track
> > > > all references through a list of all inodes referencing a character
> > > > device which is somewhat clumsy and requires list_head in each inode in
> > > > the system.
> > > > 
> > > > So make i_cdev a reference like any other.
> > > 
> > > So no rmmod for you until an inode of character device node you had
> > > closed a while ago finally gets evicted from icache?  Or am I misreading
> > > you?
> >   Yes, this is a consequence of the change. I should have noted in the
> > changelog I guess.
> 
> That consequence looks broken, IMO.
  Hum, it already behaves for block devices that way (and noone complained
- but admittedly block devices tied to strange modules are less common than
character devices). Also rmmod isn't that common IMO, but I see your point
that it's unintuitive behavior.

Alternatively we could clear i_cdev when the inode isn't open anymore (and
thus doesn't hold any references to cdev). The downside is we have to
re-lookup the character device on the first inode open. Would such a
solution look acceptable to you?

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