Message-ID: <d9def9db0704271543pd38f73ck4d46f98f77b230f@mail.gmail.com>
Date:	Sat, 28 Apr 2007 00:43:43 +0200
From:	"Markus Rechberger" <mrechberger@...il.com>
To:	"Trent Piepho" <xyzzy@...akeasy.org>
Cc:	"Michael Krufky" <mkrufky@...uxtv.org>,
	"Linux and Kernel Video" <video4linux-list@...hat.com>,
	"Linux Kernel Mailing list" <linux-kernel@...r.kernel.org>,
	"Mauro Carvalho Chehab" <mchehab@...radead.org>,
	linux-dvb-maintainer@...uxtv.org
Subject: Re: [v4l-dvb-maintainer] [GIT PATCHES] V4L/DVB updates

On 4/27/07, Trent Piepho <xyzzy@...akeasy.org> wrote:
> On Mon, 16 Apr 2007, Markus Rechberger wrote:
> > On 4/16/07, Michael Krufky <mkrufky@...uxtv.org> wrote:
> > > Adrian Bunk wrote:
> > > > On Sun, Apr 15, 2007 at 08:33:38PM -0400, Michael Krufky wrote:
> > > >> Mauro,
> > > >>
> > > >> I've been out of town for the past few days... I just got home and
> saw
> > > this:
> > > >>
> > > >>
> > > >> Mauro Carvalho Chehab wrote:
> > > >>>    - Fix 1/3 for bug 7819: fixed frontend hotplug issue
> > > >>>    - Fix 2/3 for bug 7819: demux and dvr
> > > >>>    - Fix 3/3 for bug 7819: fixed hotplugging for dvbnet
> > > >> I don't think that this is 2.6.21 material.  These patches have not
> yet
> > > >> received
> > > >> enough testing to be sent to mainline.
>
> I wish these patches had more comments about how they worked, because it's
> not clear to me exactly what they are doing or why they do it.
>
> Fix 3/3 for bug 7819: fixed hotplugging for dvbnet
>
> I'm just going to talk about this one patch, since it appears to be the
> simplest.
>
> This patch removes the 'wait_queue' memory of dvb_demux.  Why does a patch
> to dvbnet have anything to do with the demux device?  Should this change
> have been part of the previous patch, the one what fixes demux and dvr?
>

The waitqueue got added in a previous patch just ignore it..

> The patch replace the (file_operations)->release pointer with the new
> function dvb_net_close() in place of dvb_generic_release().  The first
> part of dvb_net_close() is just a cut&paste from dvb_generic_release(),
> so maybe it would be better to just call dvb_generic_release() instead?
>

no because it needs the dvb_net structure which cannot be used in the
dvb_generic_release function.

> There is also a bug:
>
> +static int dvb_net_close(struct inode *inode, struct file *file)
> +{
> +       struct dvb_device *dvbdev = file->private_data;
> +       struct dvb_net *dvbnet = dvbdev->priv;
> +
> +       if (!dvbdev)
> +               return -ENODEV;
>
> The check for !dvbdev is too late, dvbdev was already dereferenced on the
> previous line to get dvbnet, so if dvbdev is NULL you will have already
> crashed.
>

I think this check isn't needed at all actually, why should someone
release dvbdev before already?

> Is the check for !dvbdev unnecessary paranoia?  Or can dvbdev actually be
> NULL is some cases?
>
> So, w.r.t. dvbnet, my understanding is that somehow dvb_net_release() is
> called while the net device is still in use.  Is that correct?
> dvb_net_release() will end up deleting some stuff that needs to be around
> while there is still a user of the device?
>
> Would it be possible to keep dvb_net_release() from getting called in the
> first place until there are no users of the device?
>
> When looking at this code, keep in mind that users starts at 1 and goes
> down as the device is used.
>
> void dvb_net_release (struct dvb_net *dvbnet)
> {
>         int i;
>
>         dvbnet->exit = 1;
>         if (dvbnet->dvbdev->users < 1)		/* line A */
>                 wait_event(dvbnet->dvbdev->wait_queue,  /* line B */
>                                 dvbnet->dvbdev->users==1);
> 	/* line C */
>         dvb_unregister_device(dvbnet->dvbdev);
> 	[...]
> }
>
> Isn't there a race condition here?  What if there are no users of the
> device at the time line A runs, but the device is opened before line C
> runs?  After the wait_even() on line B returns there are no users, but what
> if new users opens the device after the after line B?
>

yes this is a valid argument, practically it will never occure though.
Since I also wrote that I didn't have a dvb signal at the time of
writing this I don't even know if the dvb-net implementation is ok at
all.
I don't even know if that part of the framework works and if someone
really uses it.

> Is seems like you need something to prevent new users from opening the
> device, i.e. make it so that dvbnet->exit=1 prevents the device from
> getting opened, but if that's the intention, I don't see where it's
> happening.
>
> In dvb_net_close():
>
> +       if(dvbdev->users == 1 && dvbnet->exit==1) {
> +               fops_put(file->f_op);
> +               file->f_op = NULL;
> +               wake_up(&dvbdev->wait_queue);
> +       }
>
> What is the purpose of the fops_put() call and setting file->f_op to NULL?
>

fops_put lowers the usecount on the inode, after wake_up the f_op
structure is invalid because it's just allocated memory which gets
freed by the dvb release function.

thanks for reviewing,
Markus
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