Re: [v4l-dvb-maintainer] [GIT PATCHES] V4L/DVB updates

[Trent Piepho]

On Sat, 28 Apr 2007, Markus Rechberger wrote:
> On 4/27/07, Trent Piepho <xyzzy@xxxxxxxxxxxxx> wrote:
> > On Mon, 16 Apr 2007, Markus Rechberger wrote:
> > > > >> 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..

Do the patches need to be applied all together?  If I apply just the first
two, do I get something that's broken?

> > 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.

I mean, have dvb_net_close() call dvb_generic_release() and then do the
extra stuff.

> > 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?

What about this?  I don't have any hot pluggable hardware, so how does
dvb_net_release() get called when the device disconnects?

> > 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.

fopts_put() lowers the usecount of the module that owns the inode.  I don't
see how it works to add this when it wasn't here before?  Isn't it going to
mess up the module usecount by doing a fopts_put() that's not matched by a
fopts_get()?  I didn't see anything in your patch that added a fops_put().
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