Re: [Xen-devel] [PATCH 12/29] drivers, media: convert s2255_dev.num_channels from atomic_t to refcount_t

["Reshetova, Elena" <elena.reshetova@xxxxxxxxx>]

> Hi Elena,
> 
> On Mon, Mar 06, 2017 at 04:20:59PM +0200, Elena Reshetova wrote:
> > refcount_t type and corresponding API should be
> > used instead of atomic_t when the variable is used as
> > a reference counter. This allows to avoid accidental
> > refcounter overflows that might lead to use-after-free
> > situations.
> >
> > Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> > Signed-off-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > Signed-off-by: David Windsor <dwindsor@xxxxxxxxx>
> > ---
> ...
> > @@ -1688,7 +1689,7 @@ static int s2255_probe_v4l(struct s2255_dev *dev)
> >                             "failed to register
> video device!\n");
> >                     break;
> >             }
> > -           atomic_inc(&dev->num_channels);
> > +           refcount_set(&dev->num_channels, 1);
> 
> That's not right. The loop runs four iterations and the value after the
> loop should be indeed the number of channels.

Oh, yes, I was blind here, sorry. The problem why it cannot be left as inc is 
because it would do increment from zero here, which is not allowed by 
refcount_t interface. 

> atomic_t isn't really used for reference counting here, but storing out how
> many "channels" there are per hardware device, with maximum number of four
> (4). I'd leave this driver using atomic_t.
Yes, sounds like the best thing to do. I will drop this patch. 

Thank you for reviews!

Best Regards,
Elena.
> 
> >             v4l2_info(&dev->v4l2_dev, "V4L2 device registered
> as %s\n",
> >                       video_device_node_name(&vc-
> >vdev));
> >
> 
> --
> Kind regards,
> 
> Sakari Ailus
> e-mail: sakari.ailus@xxxxxx   XMPP: sailus@xxxxxxxxxxxxxx

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