Re: [PATCH v4 01/11] firmware: raspberrypi: Keep count of all consumers

[Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>]

On Thu, Nov 12, 2020 at 07:52:14PM +0200, Andy Shevchenko wrote:
> On Thu, Nov 12, 2020 at 6:40 PM Nicolas Saenz Julienne
> <nsaenzjulienne@xxxxxxx> wrote:
> >
> > When unbinding the firmware device we need to make sure it has no
> > consumers left. Otherwise we'd leave them with a firmware handle
> > pointing at freed memory.
> >
> > Keep a reference count of all consumers and introduce rpi_firmware_put()
> > which will permit automatically decrease the reference count upon
> > unbinding consumer drivers.
> 
> ...
> 
> >  /**
> > - * rpi_firmware_get - Get pointer to rpi_firmware structure.
> >   * @firmware_node:    Pointer to the firmware Device Tree node.
> >   *
> > + * The reference to rpi_firmware has to be released with rpi_firmware_put().
> > + *
> >   * Returns NULL is the firmware device is not ready.
> >   */
> >  struct rpi_firmware *rpi_firmware_get(struct device_node *firmware_node)
> >  {
> >         struct platform_device *pdev = of_find_device_by_node(firmware_node);
> > +       struct rpi_firmware *fw;
> >
> >         if (!pdev)
> >                 return NULL;
> >
> > -       return platform_get_drvdata(pdev);
> > +       fw = platform_get_drvdata(pdev);
> > +       if (!fw)
> > +               return NULL;
> > +
> > +       if (!kref_get_unless_zero(&fw->consumers))
> > +               return NULL;
> 
> Don't we have a more traditional way of doing this, i.e.
> try_module_get() coupled with get_device() ?

get_device() will make sure that device is there, but gives no
assurances that device is bound to a driver, so it will not help with
the racy access to firmware via platform_get_drvdata() call.

Thanks.

-- 
Dmitry