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

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Date:   Thu, 12 Nov 2020 23:26:15 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        linux-pwm@...r.kernel.org,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        devicetree <devicetree@...r.kernel.org>,
        Stefan Wahren <wahrenst@....net>,
        linux-input <linux-input@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "open list:STAGING SUBSYSTEM" <devel@...verdev.osuosl.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        linux-rpi-kernel <linux-rpi-kernel@...ts.infradead.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH v4 01/11] firmware: raspberrypi: Keep count of all
 consumers

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@...e.de> 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