Re: [PATCH 2/3] gpiolib: use kref in gpio_desc

[Linus Walleij <linus.walleij@linaro.org>]

On Fri, Mar 13, 2020 at 3:47 PM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> czw., 12 mar 2020 o 11:35 Linus Walleij <linus.walleij@linaro.org> napisał(a):

> In this case I was thinking about a situation where we pass a
> requested descriptor to some other framework (nvmem in this case)
> which internally doesn't know anything about who manages this resource
> externally. Now we can of course simply not do anything about it and
> expect the user (who passed us the descriptor) to handle the resources
> correctly. But what happens if the user releases the descriptor not on
> driver detach but somewhere else for whatever reason while nvmem
> doesn't know about it? It may try to use the descriptor which will now
> be invalid. Reference counting in this case would help IMHO.

I'm so confused because I keep believing it is reference counted
elsewhere.

struct gpio_desc *d always comes from the corresponding
struct gpio_device *descs array. This:

struct gpio_device {
        int                     id;
        struct device           dev;
(...)
        struct gpio_desc        *descs;
(...)

This array is allocated in gpiochip_add_data_with_key() like this:

        gdev->descs = kcalloc(chip->ngpio, sizeof(gdev->descs[0]), GFP_KERNEL);

Then it gets free:d in gpiodevice_release():

static void gpiodevice_release(struct device *dev)
{
        struct gpio_device *gdev = dev_get_drvdata(dev);
(...)
        kfree(gdev->descs);
        kfree(gdev);
}

This is the .release function for the gdev->dev, the device inside
struct gpio_device,
i.e. the same device that contains the descs in the first place. So it
is just living
and dying with the struct gpio_device.

struct gpio_device does *NOT* die in the devm_* destructor that gets called
when someone has e.g. added a gpiochip using devm_gpiochip_add_data().

I think the above observation is crucial: the lifetime of struct gpio_chip and
struct gpio_device are decoupled. When the struct gpio_chip dies, that
just "numbs" all gpio descriptors but they stay around along with the
struct gpio_device that contain them until the last
user is gone.

The struct gpio_device reference counted with the call to get_device(&gdev->dev)
in gpiod_request() which is on the path of gpiod_get_[index]().

If a consumer gets a gpio_desc using any gpiod_get* API this gets
increased and it gets decreased at every gpiod_put() or by the
managed resources.

So should you not rather exploit this fact and just add something
like:

void gpiod_reference(struct gpio_desc *desc)
{
    struct gpio_device *gdev;

    VALIDATE_DESC(desc);
    gdev = desc->gdev;
    get_device(&gdev->dev);
}

void gpiod_unreference(struct gpio_desc *desc)
{
    struct gpio_device *gdev;

    VALIDATE_DESC(desc);
    gdev = desc->gdev;
    put_device(&gdev->dev);
}

This should make sure the desc and the backing gpio_device
stays around until all references are gone.

NB: We also issue try_module_get() on the module that drives the
GPIO, which will make it impossible to unload that module while it
has active GPIOs. I think maybe the whole logic inside gpiod_request()
is needed to properly add an extra reference to a gpiod else someone
can (theoretically) pull out the module from below.

Yours,
Linus Walleij