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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Fri, 20 Mar 2020 11:58:53 +0100
Message-ID: <CAMuHMdXWrK3QjGmAPC0aPtQ_62buiQLopyRKB-qro6HBK5bDyg@mail.gmail.com>
Subject: Re: [PATCH] gpiolib: don't call sleeping functions with a spinlock taken
To:     Bartosz Golaszewski <brgl@bgdev.pl>
Cc:     Linus Walleij <linus.walleij@linaro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>
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Hi Bartosz,

On Fri, Mar 20, 2020 at 10:31 AM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> From: Bartosz Golaszewski <bgolaszewski@baylibre.com>
>
> We must not call pinctrl_gpio_can_use_line() with the gpio_lock taken
> as it takes a mutex internally. Let's move the call before taking the
> spinlock and store the return value.
>
> This isn't perfect - there's a moment between calling
> pinctrl_gpio_can_use_line() and taking the spinlock where the situation
> can change but it isn't a regression either: previously this part wasn't
> protected at all and it only affects the information user-space is
> seeing.
>
> Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Fixes: d2ac25798208 ("gpiolib: provide a dedicated function for setting lineinfo")
> Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

Thanks for your patch!

> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -1154,8 +1154,19 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
>                                   struct gpioline_info *info)
>  {
>         struct gpio_chip *chip = desc->gdev->chip;
> +       bool ok_for_pinctrl;
>         unsigned long flags;
>
> +       /*
> +        * This function takes a mutex so we must check this before taking
> +        * the spinlock.
> +        *
> +        * FIXME: find a non-racy way to retrieve this information. Maybe a
> +        * lock common to both frameworks?
> +        */
> +       ok_for_pinctrl = pinctrl_gpio_can_use_line(
> +                               chip->base + info->line_offset);

Note that this is now called unconditionally, while before it was only called
if all previous steps in the ||-pipeline failed.

> +
>         spin_lock_irqsave(&gpio_lock, flags);
>
>         if (desc->name) {
> @@ -1182,7 +1193,7 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
>             test_bit(FLAG_USED_AS_IRQ, &desc->flags) ||
>             test_bit(FLAG_EXPORT, &desc->flags) ||
>             test_bit(FLAG_SYSFS, &desc->flags) ||
> -           !pinctrl_gpio_can_use_line(chip->base + info->line_offset))
> +           !ok_for_pinctrl)
>                 info->flags |= GPIOLINE_FLAG_KERNEL;
>         if (test_bit(FLAG_IS_OUT, &desc->flags))
>                 info->flags |= GPIOLINE_FLAG_IS_OUT;

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds