Re: [PATCH v5 3/5] gpiolib: cdev: reduce locking in gpio_desc_to_lineinfo()

[Bartosz Golaszewski <brgl@xxxxxxxx>]

On Tue, Dec 19, 2023 at 1:42 AM Kent Gibson <warthog618@xxxxxxxxx> wrote:
>
> Reduce the time holding the gpio_lock by snapshotting the desc flags,
> rather than testing them individually while holding the lock.
>
> Accept that the calculation of the used field is inherently racy, and
> only check the availability of the line from pinctrl if other checks
> pass, so avoiding the check for lines that are otherwise in use.
>
> Signed-off-by: Kent Gibson <warthog618@xxxxxxxxx>
> Reviewed-by: Andy Shevchenko <andy@xxxxxxxxxx>
> ---
>  drivers/gpio/gpiolib-cdev.c | 74 ++++++++++++++++++-------------------
>  1 file changed, 36 insertions(+), 38 deletions(-)
>
> diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> index aecc4241b6c8..9f5104d7532f 100644
> --- a/drivers/gpio/gpiolib-cdev.c
> +++ b/drivers/gpio/gpiolib-cdev.c
> @@ -2399,74 +2399,72 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
>                                   struct gpio_v2_line_info *info)
>  {
>         struct gpio_chip *gc = desc->gdev->chip;
> -       bool ok_for_pinctrl;
> -       unsigned long flags;
> +       unsigned long dflags;
>
>         memset(info, 0, sizeof(*info));
>         info->offset = gpio_chip_hwgpio(desc);
>
> -       /*
> -        * 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(gc, info->offset);
> +       scoped_guard(spinlock_irqsave, &gpio_lock) {
> +               if (desc->name)
> +                       strscpy(info->name, desc->name, sizeof(info->name));
>
> -       spin_lock_irqsave(&gpio_lock, flags);
> +               if (desc->label)
> +                       strscpy(info->consumer, desc->label,
> +                               sizeof(info->consumer));
>
> -       if (desc->name)
> -               strscpy(info->name, desc->name, sizeof(info->name));
> -
> -       if (desc->label)
> -               strscpy(info->consumer, desc->label, sizeof(info->consumer));
> +               dflags = READ_ONCE(desc->flags);
> +       }
>
>         /*
> -        * Userspace only need to know that the kernel is using this GPIO so
> -        * it can't use it.
> +        * Userspace only need know that the kernel is using this GPIO so it
> +        * can't use it.
> +        * The calculation of the used flag is slightly racy, as it may read
> +        * desc, gc and pinctrl state without a lock covering all three at
> +        * once.  Worst case if the line is in transition and the calculation
> +        * is inconsistent then it looks to the user like they performed the
> +        * read on the other side of the transition - but that can always
> +        * happen.
> +        * The definitive test that a line is available to userspace is to
> +        * request it.
>          */
> -       info->flags = 0;
> -       if (test_bit(FLAG_REQUESTED, &desc->flags) ||
> -           test_bit(FLAG_IS_HOGGED, &desc->flags) ||
> -           test_bit(FLAG_USED_AS_IRQ, &desc->flags) ||
> -           test_bit(FLAG_EXPORT, &desc->flags) ||
> -           test_bit(FLAG_SYSFS, &desc->flags) ||
> +       if (test_bit(FLAG_REQUESTED, &dflags) ||
> +           test_bit(FLAG_IS_HOGGED, &dflags) ||
> +           test_bit(FLAG_USED_AS_IRQ, &dflags) ||
> +           test_bit(FLAG_EXPORT, &dflags) ||
> +           test_bit(FLAG_SYSFS, &dflags) ||
>             !gpiochip_line_is_valid(gc, info->offset) ||
> -           !ok_for_pinctrl)
> +           !pinctrl_gpio_can_use_line(gc, info->offset))
>                 info->flags |= GPIO_V2_LINE_FLAG_USED;
>
> -       if (test_bit(FLAG_IS_OUT, &desc->flags))
> +       if (test_bit(FLAG_IS_OUT, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_OUTPUT;
>         else
>                 info->flags |= GPIO_V2_LINE_FLAG_INPUT;
>
> -       if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
> +       if (test_bit(FLAG_ACTIVE_LOW, &dflags))

One more nit: you no longer have to use atomic bitops here, you can
switch to the ones without guarantees for better performance.

Bart

>                 info->flags |= GPIO_V2_LINE_FLAG_ACTIVE_LOW;
>
> -       if (test_bit(FLAG_OPEN_DRAIN, &desc->flags))
> +       if (test_bit(FLAG_OPEN_DRAIN, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_OPEN_DRAIN;
> -       if (test_bit(FLAG_OPEN_SOURCE, &desc->flags))
> +       if (test_bit(FLAG_OPEN_SOURCE, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_OPEN_SOURCE;
>
> -       if (test_bit(FLAG_BIAS_DISABLE, &desc->flags))
> +       if (test_bit(FLAG_BIAS_DISABLE, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_BIAS_DISABLED;
> -       if (test_bit(FLAG_PULL_DOWN, &desc->flags))
> +       if (test_bit(FLAG_PULL_DOWN, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_BIAS_PULL_DOWN;
> -       if (test_bit(FLAG_PULL_UP, &desc->flags))
> +       if (test_bit(FLAG_PULL_UP, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_BIAS_PULL_UP;
>
> -       if (test_bit(FLAG_EDGE_RISING, &desc->flags))
> +       if (test_bit(FLAG_EDGE_RISING, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_EDGE_RISING;
> -       if (test_bit(FLAG_EDGE_FALLING, &desc->flags))
> +       if (test_bit(FLAG_EDGE_FALLING, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_EDGE_FALLING;
>
> -       if (test_bit(FLAG_EVENT_CLOCK_REALTIME, &desc->flags))
> +       if (test_bit(FLAG_EVENT_CLOCK_REALTIME, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME;
> -       else if (test_bit(FLAG_EVENT_CLOCK_HTE, &desc->flags))
> +       else if (test_bit(FLAG_EVENT_CLOCK_HTE, &dflags))
>                 info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE;
> -
> -       spin_unlock_irqrestore(&gpio_lock, flags);
>  }
>
>  struct gpio_chardev_data {
> --
> 2.39.2
>