Re: [PATCH 3/3] gpio: use rcu_dereference_protected() to make lockdep happy

[Paul E. McKenney]

On Tue, Feb 13, 2024 at 10:31:08AM +0100, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> 
> Lockdep with CONFIG_PROVE_RCU enabled reports false positives about
> suspicious rcu_dereference() usage. Let's silence it by using
> rcu_dereference_protected().
> 
> Fixes: d83cee3d2bb1 ("gpio: protect the pointer to gpio_chip in gpio_device with SRCU")
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-lkp/202402122234.d85cca9b-lkp@xxxxxxxxx
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> ---
>  drivers/gpio/gpiolib.c | 12 ++++++++----
>  drivers/gpio/gpiolib.h |  4 +++-
>  2 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index c1391a9a0af6..d7503376b918 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -109,7 +109,8 @@ const char *gpiod_get_label(struct gpio_desc *desc)
>  		return "interrupt";
>  
>  	return test_bit(FLAG_REQUESTED, &flags) ?
> -			rcu_dereference(desc->label) : NULL;
> +			rcu_dereference_protected(desc->label,
> +					lockdep_is_held(&desc->srcu)) : NULL;

Why not this instead?

> +			srcu_dereference(desc->label, &desc->srcu) : NULL;

And similarly for the rest of the changes.

							Thanx, Paul

>  }
>  
>  static int desc_set_label(struct gpio_desc *desc, const char *label)
> @@ -2978,7 +2979,8 @@ static int gpiod_get_raw_value_commit(const struct gpio_desc *desc)
>  
>  	guard(srcu)(&gdev->srcu);
>  
> -	gc = rcu_dereference(gdev->chip);
> +	gc = rcu_dereference_protected(gdev->chip,
> +				       lockdep_is_held(&gdev->srcu));
>  	if (!gc)
>  		return -ENODEV;
>  
> @@ -3012,7 +3014,8 @@ static bool gpio_device_chip_cmp(struct gpio_device *gdev, struct gpio_chip *gc)
>  {
>  	guard(srcu)(&gdev->srcu);
>  
> -	return gc == rcu_dereference(gdev->chip);
> +	return gc == rcu_dereference_protected(gdev->chip,
> +					       lockdep_is_held(&gdev->srcu));
>  }
>  
>  int gpiod_get_array_value_complex(bool raw, bool can_sleep,
> @@ -3593,7 +3596,8 @@ int gpiod_to_irq(const struct gpio_desc *desc)
>  	gdev = desc->gdev;
>  	/* FIXME Cannot use gpio_chip_guard due to const desc. */
>  	guard(srcu)(&gdev->srcu);
> -	gc = rcu_dereference(gdev->chip);
> +	gc = rcu_dereference_protected(gdev->chip,
> +				       lockdep_is_held(&gdev->srcu));
>  	if (!gc)
>  		return -ENODEV;
>  
> diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
> index 07443d26cbca..a857848b8955 100644
> --- a/drivers/gpio/gpiolib.h
> +++ b/drivers/gpio/gpiolib.h
> @@ -14,6 +14,7 @@
>  #include <linux/err.h>
>  #include <linux/gpio/consumer.h> /* for enum gpiod_flags */
>  #include <linux/gpio/driver.h>
> +#include <linux/lockdep.h>
>  #include <linux/module.h>
>  #include <linux/notifier.h>
>  #include <linux/srcu.h>
> @@ -202,7 +203,8 @@ DEFINE_CLASS(gpio_chip_guard,
>  
>  		_guard.gdev = desc->gdev;
>  		_guard.idx = srcu_read_lock(&_guard.gdev->srcu);
> -		_guard.gc = rcu_dereference(_guard.gdev->chip);
> +		_guard.gc = rcu_dereference_protected(_guard.gdev->chip,
> +					lockdep_is_held(&_guard.gdev->srcu));
>  
>  		_guard;
>  	     }),
> -- 
> 2.40.1
>