Re: [PATCH 20/22] gpio: protect the pointer to gpio_chip in gpio_device with SRCU

[brgl@xxxxxxxx]

On Wed, 31 Jan 2024 10:42:20 +0100, "Paul E. McKenney"
<paulmck@xxxxxxxxxx> said:
> On Wed, Jan 31, 2024 at 10:28:19AM +0100, Bartosz Golaszewski wrote:
>> On Wed, Jan 31, 2024 at 10:24 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>> >
>> > On Wed, Jan 31, 2024 at 10:02:40AM +0100, Bartosz Golaszewski wrote:
>> > > On Wed, Jan 31, 2024 at 3:21 AM kernel test robot <lkp@xxxxxxxxx> wrote:
>> > > >
>> > > > Hi Bartosz,
>> > > >
>> > > > kernel test robot noticed the following build warnings:
>> > > >
>> > > > [auto build test WARNING on brgl/gpio/for-next]
>> > > > [also build test WARNING on linus/master v6.8-rc2 next-20240130]
>> > > > [If your patch is applied to the wrong git tree, kindly drop us a note.
>> > > > And when submitting patch, we suggest to use '--base' as documented in
>> > > > https://git-scm.com/docs/git-format-patch#_base_tree_information]
>> > > >
>> > > > url:    https://github.com/intel-lab-lkp/linux/commits/Bartosz-Golaszewski/gpio-protect-the-list-of-GPIO-devices-with-SRCU/20240130-205537
>> > > > base:   https://git.kernel.org/pub/scm/linux/kernel/git/brgl/linux.git gpio/for-next
>> > > > patch link:    https://lore.kernel.org/r/20240130124828.14678-21-brgl%40bgdev.pl
>> > > > patch subject: [PATCH 20/22] gpio: protect the pointer to gpio_chip in gpio_device with SRCU
>> > > > config: x86_64-randconfig-122-20240131 (https://download.01.org/0day-ci/archive/20240131/202401311050.YNdm98Hv-lkp@xxxxxxxxx/config)
>> > > > compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
>> > > > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240131/202401311050.YNdm98Hv-lkp@xxxxxxxxx/reproduce)
>> > > >
>> > > > If you fix the issue in a separate patch/commit (i.e. not just a new version of
>> > > > the same patch/commit), kindly add following tags
>> > > > | Reported-by: kernel test robot <lkp@xxxxxxxxx>
>> > > > | Closes: https://lore.kernel.org/oe-kbuild-all/202401311050.YNdm98Hv-lkp@xxxxxxxxx/
>> > > >
>> > > > sparse warnings: (new ones prefixed by >>)
>> > > > >> drivers/gpio/gpiolib.c:444:22: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.c:444:22: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.c:444:22: sparse:    struct gpio_chip *
>> > > >    drivers/gpio/gpiolib.c:1103:9: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.c:1103:9: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.c:1103:9: sparse:    struct gpio_chip *
>> > > >    drivers/gpio/gpiolib.c:1182:22: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.c:1182:22: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.c:1182:22: sparse:    struct gpio_chip *
>> > > >    drivers/gpio/gpiolib.c:2970:14: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.c:2970:14: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.c:2970:14: sparse:    struct gpio_chip *
>> > > >    drivers/gpio/gpiolib.c:3004:22: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.c:3004:22: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.c:3004:22: sparse:    struct gpio_chip *
>> > > >    drivers/gpio/gpiolib.c:3585:14: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.c:3585:14: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.c:3585:14: sparse:    struct gpio_chip *
>> > > >    drivers/gpio/gpiolib.c:4772:14: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.c:4772:14: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.c:4772:14: sparse:    struct gpio_chip *
>> > > >    drivers/gpio/gpiolib.c:4846:14: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.c:4846:14: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.c:4846:14: sparse:    struct gpio_chip *
>> > > >    drivers/gpio/gpiolib.c: note: in included file:
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > > >> drivers/gpio/gpiolib.h:202:1: sparse: sparse: incompatible types in comparison expression (different address spaces):
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip [noderef] __rcu *
>> > > >    drivers/gpio/gpiolib.h:202:1: sparse:    struct gpio_chip *
>> > > >
>> > > > vim +444 drivers/gpio/gpiolib.c
>> > > >
>> > > >    422
>> > > >    423  /*
>> > > >    424   * Convert a GPIO name to its descriptor
>> > > >    425   * Note that there is no guarantee that GPIO names are globally unique!
>> > > >    426   * Hence this function will return, if it exists, a reference to the first GPIO
>> > > >    427   * line found that matches the given name.
>> > > >    428   */
>> > > >    429  static struct gpio_desc *gpio_name_to_desc(const char * const name)
>> > > >    430  {
>> > > >    431          struct gpio_device *gdev;
>> > > >    432          struct gpio_desc *desc;
>> > > >    433          struct gpio_chip *gc;
>> > > >    434
>> > > >    435          if (!name)
>> > > >    436                  return NULL;
>> > > >    437
>> > > >    438          guard(srcu)(&gpio_devices_srcu);
>> > > >    439
>> > > >    440          list_for_each_entry_srcu(gdev, &gpio_devices, list,
>> > > >    441                                   srcu_read_lock_held(&gpio_devices_srcu)) {
>> > > >    442                  guard(srcu)(&gdev->srcu);
>> > > >    443
>> > > >  > 444                  gc = rcu_dereference(gdev->chip);
>> > > >    445                  if (!gc)
>> > > >    446                          continue;
>> > > >    447
>> > > >    448                  for_each_gpio_desc(gc, desc) {
>> > > >    449                          if (desc->name && !strcmp(desc->name, name))
>> > > >    450                                  return desc;
>> > > >    451                  }
>> > > >    452          }
>> > > >    453
>> > > >    454          return NULL;
>> > > >    455  }
>> > > >    456
>> > > >
>> > > > --
>> > > > 0-DAY CI Kernel Test Service
>> > > > https://github.com/intel/lkp-tests/wiki
>> > >
>> > > Paul,
>> > >
>> > > Should I care about these warnings? They seem to be emitted for a lot
>> > > of RCU code already upstream. I'm not even sure how I'd go about
>> > > addressing them honestly.
>> >
>> > This is maintainer's choice.
>> >
>> > The fix would be to apply __rcu to the definition of ->chip.  The benefit
>> > is that it finds bugs where rcu-protected pointers are used without RCU
>> > primitives and vice versa.
>>
>> Ah, good point. I marked the other RCU-protected fields like
>> descriptor label but forgot this one.
>>
>> It also seems like I need to use __rcu for all function arguments
>> taking an RCU-protected pointer as argument?
>
> Not if that argument gets its value from rcu_dereference(), which is
> the usual pattern.
>
> And if you are passing an RCU-protected pointer to a function *before*
> passing it through rcu_dereference(), I would have some questions for you.
>
> Or are you instead passing a pointer to an RCU-protected pointer as an
> argument to your functions?
>
>                                                         Thanx, Paul
>

No, I'm not. Thanks for making it clear. With the following changes to the
series, the warnings are now gone:

diff --git a/drivers/gpio/gpiolib-sysfs.c b/drivers/gpio/gpiolib-sysfs.c
index 6a421309319e..15349f92d0ec 100644
--- a/drivers/gpio/gpiolib-sysfs.c
+++ b/drivers/gpio/gpiolib-sysfs.c
@@ -762,7 +762,7 @@ EXPORT_SYMBOL_GPL(gpiod_unexport);

 int gpiochip_sysfs_register(struct gpio_device *gdev)
 {
-	struct gpio_chip *chip = gdev->chip;
+	struct gpio_chip *chip;
 	struct device *parent;
 	struct device *dev;

@@ -775,6 +775,12 @@ int gpiochip_sysfs_register(struct gpio_device *gdev)
 	if (!class_is_registered(&gpio_class))
 		return 0;

+	guard(srcu)(&gdev->srcu);
+
+	chip = rcu_dereference(gdev->chip);
+	if (!chip)
+		return -ENODEV;
+
 	/*
 	 * For sysfs backward compatibility we need to preserve this
 	 * preferred parenting to the gpio_chip parent field, if set.
diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 7ecdd8cc39c5..ea0675514891 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -221,7 +221,7 @@ struct gpio_chip *gpiod_to_chip(const struct
gpio_desc *desc)
 {
 	if (!desc)
 		return NULL;
-	return desc->gdev->chip;
+	return rcu_dereference(desc->gdev->chip);
 }
 EXPORT_SYMBOL_GPL(gpiod_to_chip);

@@ -291,7 +291,7 @@ EXPORT_SYMBOL(gpio_device_get_label);
  */
 struct gpio_chip *gpio_device_get_chip(struct gpio_device *gdev)
 {
-	return gdev->chip;
+	return rcu_dereference(gdev->chip);
 }
 EXPORT_SYMBOL_GPL(gpio_device_get_chip);

@@ -742,7 +742,7 @@ static int gpiochip_setup_dev(struct gpio_device *gdev)
 		goto err_remove_device;

 	dev_dbg(&gdev->dev, "registered GPIOs %d to %d on %s\n", gdev->base,
-		gdev->base + gdev->ngpio - 1, gdev->chip->label ? : "generic");
+		gdev->base + gdev->ngpio - 1, gdev->label);

 	return 0;

@@ -866,7 +866,7 @@ int gpiochip_add_data_with_key(struct gpio_chip
*gc, void *data,
 		return -ENOMEM;
 	gdev->dev.bus = &gpio_bus_type;
 	gdev->dev.parent = gc->parent;
-	WRITE_ONCE(gdev->chip, gc);
+	rcu_assign_pointer(gdev->chip, gc);

 	gc->gpiodev = gdev;
 	gpiochip_set_data(gc, data);
diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
index c76acb8f95c6..07443d26cbca 100644
--- a/drivers/gpio/gpiolib.h
+++ b/drivers/gpio/gpiolib.h
@@ -59,7 +59,7 @@ struct gpio_device {
 	int			id;
 	struct device		*mockdev;
 	struct module		*owner;
-	struct gpio_chip	*chip;
+	struct gpio_chip __rcu	*chip;
 	struct gpio_desc	*descs;
 	int			base;
 	u16			ngpio;

Bartosz