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Message-ID: <d0636911-e15e-40fd-ac8f-200127fe433d@paulmck-laptop>
Date: Wed, 31 Jan 2024 03:00:36 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: brgl@...ev.pl
Cc: Linus Walleij <linus.walleij@...aro.org>,
Kent Gibson <warthog618@...il.com>, Alex Elder <elder@...aro.org>,
Geert Uytterhoeven <geert+renesas@...der.be>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Wolfram Sang <wsa-dev@...g-engineering.com>,
oe-kbuild-all@...ts.linux.dev, linux-gpio@...r.kernel.org,
linux-kernel@...r.kernel.org,
Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH 20/22] gpio: protect the pointer to gpio_chip in
gpio_device with SRCU
On Wed, Jan 31, 2024 at 02:17:30AM -0800, brgl@...ev.pl wrote:
> On Wed, 31 Jan 2024 10:42:20 +0100, "Paul E. McKenney"
> <paulmck@...nel.org> 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@...nel.org> 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@...el.com> 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@intel.com/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@intel.com/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@...el.com>
> >> > > > | Closes: https://lore.kernel.org/oe-kbuild-all/202401311050.YNdm98Hv-lkp@intel.com/
> >> > > >
> >> > > > 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?
>
> 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;
This is protected by an update-side lock, correct?
> 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 so, you rely on that lock's protection by replacing the above
two lines of code with something like this:
chip = rcu_dereference_protected(gdev->chip,
lockdep_is_held(&my_lock));
> + 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);
I don't know enough to comment here, but the rest of the look good to me.
> 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
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