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

["Paul E. McKenney" <paulmck@xxxxxxxxxx>]

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.

							Thanx, Paul