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

From: Bartosz Golaszewski
Date: Wed Jan 31 2024 - 04:42:23 EST

On Wed, Jan 31, 2024 at 10:28 AM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
>
> On Wed, Jan 31, 2024 at 10:24 AM Paul E. McKenney <paulmck@kernelorg> 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.
> >
> > Thanx, Paul
>
> 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?
>
> Bart

We have a deprecated, legacy function that returns the address of the
- now RCU-protected chip. This is of course dangerous but we have old
code using it. Can I somehow silence that warning as I don't want this
function to show that the returned pointer is marked with __rcu?

Bart