Re: [PATCH v2 1/2] btrfs: open code RCU for device name

[David Sterba <dsterba@suse.cz>]

On Fri, Jun 27, 2025 at 05:13:20PM +0800, kernel test robot wrote:
> Hi David,
> 
> kernel test robot noticed the following build warnings:
> 
> [auto build test WARNING on kdave/for-next]
> [also build test WARNING on next-20250626]
> [cannot apply to linus/master v6.16-rc3]
> [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/David-Sterba/btrfs-open-code-RCU-for-device-name/20250625-214017
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux.git for-next
> patch link:    https://lore.kernel.org/r/1e539dfd73debc86ddc7c1b1716f86ace14d51aa.1750858539.git.dsterba%40suse.com
> patch subject: [PATCH v2 1/2] btrfs: open code RCU for device name
> config: powerpc64-randconfig-r133-20250627 (https://download.01.org/0day-ci/archive/20250627/202506271745.xBqbUHXo-lkp@intel.com/config)
> compiler: clang version 21.0.0git (https://github.com/llvm/llvm-project e04c938cc08a90ae60440ce22d072ebc69d67ee8)
> reproduce: (https://download.01.org/0day-ci/archive/20250627/202506271745.xBqbUHXo-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@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202506271745.xBqbUHXo-lkp@intel.com/
> 
> sparse warnings: (new ones prefixed by >>)
> >> fs/btrfs/volumes.c:407:21: sparse: sparse: incorrect type in argument 1 (different address spaces) @@     expected void const *objp @@     got char const [noderef] __rcu *name @@
>    fs/btrfs/volumes.c:407:21: sparse:     expected void const *objp
>    fs/btrfs/volumes.c:407:21: sparse:     got char const [noderef] __rcu *name
> >> fs/btrfs/volumes.c:661:43: sparse: sparse: incorrect type in argument 1 (different address spaces) @@     expected char const *device_path @@     got char const [noderef] __rcu *name @@
>    fs/btrfs/volumes.c:661:43: sparse:     expected char const *device_path
>    fs/btrfs/volumes.c:661:43: sparse:     got char const [noderef] __rcu *name
> >> fs/btrfs/volumes.c:1022:34: sparse: sparse: incorrect type in assignment (different address spaces) @@     expected char const *dev_path @@     got char const [noderef] __rcu *name @@
>    fs/btrfs/volumes.c:1022:34: sparse:     expected char const *dev_path
>    fs/btrfs/volumes.c:1022:34: sparse:     got char const [noderef] __rcu *name
> >> fs/btrfs/volumes.c:1424:42: sparse: sparse: incorrect type in argument 1 (different address spaces) @@     expected char const * @@     got char const [noderef] __rcu *name @@
>    fs/btrfs/volumes.c:1424:42: sparse:     expected char const *
>    fs/btrfs/volumes.c:1424:42: sparse:     got char const [noderef] __rcu *name
>    fs/btrfs/volumes.c:2174:31: sparse: sparse: incorrect type in argument 1 (different address spaces) @@     expected char const *device_path @@     got char const [noderef] __rcu *name @@
>    fs/btrfs/volumes.c:2174:31: sparse:     expected char const *device_path
>    fs/btrfs/volumes.c:2174:31: sparse:     got char const [noderef] __rcu *name
> 
> vim +407 fs/btrfs/volumes.c
> 
> 2208a378f35fea7 Ilya Dryomov    2013-08-12  402  
> f2db4d5cb457b7d Filipe Manana   2023-04-26  403  static void btrfs_free_device(struct btrfs_device *device)
> 48dae9cf3f95acc David Sterba    2017-10-30  404  {
> bbbf7243d62d8be Nikolay Borisov 2019-03-25  405  	WARN_ON(!list_empty(&device->post_commit_list));
> 9fc9a5a2dc4bd09 David Sterba    2025-06-09  406  	/* No need to call kfree_rcu(), nothing is reading the device name. */
> 9fc9a5a2dc4bd09 David Sterba    2025-06-09 @407  	kfree(device->name);

All the warnings are for places where raw ->name is used because we know
it's safe given the context. A rcu_dereference would work and is not
necessary but will trigger the warnings. OTOH if there isn't the __rcu
annotation we can't get any checks where it would be desired.