[PATCH 2/2] btrfs: fix 64-bit division link failure

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

Some of the recent refactoring of the statfs code apparently
brought back a link failure on older gcc versions that I had
fixed before:

arm-linux-gnueabi-ld: fs/btrfs/super.o: in function `btrfs_statfs':
super.c:(.text+0xec40): undefined reference to `__aeabi_uldivmod'

I think what happened is that gcc is free to not inline a function
despite the 'inline' annotation, and when this happens it can end
up partially inlining the div_u64() helper in a way that breaks the
__builtin_constant_p() based optimization.

I only see this behavior for gcc-9, but it's possible that the same
thing happens in later versions as well when the code changes again.

Change this to __always_inline to prevent it from happening again,
and add a comment about this.

Fixes: 7e17916b35797 ("btrfs: avoid link error with CONFIG_NO_AUTO_INLINE")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 fs/btrfs/super.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index f1dd172d8d5bd..7c8ee0da0f0d1 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1902,9 +1902,9 @@ static inline void btrfs_descending_sort_devices(
 
 /*
  * The helper to calc the free space on the devices that can be used to store
- * file data.
+ * file data, always inline to avoid a link failure with gcc-9 and earlier.
  */
-static inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
+static __always_inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
 					      u64 *free_bytes)
 {
 	struct btrfs_device_info *devices_info;
-- 
2.39.2