[PATCH 6.18 009/641] device_cgroup: remove branch hint after code refactor

Greg Kroah-Hartman
2026-02-25T01:42:35Z

6.18-stable review patch. If anyone has any objections, please let me know.

------------------

From: Breno Leitao <leitao@debian.org>

[ Upstream commit 6784f274722559c0cdaaa418bc8b7b1d61c314f9 ]

commit 4ef4ac360101 ("device_cgroup: avoid access to ->i_rdev in the
common case in devcgroup_inode_permission()") reordered the checks in
devcgroup_inode_permission() to check the inode mode before checking
i_rdev, for better cache behavior.

However, the likely() annotation on the i_rdev check was not updated
to reflect the new code flow. Originally, when i_rdev was checked
first, likely(!inode->i_rdev) made sense because most inodes were(?)
regular files/directories, thus i_rdev == 0.

After the reorder, by the time we reach the i_rdev check, we have
already confirmed the inode IS a block or character device. Block and
character special files are precisely defined by having a device number
(i_rdev), so !inode->i_rdev is now the rare edge case, not the common
case.

Branch profiling confirmed this is 100% mispredicted:

correct incorrect % Function File Line
------- --------- - -------- ---- ----
0 2631904 100 devcgroup_inode_permission device_cgroup.h 24

Remove likely() to avoid giving the wrong hint to the CPU.

Fixes: 4ef4ac360101 ("device_cgroup: avoid access to ->i_rdev in the common case in devcgroup_inode_permission()")
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260107-likely_device-v1-1-0c55f83a7e47@debian.org
Reviewed-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/device_cgroup.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/device_cgroup.h b/include/linux/device_cgroup.h
index 0864773a57e8f..822085bc2d202 100644
--- a/include/linux/device_cgroup.h
+++ b/include/linux/device_cgroup.h
@@ -21,7 +21,7 @@ static inline int devcgroup_inode_permission(struct inode *inode, int mask)
if (likely(!S_ISBLK(inode->i_mode) && !S_ISCHR(inode->i_mode)))
return 0;

- if (likely(!inode->i_rdev))
+ if (!inode->i_rdev)
return 0;

if (S_ISBLK(inode->i_mode))
--
2.51.0