device_cgroup: remove branch hint after code refactor

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>

diff --git a/include/linux/device_cgroup.h b/include/linux/device_cgroup.h
index 0864773a57e8fe6cf52396fcf5640e33f6de7a97..822085bc2d202cd330483c9c66f41ab5c6a982ad 100644 (file)
--- 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))