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device_cgroup: remove branch hint after code refactor

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[ Upstream commit 6784f27472 ]

commit 4ef4ac3601 ("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: 4ef4ac3601 ("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>
This commit is contained in:
Breno Leitao 2026-01-07 06:06:36 -08:00 • committed by Greg Kroah-Hartman
commit 9251f7cfa1
1 changed files with 1 additions and 1 deletions
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