linux-kernel - [PATCH] device_cgroup: remove branch hint after code refactor

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Message-Id: <20260107-likely_device-v1-1-0c55f83a7e47@debian.org>
Date: Wed, 07 Jan 2026 06:06:36 -0800
From: Breno Leitao <leitao@...ian.org>
To: Mateusz Guzik <mjguzik@...il.com>, 
 Christian Brauner <brauner@...nel.org>
Cc: linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk, 
 rostedt@...dmis.org, linux-fsdevel@...r.kernel.org, kernel-team@...a.com, 
 Breno Leitao <leitao@...ian.org>
Subject: [PATCH] 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@...ian.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 0864773a57e8..822085bc2d20 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))

---
base-commit: f0b9d8eb98dfee8d00419aa07543bdc2c1a44fb1
change-id: 20260107-likely_device-20dae82d502d

Best regards,
--  
Breno Leitao <leitao@...ian.org>