device_cgroup: avoid access to ->i_rdev in the common case in devcgroup_inode_permission()
The routine gets called for every path component during lookup.
->i_mode is going to be cached on account of permission checks, while
->i_rdev is an area which is most likely cache-cold.
gcc 14.2 is kind enough to emit one branch:
movzwl (%rbx),%eax
mov %eax,%edx
and $0xb000,%dx
cmp $0x2000,%dx
je 11bc <inode_permission+0xec>
This patch is lazy in that I don't know if the ->i_rdev branch makes
any sense with the newly added mode check upfront. I am not changing any
semantics here though.
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/20250416221626.2710239-3-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>