kernfs: activate a new node without dropping kernfs_rwsem

kernfs_add_one() links the node in, drops the kernfs_rwsem write lock,
then calls kernfs_activate(), which takes it again.  Roots that do not
set KERNFS_ROOT_CREATE_DEACTIVATED, such as sysfs, therefore pay two
write locks for every file, directory and symlink created.

A new node has no children, so kernfs_activate() would walk only that
node.  Call kernfs_activate_one() before dropping the lock instead.  Its
two WARN_ON_ONCE()s still hold: the node was just linked, and nothing
can have changed its active count yet.  This also closes the window
where a node is linked but not yet activated.

lock_stat, creating and destroying five dummy netdevs:

                                 before    after
  kernfs_rwsem write acquires     1200      830

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Link: https://patch.msgid.link/20260913021453.21507-2-shakeel.butt@linux.dev
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions