Merge patch series "kernfs: do less work under the kernfs_rwsem write lock"

Shakeel Butt <shakeel.butt@linux.dev> says:

kernfs_rwsem is per kernfs root, so sysfs and cgroupfs have one each for
the whole machine, and every create, remove and rename takes it for
writing.  Whatever a writer does while holding it, everyone else waits
for.  These three patches take work out of that section.

Patch 1: kernfs_add_one() links the node in, drops the lock, then calls
kernfs_activate(), which takes it again.  sysfs does not create nodes
deactivated, so it pays two write locks for every file and directory.
Activate before dropping the lock.  lock_stat says 1200 -> 830 write
acquisitions for creating and destroying five dummy netdevs.

Patch 2: kernfs_rename_ns() calls kstrdup_const() under the lock.  It is
the only GFP_KERNEL allocation under the write lock in kernfs, so a
rename can enter reclaim with the lock held.  Allocate before the lock.

Patch 3: kernfs_rename_ns() also frees the old name under the lock with
kfree_rcu_mightsleep().  When its batching allocation fails, which is
what happens under memory pressure, it falls back to synchronize_rcu().
A rename can then wait out a grace period with the lock held.  Free
after the unlock.

Tested with the kernfs selftests under KASAN and lockdep.

* patches from https://patch.msgid.link/20260913021453.21507-1-shakeel.butt@linux.dev:
  kernfs: free the old name outside kernfs_rwsem
  kernfs: allocate the new name outside kernfs_rwsem
  kernfs: activate a new node without dropping kernfs_rwsem

Link: https://patch.msgid.link/20260913021453.21507-1-shakeel.butt@linux.dev
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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