[PATCH 3/3] kernfs: free the old name outside kernfs_rwsem

[Shakeel Butt <shakeel.butt@linux.dev>]

kernfs_rename_ns() frees the replaced name with kfree_rcu_mightsleep()
while still holding the kernfs_rwsem write lock.  If the batching
allocation fails, which is what happens under memory pressure,
kvfree_call_rcu() falls back to a full synchronize_rcu() before freeing.
A rename can then wait out a grace period with the write lock held, and
every create, remove and rename in the hierarchy waits with it.

The name is already unpublished by then, so nothing needs the free to
happen under the lock.  Move it past the unlock.

Assisted-by: LLM
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 fs/kernfs/dir.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 1fa288a48d7c..b071071e51b0 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1820,6 +1820,7 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
 {
 	struct kernfs_node *old_parent;
 	const char *dup_name = NULL;
+	const char *put_name = NULL;
 	struct kernfs_root *root;
 	const char *old_name;
 	bool reparent;
@@ -1905,12 +1906,14 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
 	kernfs_link_sibling(kn);
 
 	if (new_name && !is_kernel_rodata((unsigned long)old_name))
-		kfree_rcu_mightsleep(old_name);
+		put_name = old_name;
 
 	error = 0;
  out:
 	up_write(&root->kernfs_rwsem);
 	kfree_const(dup_name);
+	if (put_name)
+		kfree_rcu_mightsleep(put_name);
 	return error;
 }
 
-- 
2.53.0-Meta