[PATCH 1/3] kernfs: activate a new node without dropping kernfs_rwsem

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

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

Assisted-by: LLM
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 fs/kernfs/dir.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 07abf59f0264..8953e8a07537 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -30,6 +30,8 @@ static char kernfs_pr_cont_buf[PATH_MAX];	/* protected by pr_cont_lock */
 
 #define rb_to_kn(X) rb_entry((X), struct kernfs_node, rb)
 
+static void kernfs_activate_one(struct kernfs_node *kn);
+
 static bool __kernfs_active(struct kernfs_node *kn)
 {
 	return atomic_read(&kn->active) >= 0;
@@ -861,7 +863,6 @@ int kernfs_add_one(struct kernfs_node *kn)
 	}
 
 	up_write(&root->kernfs_iattr_rwsem);
-	up_write(&root->kernfs_rwsem);
 
 	/*
 	 * Activate the new node unless CREATE_DEACTIVATED is requested.
@@ -869,9 +870,15 @@ int kernfs_add_one(struct kernfs_node *kn)
 	 * activating the node with kernfs_activate().  A node which hasn't
 	 * been activated is not visible to userland and its removal won't
 	 * trigger deactivation.
+	 *
+	 * @kn has no children yet, so kernfs_activate() would walk only @kn.
+	 * Do it here rather than dropping the write lock and taking it again
+	 * for every new node.
 	 */
-	if (!(kernfs_root(kn)->flags & KERNFS_ROOT_CREATE_DEACTIVATED))
-		kernfs_activate(kn);
+	if (!(root->flags & KERNFS_ROOT_CREATE_DEACTIVATED))
+		kernfs_activate_one(kn);
+
+	up_write(&root->kernfs_rwsem);
 	return 0;
 
 out_unlock:
-- 
2.53.0-Meta