kernfs: change kernfs_rename_lock into a read-write lock.

kernfs_rename_lock protects a node's ->parent and thus kernfs topology.
Thus it can be used in cases that rely on a stable kernfs topology.
Change it to a read-write lock for better scalability.

Suggested by: Al Viro <viro@zeniv.linux.org.uk>

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Link: https://lore.kernel.org/r/20230309110932.2889010-4-imran.f.khan@oracle.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/dir.c

@@ -17,7 +17,7 @@
 
 #include "kernfs-internal.h"
 
-static DEFINE_SPINLOCK(kernfs_rename_lock);	/* kn->parent and ->name */
+static DEFINE_RWLOCK(kernfs_rename_lock);	/* kn->parent and ->name */
 /*
  * Don't use rename_lock to piggy back on pr_cont_buf. We don't want to
  * call pr_cont() while holding rename_lock. Because sometimes pr_cont()
@@ -196,9 +196,9 @@ int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen)
 	unsigned long flags;
 	int ret;
 
-	spin_lock_irqsave(&kernfs_rename_lock, flags);
+	read_lock_irqsave(&kernfs_rename_lock, flags);
 	ret = kernfs_name_locked(kn, buf, buflen);
-	spin_unlock_irqrestore(&kernfs_rename_lock, flags);
+	read_unlock_irqrestore(&kernfs_rename_lock, flags);
 	return ret;
 }
 
@@ -224,9 +224,9 @@ int kernfs_path_from_node(struct kernfs_node *to, struct kernfs_node *from,
 	unsigned long flags;
 	int ret;
 
-	spin_lock_irqsave(&kernfs_rename_lock, flags);
+	read_lock_irqsave(&kernfs_rename_lock, flags);
 	ret = kernfs_path_from_node_locked(to, from, buf, buflen);
-	spin_unlock_irqrestore(&kernfs_rename_lock, flags);
+	read_unlock_irqrestore(&kernfs_rename_lock, flags);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(kernfs_path_from_node);
@@ -294,10 +294,10 @@ struct kernfs_node *kernfs_get_parent(struct kernfs_node *kn)
 	struct kernfs_node *parent;
 	unsigned long flags;
 
-	spin_lock_irqsave(&kernfs_rename_lock, flags);
+	read_lock_irqsave(&kernfs_rename_lock, flags);
 	parent = kn->parent;
 	kernfs_get(parent);
-	spin_unlock_irqrestore(&kernfs_rename_lock, flags);
+	read_unlock_irqrestore(&kernfs_rename_lock, flags);
 
 	return parent;
 }
@@ -1731,7 +1731,7 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
 	kernfs_get(new_parent);
 
 	/* rename_lock protects ->parent and ->name accessors */
-	spin_lock_irq(&kernfs_rename_lock);
+	write_lock_irq(&kernfs_rename_lock);
 
 	old_parent = kn->parent;
 	kn->parent = new_parent;
@@ -1742,7 +1742,7 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
 		kn->name = new_name;
 	}
 
-	spin_unlock_irq(&kernfs_rename_lock);
+	write_unlock_irq(&kernfs_rename_lock);
 
 	kn->hash = kernfs_name_hash(kn->name, kn->ns);
 	kernfs_link_sibling(kn);