kernfs: make ->attr.open RCU protected.

After removal of kernfs_open_node->refcnt in the previous patch,
kernfs_open_node_lock can be removed as well by making ->attr.open
RCU protected. kernfs_put_open_node can delegate freeing to ->attr.open
to RCU and other readers of ->attr.open can do so under rcu_read_(un)lock.

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

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Link: https://lore.kernel.org/r/20220615021059.862643-2-imran.f.khan@oracle.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/file.c

@@ -23,16 +23,16 @@
  * for each kernfs_node with one or more open files.
  *
  * kernfs_node->attr.open points to kernfs_open_node.  attr.open is
- * protected by kernfs_open_node_lock.
+ * RCU protected.
  *
  * filp->private_data points to seq_file whose ->private points to
  * kernfs_open_file.  kernfs_open_files are chained at
  * kernfs_open_node->files, which is protected by kernfs_open_file_mutex.
  */
-static DEFINE_SPINLOCK(kernfs_open_node_lock);
 static DEFINE_MUTEX(kernfs_open_file_mutex);
 
 struct kernfs_open_node {
+	struct rcu_head		rcu_head;
 	atomic_t		event;
 	wait_queue_head_t	poll;
 	struct list_head	files; /* goes through kernfs_open_file.list */
@@ -51,6 +51,52 @@ struct kernfs_open_node {
 static DEFINE_SPINLOCK(kernfs_notify_lock);
 static struct kernfs_node *kernfs_notify_list = KERNFS_NOTIFY_EOL;
 
+/**
+ * kernfs_deref_open_node - Get kernfs_open_node corresponding to @kn.
+ *
+ * @of: associated kernfs_open_file instance.
+ * @kn: target kernfs_node.
+ *
+ * Fetch and return ->attr.open of @kn if @of->list is non empty.
+ * If @of->list is not empty we can safely assume that @of is on
+ * @kn->attr.open->files list and this guarantees that @kn->attr.open
+ * will not vanish i.e. dereferencing outside RCU read-side critical
+ * section is safe here.
+ *
+ * The caller needs to make sure that @of->list is not empty.
+ */
+static struct kernfs_open_node *
+kernfs_deref_open_node(struct kernfs_open_file *of, struct kernfs_node *kn)
+{
+	struct kernfs_open_node *on;
+
+	on = rcu_dereference_check(kn->attr.open, !list_empty(&of->list));
+
+	return on;
+}
+
+/**
+ * kernfs_deref_open_node_protected - Get kernfs_open_node corresponding to @kn
+ *
+ * @kn: target kernfs_node.
+ *
+ * Fetch and return ->attr.open of @kn when caller holds the
+ * kernfs_open_file_mutex.
+ *
+ * Update of ->attr.open happens under kernfs_open_file_mutex. So when
+ * the caller guarantees that this mutex is being held, other updaters can't
+ * change ->attr.open and this means that we can safely deref ->attr.open
+ * outside RCU read-side critical section.
+ *
+ * The caller needs to make sure that kernfs_open_file_mutex is held.
+ */
+static struct kernfs_open_node *
+kernfs_deref_open_node_protected(struct kernfs_node *kn)
+{
+	return rcu_dereference_protected(kn->attr.open,
+				lockdep_is_held(&kernfs_open_file_mutex));
+}
+
 static struct kernfs_open_file *kernfs_of(struct file *file)
 {
 	return ((struct seq_file *)file->private_data)->private;
@@ -156,8 +202,12 @@ static void kernfs_seq_stop(struct seq_file *sf, void *v)
 static int kernfs_seq_show(struct seq_file *sf, void *v)
 {
 	struct kernfs_open_file *of = sf->private;
+	struct kernfs_open_node *on = kernfs_deref_open_node(of, of->kn);
 
-	of->event = atomic_read(&of->kn->attr.open->event);
+	if (!on)
+		return -EINVAL;
+
+	of->event = atomic_read(&on->event);
 
 	return of->kn->attr.ops->seq_show(sf, v);
 }
@@ -180,6 +230,7 @@ static ssize_t kernfs_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	struct kernfs_open_file *of = kernfs_of(iocb->ki_filp);
 	ssize_t len = min_t(size_t, iov_iter_count(iter), PAGE_SIZE);
 	const struct kernfs_ops *ops;
+	struct kernfs_open_node *on;
 	char *buf;
 
 	buf = of->prealloc_buf;
@@ -201,7 +252,15 @@ static ssize_t kernfs_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		goto out_free;
 	}
 
-	of->event = atomic_read(&of->kn->attr.open->event);
+	on = kernfs_deref_open_node(of, of->kn);
+	if (!on) {
+		len = -EINVAL;
+		mutex_unlock(&of->mutex);
+		goto out_free;
+	}
+
+	of->event = atomic_read(&on->event);
+
 	ops = kernfs_ops(of->kn);
 	if (ops->read)
 		len = ops->read(of, buf, len, iocb->ki_pos);
@@ -518,36 +577,29 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
 {
 	struct kernfs_open_node *on, *new_on = NULL;
 
- retry:
 	mutex_lock(&kernfs_open_file_mutex);
-	spin_lock_irq(&kernfs_open_node_lock);
+	on = kernfs_deref_open_node_protected(kn);
-
-	if (!kn->attr.open && new_on) {
-		kn->attr.open = new_on;
-		new_on = NULL;
-	}
-
-	on = kn->attr.open;
-	if (on)
-		list_add_tail(&of->list, &on->files);
-
-	spin_unlock_irq(&kernfs_open_node_lock);
-	mutex_unlock(&kernfs_open_file_mutex);
 
 	if (on) {
-		kfree(new_on);
+		list_add_tail(&of->list, &on->files);
+		mutex_unlock(&kernfs_open_file_mutex);
 		return 0;
+	} else {
+		/* not there, initialize a new one */
+		new_on = kmalloc(sizeof(*new_on), GFP_KERNEL);
+		if (!new_on) {
+			mutex_unlock(&kernfs_open_file_mutex);
+			return -ENOMEM;
+		}
+		atomic_set(&new_on->event, 1);
+		init_waitqueue_head(&new_on->poll);
+		INIT_LIST_HEAD(&new_on->files);
+		list_add_tail(&of->list, &new_on->files);
+		rcu_assign_pointer(kn->attr.open, new_on);
 	}
+	mutex_unlock(&kernfs_open_file_mutex);
 
-	/* not there, initialize a new one and retry */
+	return 0;
-	new_on = kmalloc(sizeof(*new_on), GFP_KERNEL);
-	if (!new_on)
-		return -ENOMEM;
-
-	atomic_set(&new_on->event, 1);
-	init_waitqueue_head(&new_on->poll);
-	INIT_LIST_HEAD(&new_on->files);
-	goto retry;
 }
 
 /**
@@ -566,24 +618,25 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
 static void kernfs_unlink_open_file(struct kernfs_node *kn,
 				 struct kernfs_open_file *of)
 {
-	struct kernfs_open_node *on = kn->attr.open;
+	struct kernfs_open_node *on;
-	unsigned long flags;
 
 	mutex_lock(&kernfs_open_file_mutex);
-	spin_lock_irqsave(&kernfs_open_node_lock, flags);
+
+	on = kernfs_deref_open_node_protected(kn);
+	if (!on) {
+		mutex_unlock(&kernfs_open_file_mutex);
+		return;
+	}
 
 	if (of)
 		list_del(&of->list);
 
-	if (list_empty(&on->files))
+	if (list_empty(&on->files)) {
-		kn->attr.open = NULL;
+		rcu_assign_pointer(kn->attr.open, NULL);
-	else
+		kfree_rcu(on, rcu_head);
-		on = NULL;
+	}
 
-	spin_unlock_irqrestore(&kernfs_open_node_lock, flags);
 	mutex_unlock(&kernfs_open_file_mutex);
-
-	kfree(on);
 }
 
 static int kernfs_fop_open(struct inode *inode, struct file *file)
@@ -773,17 +826,16 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
 	 * check under kernfs_open_file_mutex will ensure bailing out if
 	 * ->attr.open became NULL while waiting for the mutex.
 	 */
-	if (!kn->attr.open)
+	if (!rcu_access_pointer(kn->attr.open))
 		return;
 
 	mutex_lock(&kernfs_open_file_mutex);
-	if (!kn->attr.open) {
+	on = kernfs_deref_open_node_protected(kn);
+	if (!on) {
 		mutex_unlock(&kernfs_open_file_mutex);
 		return;
 	}
 
-	on = kn->attr.open;
-
 	list_for_each_entry(of, &on->files, list) {
 		struct inode *inode = file_inode(of->file);
 
@@ -814,7 +866,10 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
 __poll_t kernfs_generic_poll(struct kernfs_open_file *of, poll_table *wait)
 {
 	struct kernfs_node *kn = kernfs_dentry_node(of->file->f_path.dentry);
-	struct kernfs_open_node *on = kn->attr.open;
+	struct kernfs_open_node *on = kernfs_deref_open_node(of, kn);
+
+	if (!on)
+		return EPOLLERR;
 
 	poll_wait(of->file, &on->poll, wait);
 
@@ -921,13 +976,13 @@ void kernfs_notify(struct kernfs_node *kn)
 		return;
 
 	/* kick poll immediately */
-	spin_lock_irqsave(&kernfs_open_node_lock, flags);
+	rcu_read_lock();
-	on = kn->attr.open;
+	on = rcu_dereference(kn->attr.open);
 	if (on) {
 		atomic_inc(&on->event);
 		wake_up_interruptible(&on->poll);
 	}
-	spin_unlock_irqrestore(&kernfs_open_node_lock, flags);
+	rcu_read_unlock();
 
 	/* schedule work to kick fsnotify */
 	spin_lock_irqsave(&kernfs_notify_lock, flags);

include/linux/kernfs.h

@@ -114,7 +114,7 @@ struct kernfs_elem_symlink {
 
 struct kernfs_elem_attr {
 	const struct kernfs_ops	*ops;
-	struct kernfs_open_node	*open;
+	struct kernfs_open_node __rcu	*open;
 	loff_t			size;
 	struct kernfs_node	*notify_next;	/* for kernfs_notify() */
 };