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From:   Imran Khan <imran.f.khan@oracle.com>
To:     tj@kernel.org, viro@zeniv.linux.org.uk, gregkh@linuxfoundation.org,
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Cc:     linux-kernel@vger.kernel.org
Subject: [PATCH v8 02/10] kernfs: make ->attr.open RCU protected.
Date:   Sun, 10 Apr 2022 12:37:11 +1000
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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.

So make ->attr.open RCU protected and remove global kernfs_open_node_lock.

Suggested by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
---
 fs/kernfs/file.c       | 101 ++++++++++++++++++++++-------------------
 include/linux/kernfs.h |   2 +-
 2 files changed, 55 insertions(+), 48 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index aea6968c979e..bc393dcf4efa 100644
--- a/fs/kernfs/file.c
+++ b/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 */
@@ -156,8 +156,9 @@ 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 = rcu_dereference_raw(of->kn->attr.open);
 
-	of->event = atomic_read(&of->kn->attr.open->event);
+	of->event = atomic_read(&on->event);
 
 	return of->kn->attr.ops->seq_show(sf, v);
 }
@@ -180,6 +181,7 @@ 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 +203,8 @@ 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 = rcu_dereference_raw(of->kn->attr.open);
+	of->event = atomic_read(&on->event);
 	ops = kernfs_ops(of->kn);
 	if (ops->read)
 		len = ops->read(of, buf, len, iocb->ki_pos);
@@ -519,36 +522,34 @@ 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);
-
-	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);
 
+	/**
+	 * ->attr.open changes under kernfs_open_file_mutex so we don't
+	 * need rcu_read_lock to ensure its existence.
+	 */
+	on = rcu_dereference_protected(kn->attr.open,
+				   lockdep_is_held(&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 and retry */
+		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 */
-	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;
+	return 0;
 }
 
 /**
@@ -566,24 +567,30 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
 static void kernfs_put_open_node(struct kernfs_node *kn,
 				 struct kernfs_open_file *of)
 {
-	struct kernfs_open_node *on = kn->attr.open;
-	unsigned long flags;
+	struct kernfs_open_node *on;
+
+	/* ->attr.open NULL means there are no more open files */
+	if (rcu_dereference_raw(kn->attr.open) == NULL)
+		return;
 
 	mutex_lock(&kernfs_open_file_mutex);
-	spin_lock_irqsave(&kernfs_open_node_lock, flags);
+
+	on = rcu_dereference_protected(kn->attr.open,
+				       lockdep_is_held(&kernfs_open_file_mutex));
+
+	if (!on) {
+		mutex_unlock(&kernfs_open_file_mutex);
+		return;
+	}
 
 	if (of)
 		list_del(&of->list);
 
-	if (list_empty(&on->files))
-		kn->attr.open = NULL;
-	else
-		on = NULL;
-
-	spin_unlock_irqrestore(&kernfs_open_node_lock, flags);
+	if (list_empty(&on->files)) {
+		rcu_assign_pointer(kn->attr.open, NULL);
+		kfree_rcu(on, rcu_head);
+	}
 	mutex_unlock(&kernfs_open_file_mutex);
-
-	kfree(on);
 }
 
 static int kernfs_fop_open(struct inode *inode, struct file *file)
@@ -765,12 +772,13 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
 	if (!(kn->flags & (KERNFS_HAS_MMAP | KERNFS_HAS_RELEASE)))
 		return;
 
-	on = kn->attr.open;
-	if (!on)
+	if (rcu_dereference_raw(kn->attr.open) == NULL)
 		return;
 
 	mutex_lock(&kernfs_open_file_mutex);
-	if (!kn->attr.open) {
+	on = rcu_dereference_check(kn->attr.open,
+				   lockdep_is_held(&kernfs_open_file_mutex));
+	if (!on) {
 		mutex_unlock(&kernfs_open_file_mutex);
 		return;
 	}
@@ -805,7 +813,7 @@ 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 = rcu_dereference_raw(kn->attr.open);
 
 	poll_wait(of->file, &on->poll, wait);
 
@@ -912,14 +920,13 @@ void kernfs_notify(struct kernfs_node *kn)
 		return;
 
 	/* kick poll immediately */
-	spin_lock_irqsave(&kernfs_open_node_lock, flags);
-	on = kn->attr.open;
+	rcu_read_lock();
+	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);
 	if (!kn->attr.notify_next) {
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index e2ae15a6225e..13f54f078a52 100644
--- a/include/linux/kernfs.h
+++ b/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() */
 };
-- 
2.30.2