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From:   Imran Khan <imran.f.khan@oracle.com>
To:     tj@kernel.org, viro@zeniv.linux.org.uk, gregkh@linuxfoundation.org
Cc:     linux-kernel@vger.kernel.org
Subject: [PATCH 3/5] kernfs: Change kernfs_notify_list to llist.
Date:   Thu, 28 Apr 2022 15:54:29 +1000
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At present kernfs_notify_list is implemented as a singly linked
list of kernfs_node(s), where last element points to itself and
value of ->attr.next tells if node is present on the list or not.
Both addition and deletion to list happen under kernfs_notify_lock.

Change kernfs_notify_list to llist so that addition to list can heppen
locklessly. We still need kernfs_notify_lock for consumers (kernfs_notify\
_workfn) because there can be multiple concurrent work items.

Suggested by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
---
 fs/kernfs/file.c       | 47 ++++++++++++++++++------------------------
 include/linux/kernfs.h |  2 +-
 2 files changed, 21 insertions(+), 28 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 36bff71ab263..96c8493003b6 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -38,18 +38,16 @@ struct kernfs_open_node {
 	struct list_head	files; /* goes through kernfs_open_file.list */
 };
 
-/*
- * kernfs_notify() may be called from any context and bounces notifications
- * through a work item.  To minimize space overhead in kernfs_node, the
- * pending queue is implemented as a singly linked list of kernfs_nodes.
- * The list is terminated with the self pointer so that whether a
- * kernfs_node is on the list or not can be determined by testing the next
- * pointer for NULL.
+/**
+ * attribute_to_node - get kernfs_node object corresponding to a kernfs attribute
+ * @ptr:	&struct kernfs_elem_attr
+ * @type:	struct kernfs_node
+ * @member:	name of member (i.e attr)
  */
-#define KERNFS_NOTIFY_EOL			((void *)&kernfs_notify_list)
+#define attribute_to_node(ptr, type, member)	\
+	container_of(ptr, type, member)
 
-static DEFINE_SPINLOCK(kernfs_notify_lock);
-static struct kernfs_node *kernfs_notify_list = KERNFS_NOTIFY_EOL;
+static LLIST_HEAD(kernfs_notify_list);
 
 /*
  * Raw deref RCU protected kn->attr.open.
@@ -876,18 +874,16 @@ static void kernfs_notify_workfn(struct work_struct *work)
 	struct kernfs_node *kn;
 	struct kernfs_super_info *info;
 	struct kernfs_root *root;
+	struct llist_node *free;
+	struct kernfs_elem_attr *attr;
 repeat:
 	/* pop one off the notify_list */
-	spin_lock_irq(&kernfs_notify_lock);
-	kn = kernfs_notify_list;
-	if (kn == KERNFS_NOTIFY_EOL) {
-		spin_unlock_irq(&kernfs_notify_lock);
+	free = llist_del_first(&kernfs_notify_list);
+	if (free == NULL)
 		return;
-	}
-	kernfs_notify_list = kn->attr.notify_next;
-	kn->attr.notify_next = NULL;
-	spin_unlock_irq(&kernfs_notify_lock);
 
+	attr = llist_entry(free, struct kernfs_elem_attr, notify_next);
+	kn = attribute_to_node(attr, struct kernfs_node, attr);
 	root = kernfs_root(kn);
 	/* kick fsnotify */
 	down_write(&root->kernfs_rwsem);
@@ -943,12 +939,14 @@ static void kernfs_notify_workfn(struct work_struct *work)
 void kernfs_notify(struct kernfs_node *kn)
 {
 	static DECLARE_WORK(kernfs_notify_work, kernfs_notify_workfn);
-	unsigned long flags;
 	struct kernfs_open_node *on;
 
 	if (WARN_ON(kernfs_type(kn) != KERNFS_FILE))
 		return;
 
+	/* Because we are using llist for kernfs_notify_list */
+	WARN_ON_ONCE(in_nmi());
+
 	/* kick poll immediately */
 	rcu_read_lock();
 	on = rcu_dereference(kn->attr.open);
@@ -959,14 +957,9 @@ void kernfs_notify(struct kernfs_node *kn)
 	rcu_read_unlock();
 
 	/* schedule work to kick fsnotify */
-	spin_lock_irqsave(&kernfs_notify_lock, flags);
-	if (!kn->attr.notify_next) {
-		kernfs_get(kn);
-		kn->attr.notify_next = kernfs_notify_list;
-		kernfs_notify_list = kn;
-		schedule_work(&kernfs_notify_work);
-	}
-	spin_unlock_irqrestore(&kernfs_notify_lock, flags);
+	kernfs_get(kn);
+	llist_add(&kn->attr.notify_next, &kernfs_notify_list);
+	schedule_work(&kernfs_notify_work);
 }
 EXPORT_SYMBOL_GPL(kernfs_notify);
 
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 13f54f078a52..2dd9c8df0f4f 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -116,7 +116,7 @@ struct kernfs_elem_attr {
 	const struct kernfs_ops	*ops;
 	struct kernfs_open_node __rcu	*open;
 	loff_t			size;
-	struct kernfs_node	*notify_next;	/* for kernfs_notify() */
+	struct llist_node	notify_next;	/* for kernfs_notify() */
 };
 
 /*
-- 
2.30.2