kernfs: Change kernfs_notify_list to llist.

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.

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/20220615021059.862643-3-imran.f.khan@oracle.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 22e7481..7d9138d 100644 (file)
--- 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);
 
 /**
  * kernfs_deref_open_node - Get kernfs_open_node corresponding to @kn.
@@ -902,18 +900,16 @@ 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);
@@ -969,12 +965,14 @@ repeat:
 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);
@@ -985,14 +983,9 @@ 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 13f54f0..2dd9c8d 100644 (file)
--- 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() */
 };
 
 /*