2 files changed, 58 insertions, 0 deletions

diff --git a/fs/notify/notification.c b/fs/notify/notification.c
index bc9470c7ece7..b493c378445f 100644
--- a/fs/notify/notification.c
+++ b/fs/notify/notification.c
@@ -287,6 +287,62 @@ static void initialize_event(struct fsnotify_event *event)
         INIT_LIST_HEAD(&event->private_data_list);
 }
 
+/*
+ * Caller damn well better be holding whatever mutex is protecting the
+ * old_holder->event_list.
+ */
+int fsnotify_replace_event(struct fsnotify_event_holder *old_holder,
+                           struct fsnotify_event *new_event)
+{
+        struct fsnotify_event *old_event = old_holder->event;
+        struct fsnotify_event_holder *new_holder = NULL;
+
+        /*
+         * There is one fsnotify_event_holder embedded inside each fsnotify_event.
+         * Check if we expect to be able to use that holder.  If not alloc a new
+         * holder.
+         * For the overflow event it's possible that something will use the in
+         * event holder before we get the lock so we may need to jump back and
+         * alloc a new holder, this can't happen for most events...
+         */
+        if (!list_empty(&new_event->holder.event_list)) {
+alloc_holder:
+                new_holder = fsnotify_alloc_event_holder();
+                if (!new_holder)
+                        return -ENOMEM;
+        }
+
+        spin_lock(&old_event->lock);
+        spin_lock(&new_event->lock);
+
+        if (list_empty(&new_event->holder.event_list)) {
+                if (unlikely(new_holder))
+                        fsnotify_destroy_event_holder(new_holder);
+                new_holder = &new_event->holder;
+        } else if (unlikely(!new_holder)) {
+                /* between the time we checked above and got the lock the in
+                 * event holder was used, go back and get a new one */
+                spin_unlock(&new_event->lock);
+                spin_unlock(&old_event->lock);
+                goto alloc_holder;
+        }
+
+        new_holder->event = new_event;
+        list_replace_init(&old_holder->event_list, &new_holder->event_list);
+
+        spin_unlock(&new_event->lock);
+        spin_unlock(&old_event->lock);
+
+        /* event == holder means we are referenced through the in event holder */
+        if (old_holder != &old_event->holder)
+                fsnotify_destroy_event_holder(old_holder);
+
+        fsnotify_get_event(new_event); /* on the list take reference */
+        fsnotify_put_event(old_event); /* off the list, drop reference */
+
+        return 0;
+}
+
 struct fsnotify_event *fsnotify_clone_event(struct fsnotify_event *old_event)
 {
         struct fsnotify_event *event;
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 3a7fff235539..427f6ffab127 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -365,6 +365,8 @@ extern struct fsnotify_event *fsnotify_create_event(struct inode *to_tell, __u32
 
 /* fanotify likes to change events after they are on lists... */
 extern struct fsnotify_event *fsnotify_clone_event(struct fsnotify_event *old_event);
+extern int fsnotify_replace_event(struct fsnotify_event_holder *old_holder,
+                                  struct fsnotify_event *new_event);
 
 #else