fsnotify: use an explicit flag to indicate fsnotify_destroy_mark has been called

Currently fsnotify check is mark->group is NULL to decide if
fsnotify_destroy_mark() has already been called or not.  With the upcoming
rcu work it is a heck of a lot easier to use an explicit flag than worry
about group being set to NULL.
Signed-off-by: NEric Paris <eparis@redhat.com>

fs/notify/inode_mark.c

@@ -187,7 +187,7 @@ int fsnotify_add_inode_mark(struct fsnotify_mark *mark,
 	struct hlist_node *node, *last = NULL;
 	int ret = 0;
 
-	mark->flags = FSNOTIFY_MARK_FLAG_INODE;
+	mark->flags |= FSNOTIFY_MARK_FLAG_INODE;
 
 	assert_spin_locked(&mark->lock);
 	assert_spin_locked(&group->mark_lock);

fs/notify/mark.c

@@ -121,12 +121,14 @@ void fsnotify_destroy_mark(struct fsnotify_mark *mark)
 
 	group = mark->group;
 
-	/* if !group something else already marked this to die */
-	if (!group) {
+	/* something else already called this function on this mark */
+	if (!(mark->flags & FSNOTIFY_MARK_FLAG_ALIVE)) {
 		spin_unlock(&mark->lock);
 		return;
 	}
 
+	mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
+
 	/* 1 from caller and 1 for being on i_list/g_list */
 	BUG_ON(atomic_read(&mark->refcnt) < 2);
 
@@ -141,7 +143,6 @@ void fsnotify_destroy_mark(struct fsnotify_mark *mark)
 		BUG();
 
 	list_del_init(&mark->g_list);
-	mark->group = NULL;
 
 	fsnotify_put_mark(mark); /* for i_list and g_list */
 
@@ -229,6 +230,8 @@ int fsnotify_add_mark(struct fsnotify_mark *mark,
 	spin_lock(&mark->lock);
 	spin_lock(&group->mark_lock);
 
+	mark->flags |= FSNOTIFY_MARK_FLAG_ALIVE;
+
 	mark->group = group;
 	list_add(&mark->g_list, &group->marks_list);
 	atomic_inc(&group->num_marks);
@@ -258,7 +261,7 @@ int fsnotify_add_mark(struct fsnotify_mark *mark,
 
 	return ret;
 err:
-	mark->group = NULL;
+	mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
 	list_del_init(&mark->g_list);
 	atomic_dec(&group->num_marks);
 	fsnotify_put_mark(mark);

fs/notify/vfsmount_mark.c

@@ -145,7 +145,7 @@ int fsnotify_add_vfsmount_mark(struct fsnotify_mark *mark,
 	struct hlist_node *node, *last = NULL;
 	int ret = 0;
 
-	mark->flags = FSNOTIFY_MARK_FLAG_VFSMOUNT;
+	mark->flags |= FSNOTIFY_MARK_FLAG_VFSMOUNT;
 
 	assert_spin_locked(&mark->lock);
 	assert_spin_locked(&group->mark_lock);

include/linux/fsnotify_backend.h

@@ -300,6 +300,7 @@ struct fsnotify_mark {
 #define FSNOTIFY_MARK_FLAG_VFSMOUNT		0x02
 #define FSNOTIFY_MARK_FLAG_OBJECT_PINNED	0x04
 #define FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY	0x08
+#define FSNOTIFY_MARK_FLAG_ALIVE		0x10
 	unsigned int flags;		/* vfsmount or inode mark? */
 	void (*free_mark)(struct fsnotify_mark *mark); /* called on final put+free */
 };