fsnotify: create helper fsnotify_update_sb_watchers()

We would like to count watched objects by priority group, so we will need
to update the watched object counter after adding/removing marks.

Create a helper fsnotify_update_sb_watchers() and call it after
attaching/detaching a mark, instead of fsnotify_{get,put}_sb_watchers()
only after attaching/detaching a connector.

Soon, we will use this helper to count watched objects by the highest
watching priority group.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20240317184154.1200192-7-amir73il@gmail.com>

fs/notify/mark.c

@@ -154,20 +154,23 @@ static void fsnotify_put_inode_ref(struct inode *inode)
 	iput(inode);
 }
 
-static void fsnotify_get_sb_watchers(struct fsnotify_mark_connector *conn)
+/*
+ * Grab or drop watched objects reference depending on whether the connector
+ * is attached and has any marks attached.
+ */
+static void fsnotify_update_sb_watchers(struct super_block *sb,
+					struct fsnotify_mark_connector *conn)
 {
-	struct super_block *sb = fsnotify_connector_sb(conn);
+	bool is_watched = conn->flags & FSNOTIFY_CONN_FLAG_IS_WATCHED;
+	bool has_marks = conn->obj && !hlist_empty(&conn->list);
 
-	if (sb)
+	if (has_marks && !is_watched) {
+		conn->flags |= FSNOTIFY_CONN_FLAG_IS_WATCHED;
 		fsnotify_get_sb_watched_objects(sb);
-}
-
-static void fsnotify_put_sb_watchers(struct fsnotify_mark_connector *conn)
-{
-	struct super_block *sb = fsnotify_connector_sb(conn);
-
-	if (sb)
+	} else if (!has_marks && is_watched) {
+		conn->flags &= ~FSNOTIFY_CONN_FLAG_IS_WATCHED;
 		fsnotify_put_sb_watched_objects(sb);
+	}
 }
 
 /*
@@ -266,6 +269,7 @@ static void *fsnotify_detach_connector_from_object(
 					unsigned int *type)
 {
 	fsnotify_connp_t *connp = fsnotify_object_connp(conn->obj, conn->type);
+	struct super_block *sb = fsnotify_connector_sb(conn);
 	struct inode *inode = NULL;
 
 	*type = conn->type;
@@ -285,10 +289,10 @@ static void *fsnotify_detach_connector_from_object(
 		fsnotify_conn_sb(conn)->s_fsnotify_mask = 0;
 	}
 
-	fsnotify_put_sb_watchers(conn);
 	rcu_assign_pointer(*connp, NULL);
 	conn->obj = NULL;
 	conn->type = FSNOTIFY_OBJ_TYPE_DETACHED;
+	fsnotify_update_sb_watchers(sb, conn);
 
 	return inode;
 }
@@ -340,6 +344,11 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
 		objp = fsnotify_detach_connector_from_object(conn, &type);
 		free_conn = true;
 	} else {
+		struct super_block *sb = fsnotify_connector_sb(conn);
+
+		/* Update watched objects after detaching mark */
+		if (sb)
+			fsnotify_update_sb_watchers(sb, conn);
 		objp = __fsnotify_recalc_mask(conn);
 		type = conn->type;
 	}
@@ -581,10 +590,7 @@ static int fsnotify_attach_connector_to_object(fsnotify_connp_t *connp,
 	if (cmpxchg(connp, NULL, conn)) {
 		/* Someone else created list structure for us */
 		kmem_cache_free(fsnotify_mark_connector_cachep, conn);
-		return 0;
 	}
-
-	fsnotify_get_sb_watchers(conn);
 	return 0;
 }
 
@@ -624,6 +630,7 @@ static struct fsnotify_mark_connector *fsnotify_grab_connector(
 static int fsnotify_add_mark_list(struct fsnotify_mark *mark, void *obj,
 				  unsigned int obj_type, int add_flags)
 {
+	struct super_block *sb = fsnotify_object_sb(obj, obj_type);
 	struct fsnotify_mark *lmark, *last = NULL;
 	struct fsnotify_mark_connector *conn;
 	fsnotify_connp_t *connp;
@@ -673,6 +680,7 @@ static int fsnotify_add_mark_list(struct fsnotify_mark *mark, void *obj,
 	/* mark should be the last entry.  last is the current last entry */
 	hlist_add_behind_rcu(&mark->obj_list, &last->obj_list);
 added:
+	fsnotify_update_sb_watchers(sb, conn);
 	/*
 	 * Since connector is attached to object using cmpxchg() we are
 	 * guaranteed that connector initialization is fully visible by anyone

include/linux/fsnotify_backend.h

@@ -465,6 +465,7 @@ FSNOTIFY_ITER_FUNCS(sb, SB)
 struct fsnotify_mark_connector {
 	spinlock_t lock;
 	unsigned short type;	/* Type of object [lock] */
+#define FSNOTIFY_CONN_FLAG_IS_WATCHED	0x01
 #define FSNOTIFY_CONN_FLAG_HAS_IREF	0x02
 	unsigned short flags;	/* flags [lock] */
 	union {