fsnotify: Detach mark from object list when last reference is dropped

Instead of removing mark from object list from fsnotify_detach_mark(),
remove the mark when last reference to the mark is dropped. This will
allow fanotify to wait for userspace response to event without having to
hold onto fsnotify_mark_srcu.

To avoid pinning inodes by elevated refcount (and thus e.g. delaying
file deletion) while someone holds mark reference, we detach connector
from the object also from fsnotify_destroy_marks() and not only after
removing last mark from the list as it was now.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/mark.c

@@ -49,7 +49,13 @@
  *
  * A list of notification marks relating to inode / mnt is contained in
  * fsnotify_mark_connector. That structure is alive as long as there are any
- * marks in the list and is also protected by fsnotify_mark_srcu.
+ * marks in the list and is also protected by fsnotify_mark_srcu. A mark gets
+ * detached from fsnotify_mark_connector when last reference to the mark is
+ * dropped.  Thus having mark reference is enough to protect mark->connector
+ * pointer and to make sure fsnotify_mark_connector cannot disappear. Also
+ * because we remove mark from g_list before dropping mark reference associated
+ * with that, any mark found through g_list is guaranteed to have
+ * mark->connector set until we drop group->mark_mutex.
  *
  * LIFETIME:
  * Inode marks survive between when they are added to an inode and when their
@@ -103,26 +109,16 @@ void fsnotify_get_mark(struct fsnotify_mark *mark)
 	atomic_inc(&mark->refcnt);
 }
 
-void fsnotify_put_mark(struct fsnotify_mark *mark)
-{
-	if (atomic_dec_and_test(&mark->refcnt)) {
-		spin_lock(&destroy_lock);
-		list_add(&mark->g_list, &destroy_list);
-		spin_unlock(&destroy_lock);
-		queue_delayed_work(system_unbound_wq, &reaper_work,
-				   FSNOTIFY_REAPER_DELAY);
-	}
-}
-
 static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
 {
 	u32 new_mask = 0;
 	struct fsnotify_mark *mark;
 
 	assert_spin_locked(&conn->lock);
-	hlist_for_each_entry(mark, &conn->list, obj_list)
-		new_mask |= mark->mask;
-
+	hlist_for_each_entry(mark, &conn->list, obj_list) {
+		if (mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)
+			new_mask |= mark->mask;
+	}
 	if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
 		conn->inode->i_fsnotify_mask = new_mask;
 	else if (conn->flags & FSNOTIFY_OBJ_TYPE_VFSMOUNT)
@@ -131,8 +127,9 @@ static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
 
 /*
  * Calculate mask of events for a list of marks. The caller must make sure
- * connector cannot disappear under us (usually by holding a mark->lock or
- * mark->group->mark_mutex for a mark on this list).
+ * connector and connector->inode cannot disappear under us.  Callers achieve
+ * this by holding a mark->lock or mark->group->mark_mutex for a mark on this
+ * list.
  */
 void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
 {
@@ -164,7 +161,6 @@ static void fsnotify_connector_destroy_workfn(struct work_struct *work)
 	}
 }
 
-
 static struct inode *fsnotify_detach_connector_from_object(
 					struct fsnotify_mark_connector *conn)
 {
@@ -187,14 +183,34 @@ static struct inode *fsnotify_detach_connector_from_object(
 	return inode;
 }
 
-static struct inode *fsnotify_detach_from_object(struct fsnotify_mark *mark)
+static void fsnotify_final_mark_destroy(struct fsnotify_mark *mark)
+{
+	if (mark->group)
+		fsnotify_put_group(mark->group);
+	mark->free_mark(mark);
+}
+
+void fsnotify_put_mark(struct fsnotify_mark *mark)
 {
 	struct fsnotify_mark_connector *conn;
 	struct inode *inode = NULL;
 	bool free_conn = false;
 
+	/* Catch marks that were actually never attached to object */
+	if (!mark->connector) {
+		if (atomic_dec_and_test(&mark->refcnt))
+			fsnotify_final_mark_destroy(mark);
+		return;
+	}
+
+	/*
+	 * We have to be careful so that traversals of obj_list under lock can
+	 * safely grab mark reference.
+	 */
+	if (!atomic_dec_and_lock(&mark->refcnt, &mark->connector->lock))
+		return;
+
 	conn = mark->connector;
-	spin_lock(&conn->lock);
 	hlist_del_init_rcu(&mark->obj_list);
 	if (hlist_empty(&conn->list)) {
 		inode = fsnotify_detach_connector_from_object(conn);
@@ -205,6 +221,8 @@ static struct inode *fsnotify_detach_from_object(struct fsnotify_mark *mark)
 	mark->connector = NULL;
 	spin_unlock(&conn->lock);
 
+	iput(inode);
+
 	if (free_conn) {
 		spin_lock(&destroy_lock);
 		conn->destroy_next = connector_destroy_list;
@@ -212,20 +230,31 @@ static struct inode *fsnotify_detach_from_object(struct fsnotify_mark *mark)
 		spin_unlock(&destroy_lock);
 		queue_work(system_unbound_wq, &connector_reaper_work);
 	}
-
-	return inode;
+	/*
+	 * Note that we didn't update flags telling whether inode cares about
+	 * what's happening with children. We update these flags from
+	 * __fsnotify_parent() lazily when next event happens on one of our
+	 * children.
+	 */
+	spin_lock(&destroy_lock);
+	list_add(&mark->g_list, &destroy_list);
+	spin_unlock(&destroy_lock);
+	queue_delayed_work(system_unbound_wq, &reaper_work,
+			   FSNOTIFY_REAPER_DELAY);
 }
 
 /*
- * Remove mark from inode / vfsmount list, group list, drop inode reference
- * if we got one.
+ * Mark mark as detached, remove it from group list. Mark still stays in object
+ * list until its last reference is dropped. Note that we rely on mark being
+ * removed from group list before corresponding reference to it is dropped. In
+ * particular we rely on mark->connector being valid while we hold
+ * group->mark_mutex if we found the mark through g_list.
  *
  * Must be called with group->mark_mutex held. The caller must either hold
  * reference to the mark or be protected by fsnotify_mark_srcu.
  */
 void fsnotify_detach_mark(struct fsnotify_mark *mark)
 {
-	struct inode *inode = NULL;
 	struct fsnotify_group *group = mark->group;
 
 	WARN_ON_ONCE(!mutex_is_locked(&group->mark_mutex));
@@ -234,31 +263,15 @@ void fsnotify_detach_mark(struct fsnotify_mark *mark)
 			!!(mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED));
 
 	spin_lock(&mark->lock);
-
 	/* something else already called this function on this mark */
 	if (!(mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
 		spin_unlock(&mark->lock);
 		return;
 	}
-
 	mark->flags &= ~FSNOTIFY_MARK_FLAG_ATTACHED;
-
-	inode = fsnotify_detach_from_object(mark);
-
-	/*
-	 * Note that we didn't update flags telling whether inode cares about
-	 * what's happening with children. We update these flags from
-	 * __fsnotify_parent() lazily when next event happens on one of our
-	 * children.
-	 */
-
 	list_del_init(&mark->g_list);
-
 	spin_unlock(&mark->lock);
 
-	if (inode)
-		iput(inode);
-
 	atomic_dec(&group->num_marks);
 
 	/* Drop mark reference acquired in fsnotify_add_mark_locked() */
@@ -458,7 +471,9 @@ restart:
 	hlist_for_each_entry(lmark, &conn->list, obj_list) {
 		last = lmark;
 
-		if ((lmark->group == mark->group) && !allow_dups) {
+		if ((lmark->group == mark->group) &&
+		    (lmark->flags & FSNOTIFY_MARK_FLAG_ATTACHED) &&
+		    !allow_dups) {
 			err = -EEXIST;
 			goto out_err;
 		}
@@ -509,7 +524,7 @@ int fsnotify_add_mark_locked(struct fsnotify_mark *mark,
 	mark->group = group;
 	list_add(&mark->g_list, &group->marks_list);
 	atomic_inc(&group->num_marks);
-	fsnotify_get_mark(mark); /* for i_list and g_list */
+	fsnotify_get_mark(mark); /* for g_list */
 	spin_unlock(&mark->lock);
 
 	ret = fsnotify_add_mark_list(mark, inode, mnt, allow_dups);
@@ -557,7 +572,8 @@ struct fsnotify_mark *fsnotify_find_mark(
 		return NULL;
 
 	hlist_for_each_entry(mark, &conn->list, obj_list) {
-		if (mark->group == group) {
+		if (mark->group == group &&
+		    (mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
 			fsnotify_get_mark(mark);
 			spin_unlock(&conn->lock);
 			return mark;
@@ -637,23 +653,38 @@ void fsnotify_detach_group_marks(struct fsnotify_group *group)
 void fsnotify_destroy_marks(struct fsnotify_mark_connector __rcu **connp)
 {
 	struct fsnotify_mark_connector *conn;
-	struct fsnotify_mark *mark;
+	struct fsnotify_mark *mark, *old_mark = NULL;
+	struct inode *inode;
 
-	while ((conn = fsnotify_grab_connector(connp))) {
-		/*
-		 * We have to be careful since we can race with e.g.
-		 * fsnotify_clear_marks_by_group() and once we drop the list
-		 * lock, mark can get removed from the obj_list and destroyed.
-		 * But we are holding mark reference so mark cannot be freed
-		 * and calling fsnotify_destroy_mark() more than once is fine.
-		 */
-		mark = hlist_entry(conn->list.first, struct fsnotify_mark,
-				   obj_list);
+	conn = fsnotify_grab_connector(connp);
+	if (!conn)
+		return;
+	/*
+	 * We have to be careful since we can race with e.g.
+	 * fsnotify_clear_marks_by_group() and once we drop the conn->lock, the
+	 * list can get modified. However we are holding mark reference and
+	 * thus our mark cannot be removed from obj_list so we can continue
+	 * iteration after regaining conn->lock.
+	 */
+	hlist_for_each_entry(mark, &conn->list, obj_list) {
 		fsnotify_get_mark(mark);
 		spin_unlock(&conn->lock);
+		if (old_mark)
+			fsnotify_put_mark(old_mark);
+		old_mark = mark;
 		fsnotify_destroy_mark(mark, mark->group);
-		fsnotify_put_mark(mark);
+		spin_lock(&conn->lock);
 	}
+	/*
+	 * Detach list from object now so that we don't pin inode until all
+	 * mark references get dropped. It would lead to strange results such
+	 * as delaying inode deletion or blocking unmount.
+	 */
+	inode = fsnotify_detach_connector_from_object(conn);
+	spin_unlock(&conn->lock);
+	if (old_mark)
+		fsnotify_put_mark(old_mark);
+	iput(inode);
 }
 
 /*
@@ -686,9 +717,7 @@ void fsnotify_mark_destroy_list(void)
 
 	list_for_each_entry_safe(mark, next, &private_destroy_list, g_list) {
 		list_del_init(&mark->g_list);
-		if (mark->group)
-			fsnotify_put_group(mark->group);
-		mark->free_mark(mark);
+		fsnotify_final_mark_destroy(mark);
 	}
 }
 

include/linux/fsnotify_backend.h

@@ -245,9 +245,9 @@ struct fsnotify_mark {
 	struct list_head g_list;
 	/* Protects inode / mnt pointers, flags, masks */
 	spinlock_t lock;
-	/* List of marks for inode / vfsmount [connector->lock] */
+	/* List of marks for inode / vfsmount [connector->lock, mark ref] */
 	struct hlist_node obj_list;
-	/* Head of list of marks for an object [mark->lock, group->mark_mutex] */
+	/* Head of list of marks for an object [mark ref] */
 	struct fsnotify_mark_connector *connector;
 	/* Events types to ignore [mark->lock, group->mark_mutex] */
 	__u32 ignored_mask;

kernel/audit_tree.c

@@ -172,27 +172,18 @@ static unsigned long inode_to_key(const struct inode *inode)
 /*
  * Function to return search key in our hash from chunk. Key 0 is special and
  * should never be present in the hash.
- *
- * Must be called with chunk->mark.lock held to protect from connector
- * becoming NULL.
  */
-static unsigned long __chunk_to_key(struct audit_chunk *chunk)
+static unsigned long chunk_to_key(struct audit_chunk *chunk)
 {
-	if (!chunk->mark.connector)
+	/*
+	 * We have a reference to the mark so it should be attached to a
+	 * connector.
+	 */
+	if (WARN_ON_ONCE(!chunk->mark.connector))
 		return 0;
 	return (unsigned long)chunk->mark.connector->inode;
 }
 
-static unsigned long chunk_to_key(struct audit_chunk *chunk)
-{
-	unsigned long key;
-
-	spin_lock(&chunk->mark.lock);
-	key = __chunk_to_key(chunk);
-	spin_unlock(&chunk->mark.lock);
-	return key;
-}
-
 static inline struct list_head *chunk_hash(unsigned long key)
 {
 	unsigned long n = key / L1_CACHE_BYTES;
@@ -202,7 +193,7 @@ static inline struct list_head *chunk_hash(unsigned long key)
 /* hash_lock & entry->lock is held by caller */
 static void insert_hash(struct audit_chunk *chunk)
 {
-	unsigned long key = __chunk_to_key(chunk);
+	unsigned long key = chunk_to_key(chunk);
 	struct list_head *list;
 
 	if (!(chunk->mark.flags & FSNOTIFY_MARK_FLAG_ATTACHED))
@@ -263,6 +254,10 @@ static void untag_chunk(struct node *p)
 
 	mutex_lock(&entry->group->mark_mutex);
 	spin_lock(&entry->lock);
+	/*
+	 * mark_mutex protects mark from getting detached and thus also from
+	 * mark->connector->inode getting NULL.
+	 */
 	if (chunk->dead || !(entry->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
 		spin_unlock(&entry->lock);
 		mutex_unlock(&entry->group->mark_mutex);
@@ -423,6 +418,10 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)
 
 	mutex_lock(&old_entry->group->mark_mutex);
 	spin_lock(&old_entry->lock);
+	/*
+	 * mark_mutex protects mark from getting detached and thus also from
+	 * mark->connector->inode getting NULL.
+	 */
 	if (!(old_entry->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
 		/* old_entry is being shot, lets just lie */
 		spin_unlock(&old_entry->lock);