bcachefs: rcu protect trans->paths

Upcoming patches are going to be changing trans->paths to a
reallocatable buffer. We need to guard against use after free when it's
used by other threads; this introduces RCU protection to those paths and
changes them to check for trans->paths == NULL

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -2920,6 +2920,8 @@ struct btree_trans *__bch2_trans_get(struct bch_fs *c, unsigned fn_idx)
 	trans->sorted		= trans->_sorted;
 	trans->paths		= trans->_paths;
 
+	*trans_paths_nr(trans->paths) = BTREE_ITER_MAX;
+
 	trans->paths_allocated[0] = 1;
 
 	s = btree_trans_stats(trans);
@@ -3074,12 +3076,14 @@ bch2_btree_bkey_cached_common_to_text(struct printbuf *out,
 
 void bch2_btree_trans_to_text(struct printbuf *out, struct btree_trans *trans)
 {
-	struct btree_path *path;
 	struct btree_bkey_cached_common *b;
 	static char lock_types[] = { 'r', 'i', 'w' };
 	struct task_struct *task = READ_ONCE(trans->locking_wait.task);
 	unsigned l, idx;
 
+	/* before rcu_read_lock(): */
+	bch2_printbuf_make_room(out, 4096);
+
 	if (!out->nr_tabstops) {
 		printbuf_tabstop_push(out, 16);
 		printbuf_tabstop_push(out, 32);
@@ -3087,7 +3091,18 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct btree_trans *trans)
 
 	prt_printf(out, "%i %s\n", task ? task->pid : 0, trans->fn);
 
-	trans_for_each_path(trans, path, idx) {
+	/* trans->paths is rcu protected vs. freeing */
+	rcu_read_lock();
+	out->atomic++;
+
+	struct btree_path *paths = rcu_dereference(trans->paths);
+	if (!paths)
+		goto out;
+
+	unsigned long *paths_allocated = trans_paths_allocated(paths);
+
+	trans_for_each_path_idx_from(paths_allocated, *trans_paths_nr(paths), idx, 1) {
+		struct btree_path *path = paths + idx;
 		if (!path->nodes_locked)
 			continue;
 
@@ -3120,6 +3135,9 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct btree_trans *trans)
 		bch2_btree_bkey_cached_common_to_text(out, b);
 		prt_newline(out);
 	}
+out:
+	--out->atomic;
+	rcu_read_unlock();
 }
 
 void bch2_fs_btree_iter_exit(struct bch_fs *c)

fs/bcachefs/btree_iter.h

@@ -63,6 +63,22 @@ static inline void btree_trans_sort_paths(struct btree_trans *trans)
 	__bch2_btree_trans_sort_paths(trans);
 }
 
+static inline unsigned long *trans_paths_nr(struct btree_path *paths)
+{
+	return &container_of(paths, struct btree_trans_paths, paths[0])->nr_paths;
+}
+
+static inline unsigned long *trans_paths_allocated(struct btree_path *paths)
+{
+	unsigned long *v = trans_paths_nr(paths);
+	return v - BITS_TO_LONGS(*v);
+}
+
+#define trans_for_each_path_idx_from(_paths_allocated, _nr, _idx, _start)\
+	for (_idx = _start;						\
+	     (_idx = find_next_bit(_paths_allocated, _nr, _idx)) < _nr;	\
+	     _idx++)
+
 static inline struct btree_path *
 __trans_next_path(struct btree_trans *trans, unsigned *idx)
 {

fs/bcachefs/btree_locking.c

@@ -281,9 +281,8 @@ int bch2_check_for_deadlock(struct btree_trans *trans, struct printbuf *cycle)
 	struct lock_graph g;
 	struct trans_waiting_for_lock *top;
 	struct btree_bkey_cached_common *b;
-	struct btree_path *path;
-	unsigned path_idx;
-	int ret;
+	btree_path_idx_t path_idx;
+	int ret = 0;
 
 	g.nr = 0;
 
@@ -296,13 +295,26 @@ int bch2_check_for_deadlock(struct btree_trans *trans, struct printbuf *cycle)
 	}
 
 	lock_graph_down(&g, trans);
+
+	/* trans->paths is rcu protected vs. freeing */
+	rcu_read_lock();
+	if (cycle)
+		cycle->atomic++;
 next:
 	if (!g.nr)
-		return 0;
+		goto out;
 
 	top = &g.g[g.nr - 1];
 
-	trans_for_each_path_from(top->trans, path, path_idx, top->path_idx) {
+	struct btree_path *paths = rcu_dereference(top->trans->paths);
+	if (!paths)
+		goto up;
+
+	unsigned long *paths_allocated = trans_paths_allocated(paths);
+
+	trans_for_each_path_idx_from(paths_allocated, *trans_paths_nr(paths),
+				     path_idx, top->path_idx) {
+		struct btree_path *path = paths + path_idx;
 		if (!path->nodes_locked)
 			continue;
 
@@ -368,18 +380,23 @@ next:
 
 				ret = lock_graph_descend(&g, trans, cycle);
 				if (ret)
-					return ret;
+					goto out;
 				goto next;
 
 			}
 			raw_spin_unlock(&b->lock.wait_lock);
 		}
 	}
-
+up:
 	if (g.nr > 1 && cycle)
 		print_chain(cycle, &g);
 	lock_graph_up(&g);
 	goto next;
+out:
+	if (cycle)
+		--cycle->atomic;
+	rcu_read_unlock();
+	return ret;
 }
 
 int bch2_six_check_for_deadlock(struct six_lock *lock, void *p)

fs/bcachefs/btree_types.h

@@ -372,12 +372,17 @@ struct btree_trans_commit_hook {
 
 #define BTREE_TRANS_MAX_LOCK_HOLD_TIME_NS	10000
 
+struct btree_trans_paths {
+	unsigned long		nr_paths;
+	struct btree_path	paths[];
+};
+
 struct btree_trans {
 	struct bch_fs		*c;
 
 	unsigned long		*paths_allocated;
-	u8			*sorted;
 	struct btree_path	*paths;
+	u8			*sorted;
 
 	void			*mem;
 	unsigned		mem_top;
@@ -431,8 +436,9 @@ struct btree_trans {
 	struct replicas_delta_list *fs_usage_deltas;
 
 	unsigned long		_paths_allocated[BITS_TO_LONGS(BTREE_ITER_MAX)];
-	u8			_sorted[BTREE_ITER_MAX + 8];
+	struct btree_trans_paths trans_paths;
 	struct btree_path	_paths[BTREE_ITER_MAX];
+	u8			_sorted[BTREE_ITER_MAX + 8];
 };
 
 static inline struct btree_path *btree_iter_path(struct btree_trans *trans, struct btree_iter *iter)