bcachefs: trans_for_each_path_safe()

bch2_btree_trans_to_text() is used on btree_trans objects that are owned
by different threads - when printing out deadlock cycles - so we need a
safe version of trans_for_each_path(), else we race with seeing a
btree_path that was just allocated and not fully initialized:
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -2914,6 +2914,10 @@ static void bch2_trans_alloc_paths(struct btree_trans *trans, struct bch_fs *c)
 #endif
 	if (!p)
 		p = mempool_alloc(&trans->c->btree_paths_pool, GFP_NOFS);
+	/*
+	 * paths need to be zeroed, bch2_check_for_deadlock looks at paths in
+	 * other threads
+	 */
 
 	trans->paths		= p; p += paths_bytes;
 	trans->updates		= p; p += updates_bytes;
@@ -3111,7 +3115,7 @@ 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' };
-	unsigned l;
+	unsigned l, idx;
 
 	if (!out->nr_tabstops) {
 		printbuf_tabstop_push(out, 16);
@@ -3120,7 +3124,7 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct btree_trans *trans)
 
 	prt_printf(out, "%i %s\n", trans->locking_wait.task->pid, trans->fn);
 
-	trans_for_each_path(trans, path) {
+	trans_for_each_path_safe(trans, path, idx) {
 		if (!path->nodes_locked)
 			continue;
 

fs/bcachefs/btree_iter.h

@@ -89,6 +89,35 @@ __trans_next_path(struct btree_trans *trans, unsigned idx)
 #define trans_for_each_path(_trans, _path)				\
 	trans_for_each_path_from(_trans, _path, 0)
 
+static inline struct btree_path *
+__trans_next_path_safe(struct btree_trans *trans, unsigned *idx)
+{
+	u64 l;
+
+	if (*idx == BTREE_ITER_MAX)
+		return NULL;
+
+	l = trans->paths_allocated >> *idx;
+	if (!l)
+		return NULL;
+
+	*idx += __ffs64(l);
+	EBUG_ON(*idx >= BTREE_ITER_MAX);
+	return &trans->paths[*idx];
+}
+
+/*
+ * This version is intended to be safe for use on a btree_trans that is owned by
+ * another thread, for bch2_btree_trans_to_text();
+ */
+#define trans_for_each_path_safe_from(_trans, _path, _idx, _start)	\
+	for (_idx = _start;						\
+	     (_path = __trans_next_path_safe((_trans), &_idx));		\
+	     _idx++)
+
+#define trans_for_each_path_safe(_trans, _path, _idx)			\
+	trans_for_each_path_safe_from(_trans, _path, _idx, 0)
+
 static inline struct btree_path *next_btree_path(struct btree_trans *trans, struct btree_path *path)
 {
 	unsigned idx = path ? path->sorted_idx + 1 : 0;

fs/bcachefs/btree_locking.c

@@ -255,6 +255,7 @@ int bch2_check_for_deadlock(struct btree_trans *trans, struct printbuf *cycle)
 	struct trans_waiting_for_lock *top;
 	struct btree_bkey_cached_common *b;
 	struct btree_path *path;
+	unsigned path_idx;
 	int ret;
 
 	if (trans->lock_must_abort) {
@@ -273,12 +274,12 @@ int bch2_check_for_deadlock(struct btree_trans *trans, struct printbuf *cycle)
 
 	top = &g.g[g.nr - 1];
 
-	trans_for_each_path_from(top->trans, path, top->path_idx) {
+	trans_for_each_path_safe_from(top->trans, path, path_idx, top->path_idx) {
 		if (!path->nodes_locked)
 			continue;
 
-		if (top->path_idx != path->idx) {
-			top->path_idx		= path->idx;
+		if (path_idx != top->path_idx) {
+			top->path_idx		= path_idx;
 			top->level		= 0;
 			top->lock_start_time	= 0;
 		}