bcachefs: Improve btree_deadlock debugfs output

This changes bch2_check_for_deadlock() to print the longest chains it
finds - when we have a deadlock because the cycle detector isn't finding
something, this will let us see what it's missing.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_locking.c

@@ -71,11 +71,6 @@ struct lock_graph {
 	unsigned			nr;
 };
 
-static void lock_graph_pop(struct lock_graph *g)
-{
-	closure_put(&g->g[--g->nr].trans->ref);
-}
-
 static noinline void print_cycle(struct printbuf *out, struct lock_graph *g)
 {
 	struct trans_waiting_for_lock *i;
@@ -87,6 +82,18 @@ static noinline void print_cycle(struct printbuf *out, struct lock_graph *g)
 		bch2_btree_trans_to_text(out, i->trans);
 }
 
+static noinline void print_chain(struct printbuf *out, struct lock_graph *g)
+{
+	struct trans_waiting_for_lock *i;
+
+	for (i = g->g; i != g->g + g->nr; i++) {
+		if (i != g->g)
+			prt_str(out, "<- ");
+		prt_printf(out, "%u ", i->trans->locking_wait.task->pid);
+	}
+	prt_newline(out);
+}
+
 static int abort_lock(struct lock_graph *g, struct trans_waiting_for_lock *i)
 {
 	int ret;
@@ -134,6 +141,21 @@ static noinline int break_cycle(struct lock_graph *g)
 	BUG();
 }
 
+static void lock_graph_pop(struct lock_graph *g)
+{
+	closure_put(&g->g[--g->nr].trans->ref);
+}
+
+static void lock_graph_pop_above(struct lock_graph *g, struct trans_waiting_for_lock *above,
+				 struct printbuf *cycle)
+{
+	if (g->nr > 1 && cycle)
+		print_chain(cycle, g);
+
+	while (g->g + g->nr > above)
+		lock_graph_pop(g);
+}
+
 static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans,
 			      struct printbuf *cycle)
 {
@@ -142,9 +164,8 @@ static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans,
 	int ret = 0;
 
 	for (i = g->g; i < g->g + g->nr; i++) {
-		if (i->trans->locking != i->node_want)
-			while (g->g + g->nr >= i) {
-				lock_graph_pop(g);
+		if (i->trans->locking != i->node_want) {
+			lock_graph_pop_above(g, i - 1, cycle);
 			return 0;
 		}
 
@@ -185,20 +206,19 @@ static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans,
 
 	return 0;
 deadlock:
-	while (g->nr)
-		lock_graph_pop(g);
+	lock_graph_pop_above(g, g->g, cycle);
 	return ret;
 }
 
-static noinline void lock_graph_remove_non_waiters(struct lock_graph *g)
+static noinline void lock_graph_remove_non_waiters(struct lock_graph *g,
+						   struct printbuf *cycle)
 {
 	struct trans_waiting_for_lock *i;
 
 	for (i = g->g + 1; i < g->g + g->nr; i++)
 		if (i->trans->locking != i->node_want ||
 		    i->trans->locking_wait.start_time != i[-1].lock_start_time) {
-			while (g->g + g->nr >= i)
-				lock_graph_pop(g);
+			lock_graph_pop_above(g, i - 1, cycle);
 			return;
 		}
 	BUG();
@@ -252,7 +272,7 @@ int bch2_check_for_deadlock(struct btree_trans *trans, struct printbuf *cycle)
 			b = &READ_ONCE(path->l[top->level].b)->c;
 
 			if (unlikely(IS_ERR_OR_NULL(b))) {
-				lock_graph_remove_non_waiters(&g);
+				lock_graph_remove_non_waiters(&g, cycle);
 				goto next;
 			}
 
@@ -286,6 +306,8 @@ int bch2_check_for_deadlock(struct btree_trans *trans, struct printbuf *cycle)
 		}
 	}
 
+	if (g.nr > 1 && cycle)
+		print_chain(cycle, &g);
 	lock_graph_pop(&g);
 	goto next;
 }

fs/bcachefs/debug.c

@@ -725,10 +725,17 @@ static ssize_t bch2_btree_deadlock_read(struct file *file, char __user *buf,
 		goto out;
 
 	mutex_lock(&c->btree_trans_lock);
-	list_for_each_entry(trans, &c->btree_trans_list, list)
-		if (bch2_check_for_deadlock(trans, &i->buf)) {
-			i->iter = 1;
-			break;
+	list_for_each_entry(trans, &c->btree_trans_list, list) {
+		if (trans->locking_wait.task->pid <= i->iter)
+			continue;
+
+		ret = flush_buf(i);
+		if (ret)
+			return ret;
+
+		bch2_check_for_deadlock(trans, &i->buf);
+
+		i->iter = trans->locking_wait.task->pid;
 	}
 	mutex_unlock(&c->btree_trans_lock);
 out: