bcachefs: Print deadlock cycle in debugfs

In the event that we're not finished debugging the cycle detector, this
adds a new file to debugfs that shows what the cycle detector finds, if
anything. By comparing this with btree_transactions, which shows held
locks for every btree_transaction, we'll be able to determine if it's
the cycle detector that's buggy or something else.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -2992,7 +2992,6 @@ bch2_btree_path_node_to_text(struct printbuf *out,
 		   c.n[0], c.n[1], c.n[2], pid);
 }
 
-#ifdef CONFIG_BCACHEFS_DEBUG_TRANSACTIONS
 void bch2_btree_trans_to_text(struct printbuf *out, struct btree_trans *trans)
 {
 	struct btree_path *path;
@@ -3041,7 +3040,6 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct btree_trans *trans)
 		prt_printf(out, "\n");
 	}
 }
-#endif
 
 void bch2_fs_btree_iter_exit(struct bch_fs *c)
 {

fs/bcachefs/btree_locking.c

@@ -76,6 +76,17 @@ 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;
+
+	prt_printf(out, "Found lock cycle (%u entries):", g->nr);
+	prt_newline(out);
+
+	for (i = g->g; i < g->g + g->nr; i++)
+		bch2_btree_trans_to_text(out, i->trans);
+}
+
 static int abort_lock(struct lock_graph *g, struct trans_waiting_for_lock *i)
 {
 	int ret;
@@ -122,7 +133,8 @@ static noinline int break_cycle(struct lock_graph *g)
 	BUG();
 }
 
-static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans)
+static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans,
+			      struct printbuf *cycle)
 {
 	struct btree_trans *orig_trans = g->g->trans;
 	struct trans_waiting_for_lock *i;
@@ -136,7 +148,14 @@ static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans)
 			}
 
 		if (i->trans == trans) {
-			ret = break_cycle(g);
+			if (cycle) {
+				/* Only checking: */
+				print_cycle(cycle, g);
+				ret = -1;
+			} else {
+				ret = break_cycle(g);
+			}
+
 			if (ret)
 				goto deadlock;
 			/*
@@ -170,19 +189,6 @@ static int lock_graph_descend(struct lock_graph *g, struct btree_trans *trans)
 	return ret;
 }
 
-#if 0
-static void print_cycle(struct printbuf *out, struct lock_graph *g)
-{
-	struct trans_waiting_for_lock *i;
-
-	prt_str(out, "Found lock cycle:");
-	prt_newline(out);
-
-	for (i = g->g; i < g->g + g->nr; i++)
-		bch2_btree_trans_to_text(out, i->trans);
-}
-#endif
-
 static noinline void lock_graph_remove_non_waiters(struct lock_graph *g)
 {
 	struct trans_waiting_for_lock *i;
@@ -202,7 +208,7 @@ static bool lock_type_conflicts(enum six_lock_type t1, enum six_lock_type t2)
 	return t1 + t2 > 1;
 }
 
-static int check_for_deadlock(struct btree_trans *trans)
+int bch2_check_for_deadlock(struct btree_trans *trans, struct printbuf *cycle)
 {
 	struct lock_graph g;
 	struct trans_waiting_for_lock *top;
@@ -214,7 +220,7 @@ static int check_for_deadlock(struct btree_trans *trans)
 		return btree_trans_restart(trans, BCH_ERR_transaction_restart_would_deadlock);
 
 	g.nr = 0;
-	ret = lock_graph_descend(&g, trans);
+	ret = lock_graph_descend(&g, trans, cycle);
 	BUG_ON(ret);
 next:
 	if (!g.nr)
@@ -265,7 +271,7 @@ static int check_for_deadlock(struct btree_trans *trans)
 				    !lock_type_conflicts(lock_held, trans->locking_wait.lock_want))
 					continue;
 
-				ret = lock_graph_descend(&g, trans);
+				ret = lock_graph_descend(&g, trans, cycle);
 				raw_spin_unlock(&b->lock.wait_lock);
 
 				if (ret)
@@ -285,7 +291,7 @@ int bch2_six_check_for_deadlock(struct six_lock *lock, void *p)
 {
 	struct btree_trans *trans = p;
 
-	return check_for_deadlock(trans);
+	return bch2_check_for_deadlock(trans, NULL);
 }
 
 int __bch2_btree_node_lock_write(struct btree_trans *trans,

fs/bcachefs/btree_locking.h

@@ -426,6 +426,7 @@ struct six_lock_count bch2_btree_node_lock_counts(struct btree_trans *,
 				struct btree_bkey_cached_common *b,
 				unsigned);
 
+int bch2_check_for_deadlock(struct btree_trans *, struct printbuf *);
 
 #ifdef CONFIG_BCACHEFS_DEBUG
 void bch2_btree_path_verify_locks(struct btree_path *);

fs/bcachefs/debug.c

@@ -11,6 +11,7 @@
 #include "btree_cache.h"
 #include "btree_io.h"
 #include "btree_iter.h"
+#include "btree_locking.h"
 #include "btree_update.h"
 #include "buckets.h"
 #include "debug.h"
@@ -708,6 +709,45 @@ static const struct file_operations lock_held_stats_op = {
 	.read = lock_held_stats_read,
 };
 
+static ssize_t bch2_btree_deadlock_read(struct file *file, char __user *buf,
+					    size_t size, loff_t *ppos)
+{
+	struct dump_iter *i = file->private_data;
+	struct bch_fs *c = i->c;
+	struct btree_trans *trans;
+	ssize_t ret = 0;
+
+	i->ubuf = buf;
+	i->size	= size;
+	i->ret	= 0;
+
+	if (i->iter)
+		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;
+		}
+	mutex_unlock(&c->btree_trans_lock);
+out:
+	if (i->buf.allocation_failure)
+		ret = -ENOMEM;
+
+	if (!ret)
+		ret = flush_buf(i);
+
+	return ret ?: i->ret;
+}
+
+static const struct file_operations btree_deadlock_ops = {
+	.owner		= THIS_MODULE,
+	.open		= bch2_dump_open,
+	.release	= bch2_dump_release,
+	.read		= bch2_btree_deadlock_read,
+};
+
 void bch2_fs_debug_exit(struct bch_fs *c)
 {
 	if (!IS_ERR_OR_NULL(c->fs_debug_dir))
@@ -741,6 +781,9 @@ void bch2_fs_debug_init(struct bch_fs *c)
 	debugfs_create_file("btree_transaction_stats", 0400, c->fs_debug_dir,
 			    c, &lock_held_stats_op);
 
+	debugfs_create_file("btree_deadlock", 0400, c->fs_debug_dir,
+			    c->btree_debug, &btree_deadlock_ops);
+
 	c->btree_debug_dir = debugfs_create_dir("btrees", c->fs_debug_dir);
 	if (IS_ERR_OR_NULL(c->btree_debug_dir))
 		return;