bcachefs: Improve trace_trans_restart_would_deadlock

In the CI, we're seeing tests failing due to excessive would_deadlock
transaction restarts - the tracepoint now includes the lock cycle that
occured.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -3054,6 +3054,7 @@ 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' };
+	struct task_struct *task = READ_ONCE(trans->locking_wait.task);
 	unsigned l, idx;
 
 	if (!out->nr_tabstops) {
@@ -3061,7 +3062,7 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct btree_trans *trans)
 		printbuf_tabstop_push(out, 32);
 	}
 
-	prt_printf(out, "%i %s\n", trans->locking_wait.task->pid, trans->fn);
+	prt_printf(out, "%i %s\n", task ? task->pid : 0, trans->fn);
 
 	trans_for_each_path_safe(trans, path, idx) {
 		if (!path->nodes_locked)

fs/bcachefs/btree_locking.c

@@ -142,10 +142,28 @@ static bool lock_graph_remove_non_waiters(struct lock_graph *g)
 	return false;
 }
 
+static void trace_would_deadlock(struct lock_graph *g, struct btree_trans *trans,
+				 unsigned long ip)
+{
+	struct bch_fs *c = trans->c;
+
+	count_event(c, trans_restart_would_deadlock);
+
+	if (trace_trans_restart_would_deadlock_enabled()) {
+		struct printbuf buf = PRINTBUF;
+
+		buf.atomic++;
+		print_cycle(&buf, g);
+
+		trace_trans_restart_would_deadlock(trans, ip, buf.buf);
+		printbuf_exit(&buf);
+	}
+}
+
 static int abort_lock(struct lock_graph *g, struct trans_waiting_for_lock *i)
 {
 	if (i == g->g) {
-		trace_and_count(i->trans->c, trans_restart_would_deadlock, i->trans, _RET_IP_);
+		trace_would_deadlock(g, i->trans, _RET_IP_);
 		return btree_trans_restart(i->trans, BCH_ERR_transaction_restart_would_deadlock);
 	} else {
 		i->trans->lock_must_abort = true;
@@ -266,15 +284,16 @@ int bch2_check_for_deadlock(struct btree_trans *trans, struct printbuf *cycle)
 	unsigned path_idx;
 	int ret;
 
+	g.nr = 0;
+
 	if (trans->lock_must_abort) {
 		if (cycle)
 			return -1;
 
-		trace_and_count(trans->c, trans_restart_would_deadlock, trans, _RET_IP_);
+		trace_would_deadlock(&g, trans, _RET_IP_);
 		return btree_trans_restart(trans, BCH_ERR_transaction_restart_would_deadlock);
 	}
 
-	g.nr = 0;
 	lock_graph_down(&g, trans);
 next:
 	if (!g.nr)

fs/bcachefs/trace.h

@@ -1205,10 +1205,11 @@ DEFINE_EVENT(transaction_restart_iter,	trans_restart_memory_allocation_failure,
 	TP_ARGS(trans, caller_ip, path)
 );
 
-DEFINE_EVENT(transaction_event,	trans_restart_would_deadlock,
+DEFINE_EVENT(trans_str, trans_restart_would_deadlock,
 	TP_PROTO(struct btree_trans *trans,
-		 unsigned long caller_ip),
-	TP_ARGS(trans, caller_ip)
+		 unsigned long caller_ip,
+		 const char *cycle),
+	TP_ARGS(trans, caller_ip, cycle)
 );
 
 DEFINE_EVENT(transaction_event,	trans_restart_would_deadlock_recursion_limit,