bcachefs: Make btree_deadlock_to_text() clearer

btree_deadlock_to_text() searches the list of btree transactions to find
a deadlock - when it finds one it's done; it's not like other *_read()
functions that's printing each object.

Factor out btree_deadlock_to_text() to make this clearer.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/debug.c

@@ -802,48 +802,54 @@ static const struct file_operations btree_transaction_stats_op = {
 	.read		= btree_transaction_stats_read,
 };
 
-static ssize_t bch2_btree_deadlock_read(struct file *file, char __user *buf,
-					    size_t size, loff_t *ppos)
+/* walk btree transactions until we find a deadlock and print it */
+static void btree_deadlock_to_text(struct printbuf *out, struct bch_fs *c)
 {
-	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;
+	pid_t iter = 0;
 restart:
 	seqmutex_lock(&c->btree_trans_lock);
 	list_for_each_entry(trans, &c->btree_trans_list, list) {
 		struct task_struct *task = READ_ONCE(trans->locking_wait.task);
 
-		if (!task || task->pid <= i->iter)
+		if (!task || task->pid <= iter)
 			continue;
 
-		closure_get(&trans->ref);
-		u32 seq = seqmutex_unlock(&c->btree_trans_lock);
+		iter = task->pid;
 
-		ret = flush_buf(i);
-		if (ret) {
-			closure_put(&trans->ref);
-			goto out;
-		}
+		closure_get(&trans->ref);
 
-		bch2_check_for_deadlock(trans, &i->buf);
+		u32 seq = seqmutex_unlock(&c->btree_trans_lock);
 
-		i->iter = task->pid;
+		bool found = bch2_check_for_deadlock(trans, out) != 0;
 
 		closure_put(&trans->ref);
 
+		if (found)
+			return;
+
 		if (!seqmutex_relock(&c->btree_trans_lock, seq))
 			goto restart;
 	}
 	seqmutex_unlock(&c->btree_trans_lock);
-out:
+}
+
+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;
+	ssize_t ret = 0;
+
+	i->ubuf = buf;
+	i->size	= size;
+	i->ret	= 0;
+
+	if (!i->iter) {
+		btree_deadlock_to_text(&i->buf, c);
+		i->iter++;
+	}
+
 	if (i->buf.allocation_failure)
 		ret = -ENOMEM;