diff --git a/fs/libfs.c b/fs/libfs.c
index 4910a36..cd22319 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -575,6 +575,21 @@
  * possibly a read which collects the result - which is stored in a
  * file-local buffer.
  */
+
+void simple_transaction_set(struct file *file, size_t n)
+{
+	struct simple_transaction_argresp *ar = file->private_data;
+
+	BUG_ON(n > SIMPLE_TRANSACTION_LIMIT);
+
+	/*
+	 * The barrier ensures that ar->size will really remain zero until
+	 * ar->data is ready for reading.
+	 */
+	smp_mb();
+	ar->size = n;
+}
+
 char *simple_transaction_get(struct file *file, const char __user *buf, size_t size)
 {
 	struct simple_transaction_argresp *ar;
@@ -820,6 +835,7 @@
 EXPORT_SYMBOL(simple_unlink);
 EXPORT_SYMBOL(simple_read_from_buffer);
 EXPORT_SYMBOL(memory_read_from_buffer);
+EXPORT_SYMBOL(simple_transaction_set);
 EXPORT_SYMBOL(simple_transaction_get);
 EXPORT_SYMBOL(simple_transaction_read);
 EXPORT_SYMBOL(simple_transaction_release);