--- a/drivers/cbus/retu.c
+++ b/drivers/cbus/retu.c
@@ -120,7 +120,9 @@ void retu_write_reg(struct device *child
 {
 	struct retu		*retu = dev_get_drvdata(child->parent);
 
+	mutex_lock(&retu->mutex);
 	__retu_write_reg(retu, reg, val);
+	mutex_unlock(&retu->mutex);
 }
 EXPORT_SYMBOL_GPL(retu_write_reg);
 
