Index: linux-2.6.38-rc6/drivers/cbus/retu.c
===================================================================
--- linux-2.6.38-rc6.orig/drivers/cbus/retu.c	2011-02-26 23:23:34.014429906 +0100
+++ linux-2.6.38-rc6/drivers/cbus/retu.c	2011-02-26 23:25:18.905358127 +0100
@@ -120,7 +120,9 @@
 {
 	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);