+ * ipc/mqueue.c for the details. The correct serialization
+ * ensures that a receiver cannot continue without the wakeup
+ * being visibible _before_ setting r_msg:
+ *
+ * CPU 0 CPU 1
+ * <loop receiver>
+ * smp_rmb(); (A) <-- pair -. <waker thread>
+ * <load ->r_msg> | msr->r_msg = NULL;
+ * | wake_up_process();
+ * <continue> `------> smp_wmb(); (B)
+ * msr->r_msg = msg;
+ *
+ * Where (A) orders the message value read and where (B) orders
+ * the write to the r_msg -- done in both pipelined_send and
+ * expunge_all.