+/*
+ * Returns true if request was not in flight.
+ */
+static inline bool blk_mq_mark_complete(struct request *rq)
+{
+	return (cmpxchg(&rq->state, MQ_RQ_IN_FLIGHT, MQ_RQ_COMPLETE) !=
+			MQ_RQ_IN_FLIGHT);
+}

This needs a much better comment describing when and how to use it.

Also the outer braces in the return statement are not required.