blk-mq: Simplify request completion state

There are no more users relying on blk-mq request states to prevent
double completions, so replace the relatively expensive cmpxchg operation
with WRITE_ONCE.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -568,9 +568,7 @@ static void __blk_mq_complete_request(struct request *rq)
 	bool shared = false;
 	int cpu;
 
-	if (!blk_mq_mark_complete(rq))
-		return;
-
+	WRITE_ONCE(rq->state, MQ_RQ_COMPLETE);
 	/*
 	 * Most of single queue controllers, there is only one irq vector
 	 * for handling IO completion, and the only irq's affinity is set

include/linux/blk-mq.h

@@ -329,20 +329,6 @@ void blk_mq_quiesce_queue_nowait(struct request_queue *q);
 
 unsigned int blk_mq_rq_cpu(struct request *rq);
 
-/**
- * blk_mq_mark_complete() - Set request state to complete
- * @rq: request to set to complete state
- *
- * Returns true if request state was successfully set to complete. If
- * successful, the caller is responsibile for seeing this request is ended, as
- * blk_mq_complete_request will not work again.
- */
-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;
-}
-
 /*
  * Driver command data is immediately after the request. So subtract request
  * size to get back to the original request, add request size to get the PDU.