block: return whether or not to unplug through boolean

Instead of returning the same queue request through a request pointer,
use a boolean to accomplish the same.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -1067,9 +1067,8 @@ static enum bio_merge_status blk_attempt_bio_merge(struct request_queue *q,
  * @q: request_queue new bio is being queued at
  * @bio: new bio being queued
  * @nr_segs: number of segments in @bio
- * @same_queue_rq: pointer to &struct request that gets filled in when
- * another request associated with @q is found on the plug list
- * (optional, may be %NULL)
+ * @same_queue_rq: output value, will be true if there's an existing request
+ * from the passed in @q already in the plug list
  *
  * Determine whether @bio being queued on @q can be merged with the previous
  * request on %current's plugged list.  Returns %true if merge was successful,
@@ -1085,7 +1084,7 @@ static enum bio_merge_status blk_attempt_bio_merge(struct request_queue *q,
  * Caller must ensure !blk_queue_nomerges(q) beforehand.
  */
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
-		unsigned int nr_segs, struct request **same_queue_rq)
+		unsigned int nr_segs, bool *same_queue_rq)
 {
 	struct blk_plug *plug;
 	struct request *rq;
@@ -1096,12 +1095,12 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
 
 	/* check the previously added entry for a quick merge attempt */
 	rq = list_last_entry(&plug->mq_list, struct request, queuelist);
-	if (rq->q == q && same_queue_rq) {
+	if (rq->q == q) {
 		/*
 		 * Only blk-mq multiple hardware queues case checks the rq in
 		 * the same queue, there should be only one such rq in a queue
 		 */
-		*same_queue_rq = rq;
+		*same_queue_rq = true;
 	}
 	if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) == BIO_MERGE_OK)
 		return true;

block/blk-mq.c

@@ -2432,7 +2432,7 @@ void blk_mq_submit_bio(struct bio *bio)
 	const int is_flush_fua = op_is_flush(bio->bi_opf);
 	struct request *rq;
 	struct blk_plug *plug;
-	struct request *same_queue_rq = NULL;
+	bool same_queue_rq = false;
 	unsigned int nr_segs = 1;
 	blk_status_t ret;
 
@@ -2525,26 +2525,28 @@ void blk_mq_submit_bio(struct bio *bio)
 		/* Insert the request at the IO scheduler queue */
 		blk_mq_sched_insert_request(rq, false, true, true);
 	} else if (plug && !blk_queue_nomerges(q)) {
+		struct request *next_rq = NULL;
+
 		/*
 		 * We do limited plugging. If the bio can be merged, do that.
 		 * Otherwise the existing request in the plug list will be
 		 * issued. So the plug list will have one request at most
 		 * The plug list might get flushed before this. If that happens,
 		 * the plug list is empty, and same_queue_rq is invalid.
 		 */
-		if (list_empty(&plug->mq_list))
-			same_queue_rq = NULL;
 		if (same_queue_rq) {
-			list_del_init(&same_queue_rq->queuelist);
+			next_rq = list_last_entry(&plug->mq_list,
+							struct request,
+							queuelist);
+			list_del_init(&next_rq->queuelist);
 			plug->rq_count--;
 		}
 		blk_add_rq_to_plug(plug, rq);
 		trace_block_plug(q);
 
-		if (same_queue_rq) {
+		if (next_rq) {
 			trace_block_unplug(q, 1, true);
-			blk_mq_try_issue_directly(same_queue_rq->mq_hctx,
-						  same_queue_rq);
+			blk_mq_try_issue_directly(next_rq->mq_hctx, next_rq);
 		}
 	} else if ((q->nr_hw_queues > 1 && is_sync) ||
 		   !rq->mq_hctx->dispatch_busy) {

block/blk.h

@@ -218,7 +218,7 @@ void blk_add_timer(struct request *req);
 void blk_print_req_error(struct request *req, blk_status_t status);
 
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
-		unsigned int nr_segs, struct request **same_queue_rq);
+		unsigned int nr_segs, bool *same_queue_rq);
 bool blk_bio_list_merge(struct request_queue *q, struct list_head *list,
 			struct bio *bio, unsigned int nr_segs);