block: avoid unnecessary plug list flush

get_request_wait() could sleep and flush the plug list.  If the list is
already flushed, don't flush again.
Signed-off-by: NShaohua Li <shaohua.li@intel.com>
Reviewed-by: NNamhyung Kim <namhyung@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -1379,7 +1379,8 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
 		 */
 		if (list_empty(&plug->list))
 			trace_block_plug(q);
-		else if (!plug->should_sort) {
+		else {
+			if (!plug->should_sort) {
 				struct request *__rq;
 
 				__rq = list_entry_rq(plug->list.prev);
@@ -1388,6 +1389,7 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
 			}
 			if (request_count >= BLK_MAX_REQUEST_COUNT)
 				blk_flush_plug_list(plug, false);
+		}
 		list_add_tail(&req->queuelist, &plug->list);
 		drive_stat_acct(req, 1);
 	} else {