3 files changed, 23 insertions, 9 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index 9dcfb8ec554b..f0be754c7781 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1522,7 +1522,8 @@ bool bio_attempt_front_merge(struct request_queue *q, struct request *req,
  * Caller must ensure !blk_queue_nomerges(q) beforehand.
  */
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
-                            unsigned int *request_count)
+                            unsigned int *request_count,
+                            struct request **same_queue_rq)
 {
         struct blk_plug *plug;
         struct request *rq;
@@ -1542,8 +1543,16 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
         list_for_each_entry_reverse(rq, plug_list, queuelist) {
                 int el_ret;
 
-                if (rq->q == q)
+                if (rq->q == q) {
                         (*request_count)++;
+                        /*
+                         * Only blk-mq multiple hardware queues case checks the
+                         * rq in the same queue, there should be only one such
+                         * rq in a queue
+                         **/
+                        if (same_queue_rq)
+                                *same_queue_rq = rq;
+                }
 
                 if (rq->q != q || !blk_rq_merge_ok(rq, bio))
                         continue;
@@ -1608,7 +1617,7 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
          * any locks.
          */
         if (!blk_queue_nomerges(q) &&
-            blk_attempt_plug_merge(q, bio, &request_count))
+            blk_attempt_plug_merge(q, bio, &request_count, NULL))
                 return;
 
         spin_lock_irq(q->queue_lock);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 902c2eb9a0e7..31df47443699 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1269,6 +1269,7 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
         struct request *rq;
         unsigned int request_count = 0;
         struct blk_plug *plug;
+        struct request *same_queue_rq = NULL;
 
         blk_queue_bounce(q, &bio);
 
@@ -1278,7 +1279,7 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
         }
 
         if (!is_flush_fua && !blk_queue_nomerges(q) &&
-            blk_attempt_plug_merge(q, bio, &request_count))
+            blk_attempt_plug_merge(q, bio, &request_count, &same_queue_rq))
                 return;
 
         rq = blk_mq_map_request(q, bio, &data);
@@ -1309,9 +1310,12 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
                  * issued. So the plug list will have one request at most
                  */
                 if (plug) {
-                        if (!list_empty(&plug->mq_list)) {
-                                old_rq = list_first_entry(&plug->mq_list,
-                                        struct request, queuelist);
+                        /*
+                         * The plug list might get flushed before this. If that
+                         * happens, same_queue_rq is invalid and plug list is empty
+                         **/
+                        if (same_queue_rq && !list_empty(&plug->mq_list)) {
+                                old_rq = same_queue_rq;
                                 list_del_init(&old_rq->queuelist);
                         }
                         list_add_tail(&rq->queuelist, &plug->mq_list);
@@ -1360,7 +1364,7 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio)
         }
 
         if (!is_flush_fua && !blk_queue_nomerges(q) &&
-            blk_attempt_plug_merge(q, bio, &request_count))
+            blk_attempt_plug_merge(q, bio, &request_count, NULL))
                 return;
 
         rq = blk_mq_map_request(q, bio, &data);
diff --git a/block/blk.h b/block/blk.h
index 4b48d55e588e..026d9594142b 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -78,7 +78,8 @@ bool bio_attempt_front_merge(struct request_queue *q, struct request *req,
 bool bio_attempt_back_merge(struct request_queue *q, struct request *req,
                             struct bio *bio);
 bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
-                            unsigned int *request_count);
+                            unsigned int *request_count,
+                            struct request **same_queue_rq);
 
 void blk_account_io_start(struct request *req, bool new_io);
 void blk_account_io_completion(struct request *req, unsigned int bytes);