Since commit 1d9bd5161ba3 ("blk-mq: replace timeout synchronization with a
RCU and generation based scheme") I observe lockdep complaints (full
message attached):

[   21.763369]        CPU0                    CPU1
[   21.763370]        ----                    ----
[   21.763371]   lock(&rq->gstate_seq);
[   21.763374]                                local_irq_disable();
[   21.763375]                                lock(&(&dq->lock)->rlock);
[   21.763377]                                lock(&rq->gstate_seq);
[   21.763379]   <Interrupt>
[   21.763380]     lock(&(&dq->lock)->rlock);
[   21.763382] 
                *** DEADLOCK ***


This only happens in combination with DASD (s390 specific) and another
blk-mq driver (scsi, null_blk). The distinctive behavior of DASD is that
it calls blk_mq_start_request with irqs disabled.

This is a false positive since gstate_seq on CPU0 is from a different
request queue / block driver than gstate_seq on CPU1.

Possible fixes are to use local_irq_save/restore in blk_mq_start_request.
Or, since it's a false positive, teach lockdep that these are different
locks - like below:

------>8
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0dc9e341c2a7..d1a2c2fa015d 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2040,7 +2040,7 @@ static int blk_mq_init_request(struct blk_mq_tag_set *set, struct request *rq,
 			return ret;
 	}
 
-	seqcount_init(&rq->gstate_seq);
+	__seqcount_init(&rq->gstate_seq, "&rq->gstate_seq", set->rq_seqcount_key);
 	u64_stats_init(&rq->aborted_gstate_sync);
 	return 0;
 }
@@ -2774,7 +2774,7 @@ static int blk_mq_update_queue_map(struct blk_mq_tag_set *set)
  * requested depth down, if if it too large. In that case, the set
  * value will be stored in set->queue_depth.
  */
-int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
+int __blk_mq_alloc_tag_set(struct blk_mq_tag_set *set, struct lock_class_key *key)
 {
 	int ret;
 
@@ -2825,6 +2825,7 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
 	if (!set->mq_map)
 		goto out_free_tags;
 
+	set->rq_seqcount_key = key;
 	ret = blk_mq_update_queue_map(set);
 	if (ret)
 		goto out_free_mq_map;
@@ -2846,7 +2847,7 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
 	set->tags = NULL;
 	return ret;
 }
-EXPORT_SYMBOL(blk_mq_alloc_tag_set);
+EXPORT_SYMBOL(__blk_mq_alloc_tag_set);
 
 void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
 {
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index e3986f4b3461..1fb8bc1fdb07 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -85,6 +85,8 @@ struct blk_mq_tag_set {
 
 	struct mutex		tag_list_lock;
 	struct list_head	tag_list;
+
+	struct lock_class_key	*rq_seqcount_key;
 };
 
 struct blk_mq_queue_data {
@@ -201,7 +203,15 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
 int blk_mq_register_dev(struct device *, struct request_queue *);
 void blk_mq_unregister_dev(struct device *, struct request_queue *);
 
-int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set);
+int __blk_mq_alloc_tag_set(struct blk_mq_tag_set *set, struct lock_class_key *key);
+
+static inline int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
+{
+	static struct lock_class_key _seqcount_key;
+
+	return __blk_mq_alloc_tag_set(set, &_seqcount_key);
+}
+
 void blk_mq_free_tag_set(struct blk_mq_tag_set *set);
 
 void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule);