Comment # 36 on bug 1199355 from Hannes Reinecke

Next try:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 84d749511f55..e6f15ac9afd0 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2769,6 +2769,10 @@ static inline struct request
*blk_mq_get_cached_request(struct request_queue *q,
        rq = rq_list_peek(&plug->cached_rq);
        if (!rq || rq->q != q)
                return NULL;
+       if (WARN_ON(!rq->mq_hctx)) {
+               plug->cached_rq = rq_list_next(rq);
+               return NULL;
+       }

        if (blk_mq_attempt_bio_merge(q, *bio, nsegs)) {
                *bio = NULL;

(and remove the debug patch).
seife?