blk-mq: blk_mq_tag_to_rq should handle flush request

flush request is special, which borrows the tag from the parent
request. Hence blk_mq_tag_to_rq needs special handling to return
the flush request from the tag.
Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-flush.c

@@ -223,8 +223,10 @@ static void flush_end_io(struct request *flush_rq, int error)
 	struct request *rq, *n;
 	unsigned long flags = 0;
 
-	if (q->mq_ops)
+	if (q->mq_ops) {
 		spin_lock_irqsave(&q->mq_flush_lock, flags);
+		q->flush_rq->cmd_flags = 0;
+	}
 
 	running = &q->flush_queue[q->flush_running_idx];
 	BUG_ON(q->flush_pending_idx == q->flush_running_idx);

block/blk-mq.c

@@ -541,9 +541,15 @@ void blk_mq_kick_requeue_list(struct request_queue *q)
 }
 EXPORT_SYMBOL(blk_mq_kick_requeue_list);
 
-struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
+struct request *blk_mq_tag_to_rq(struct blk_mq_hw_ctx *hctx, unsigned int tag)
 {
-	return tags->rqs[tag];
+	struct request_queue *q = hctx->queue;
+
+	if ((q->flush_rq->cmd_flags & REQ_FLUSH_SEQ) &&
+	    q->flush_rq->tag == tag)
+		return q->flush_rq;
+
+	return hctx->tags->rqs[tag];
 }
 EXPORT_SYMBOL(blk_mq_tag_to_rq);
 
@@ -572,7 +578,7 @@ static void blk_mq_timeout_check(void *__data, unsigned long *free_tags)
 		if (tag >= hctx->tags->nr_tags)
 			break;
 
-		rq = blk_mq_tag_to_rq(hctx->tags, tag++);
+		rq = blk_mq_tag_to_rq(hctx, tag++);
 		if (rq->q != hctx->queue)
 			continue;
 		if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))

include/linux/blk-mq.h

@@ -154,7 +154,7 @@ void blk_mq_free_request(struct request *rq);
 bool blk_mq_can_queue(struct blk_mq_hw_ctx *);
 struct request *blk_mq_alloc_request(struct request_queue *q, int rw,
 		gfp_t gfp, bool reserved);
-struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag);
+struct request *blk_mq_tag_to_rq(struct blk_mq_hw_ctx *hctx, unsigned int tag);
 
 struct blk_mq_hw_ctx *blk_mq_map_queue(struct request_queue *, const int ctx_index);
 struct blk_mq_hw_ctx *blk_mq_alloc_single_hw_queue(struct blk_mq_tag_set *, unsigned int, int);