The problem here is the following: blk_finish_request must always be called with the queue lock held, it even has an assert. Without blk-mq used by dm-rq, dm uses the block softirq to execute the completion, which means we always have a different execution context and can take the queue lock again without issuesi. With blk-mq used by dm-rq, the the dm .complete handler that is the rough equivalent of the softirq handler is called either directly if were are on the same CPU, or using a IPI (hardirq) if not. If this handler gets called from a legacy request function it will be called with the queue_lock held, but if it's called from a blk-mq driver or actually uses the IPI no lock will be held. When I did my blk-mq only for dm-mpath WIP patch my solution to that was that I removed the ->complete handler entirely and just ran the whole dm completion from the original hardirq context. With that change I know that for blk-mq we'll never hold the queue_lock (and the blk-mq request free path doesn't care), and for legacy we always hold it, so __blk_put_request can always be used.
