Hi Ming,
Yeah, the above change is correct, for any canceling requests in this way we should use blk_mq_quiesce_queue().
I still don't understand why should blk_mq_flush_busy_ctxs hit a NULL deref if we don't touch the tagset... Also, I'm wandering in what case we shouldn't use blk_mq_quiesce_queue()? Maybe we should unexport blk_mq_stop_hw_queues() and blk_mq_start_stopped_hw_queues() and use the quiesce/unquiesce equivalent always? The only fishy usage is in nvme_fc_start_fcp_op() where if submission failed the code stop the hw queues and delays it, but I think it should be handled differently..
