[PATCH v2 1/2] blk-mq: add tagset quiesce interface

[Sagi Grimberg]

>> nvme-tcp will opportunistically try a network send directly from
>> .queue_rq(), but always with MSG_DONTWAIT, so that is not a problem.
>>
>> nbd though can block in .queue_rq() with blocking network sends, however
>> afaict nbd allocates a tagset per nbd_device, and also a request_queue
>> per device, so its effectively the same if the srcu is in the tagset or
>> in the request_queue.
> 
> Yes, the only multiple request_queue per tag_set cases right now
> are nvme-tcp and mmc.  There have been patches from iSCSI, but they
> seem to be stuck.

iscsi defers network sends to a workqueue anyways, and no scsi lld sets
BLK_MQ_F_BLOCKING.