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

[Sagi Grimberg]

>> Having to allocate a struct rcu_synchronize for each of the potentially
>> many queues here is a bit sad.
>>
>> Pull just explained the start_poll_synchronize_rcu interfaces at ALPSS
>> last week, so I wonder if something like that would also be feasible
>> for SRCU, as that would come in really handy here.
> 
> Alternatively I wonder if we could simply use a single srcu_struct
> in the tag_set instead of one per-queue.  That would potentially
> increase the number of read side critical sections
> blk_mq_wait_quiesce_done would have to wait for in tagsets with
> multiple queues, but I wonder how much overhead that would be in
> practive.

It used to be on the hctx actually. It was moved to the request_queue
in an attempt to make parallel quiesce that didn't eventually complete
afaict.

For nvme, we never really quiesce a single namespace, so it will be a
net positive IMO. I also think that quiesce is really more a tagset
operation so I would think that its a more natural location.

The only question in my mind is regarding patch 2/2 with the subtle
ordering of nvme_set_queue_dying...