[PATCH] nvme-core: reduce io pause time when fail over

[Sagi Grimberg]

>> To reduce io pause time, nvme_stop_queues use
>> blk_mq_quiesce_queue_nowait to quiesce the queue, nvme_stop_queues wait
>> all ongoing dispatches completed after all queues has been quiesced.
> 
> The comment above blk_mq_quiesce_queue_nowait() wants to remove this
> function. I'm not sure we should be introducing more users if that's the
> case.
> 
> Using synchronize_rcu() at the end may be looking to much into blk-mq's
> internal quiesce implementation. It happens to be the right thing to do
> for non-blocking hctx, so this patch assumes that will be true for any
> nvme request_queue.

nvme-tcp became a block hctx since we optimize to do network sends from
inside queue_rq. So this should either split into two functions or
nvme-core needs to look into BLK_MQ_F_BLOCKING and do the right thing.