[PATCH 5/5] blk-mq: support nested blk_mq_quiesce_queue()

[Sagi Grimberg]

On 9/29/21 7:15 AM, Ming Lei wrote:
> Turns out that blk_mq_freeze_queue() isn't stronger[1] than
> blk_mq_quiesce_queue() because dispatch may still be in-progress after
> queue is frozen, and in several cases, such as switching io scheduler,
> updating nr_requests & wbt latency, we still need to quiesce queue as a
> supplement of freezing queue.
> 
> As we need to extend uses of blk_mq_quiesce_queue(), it is inevitable
> for us to need support nested quiesce, especailly we can't let
> unquiesce happen when there is quiesce originated from other contexts.

The serialization need is clear, but why is the nesting required?
In other words what is the harm is running the hw queue every time
we unquiesce?