[PATCH v5 1/2] blk-mq: add tagset quiesce interface

[Sagi Grimberg]

>>>> I like the tagset based interface.  But the idea of doing a per-hctx
>>>> allocation and wait doesn't seem very scalable.
>>>>
>>>> Paul, do you have any good idea for an interface that waits on
>>>> multiple srcu heads?  As far as I can tell we could just have a single
>>>> global completion and counter, and each call_srcu would just just
>>>> decrement it and then the final one would do the wakeup.  It would just
>>>> be great to figure out a way to keep the struct rcu_synchronize and
>>>> counter on stack to avoid an allocation.
>>>>
>>>> But if we can't do with an on-stack object I'd much rather just embedd
>>>> the rcu_head in the hw_ctx.
>>>
>>> I think we can do that, please see the following patch which is against Sagi's V5:
>>
>> I don't think you can send a single rcu_head to multiple call_srcu calls.
> 
> OK, then one variant is to put the rcu_head into blk_mq_hw_ctx, and put
> rcu_synchronize into blk_mq_tag_set.

I can cook up a spin, but I still hate the fact that I have a queue that
ends up quiesced which I didn't want it to...