[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,

Nope.. spoke too soon, the rcu_head needs to be in a context that has
access to the counter (which is what you called blk_mq_srcu_sync).
you want to add also a pointer to hctx? that is almost as big as
rcu_synchronize...