[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.