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

[Christoph Hellwig]

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.