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...
