Re: [PATCH 1/2] blk-mq: Export iterating all tagged requests

[Keith Busch <keith.busch@intel.com>]

On Tue, Dec 04, 2018 at 09:38:29AM -0800, Sagi Grimberg wrote:
> 
> > > > Yes, I'm very much in favour of this, too.
> > > > We always have this IMO slightly weird notion of stopping the queue, set
> > > > some error flags in the driver, then _restarting_ the queue, just so
> > > > that the driver then sees the error flag and terminates the requests.
> > > > Which I always found quite counter-intuitive.
> > > 
> > > What about requests that come in after the iteration runs? how are those
> > > terminated?
> > 
> > If we've reached a dead state, I think you'd want to start a queue freeze
> > before running the terminating iterator.
> 
> Its not necessarily dead, in fabrics we need to handle disconnections
> that last for a while before we are able to reconnect (for a variety of
> reasons) and we need a way to fail I/O for failover (or requeue, or
> block its up to the upper layer). Its less of a "last resort" action
> like in the pci case.
> 
> Does this guarantee that after freeze+iter we won't get queued with any
> other request? If not then we still need to unfreeze and fail at
> queue_rq.

It sounds like there are different scenarios to consider.

For the dead controller, we call blk_cleanup_queue() at the end which
ends callers who blocked on entering.

If you're doing a failover, you'd replace the freeze with a current path
update in order to prevent new requests from entering.

In either case, you don't need checks in queue_rq. The queue_rq check
is redundant with the quiesce state that blk-mq already provides.

Once quiesced, the proposed iterator can handle the final termination
of the request, perform failover, or some other lld specific action
depending on your situation.