[PATCH V4 0/2] fixes for the updating nr_hw_queues

[Jianchao Wang <jianchao.w.wang@oracle.com>]

Hi Jens

Two fixes for updating nr_hw_queues.

The first patch fixes the following scenario:
io scheduler (kyber) depends on the mapping between ctx and hctx.
When update nr_hw_queues, io scheduler's init_hctx will be
invoked before the mapping is adapted correctly, this would cause
panic in kyber.

The second patch fixes the following scenario:
part_in_flight/rw will invoke blk_mq_in_flight/rw to account the
inflight requests. It will access the queue_hw_ctx and nr_hw_queues
w/o any protection. When updating nr_hw_queues and blk_mq_in_flight
/rw occur concurrently, panic comes up.

V4:
 - remove the elv_type in request_queue and introduce blk_mq_qe_pair
   to cache the elevator_type associated with one request_queue. And
   add two new helper interfaces blk_mq_elv_switch_none/back to carry
   out the elevator_type caching and switching work.
 - Add Ming's Reviewd-by on 2nd patch and remove it from 1st patch due
   to the reworking.
 - comment modification.

V3:
 - move the rcu and q_usage_counter checking into blk_mq_queue_tag_busy_iter
   as suggestion of Ming.
 - Add more comment about the __module_get in 1st patch.
 - Add Ming's Reviewd-by on 1st patch.

V2:
 - remove blk_mq_sched_init/exit_hctx in patch 1.
 - use q_usage_counter instead of adding new member suggested by
   Ming in patch 2.
 - comment modification.

 block/blk-mq-sched.c | 44 --------------------------------------------
 block/blk-mq-sched.h |  5 -----
 block/blk-mq-tag.c   | 14 +++++++++++++-
 block/blk-mq.c       | 96 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--------
 block/blk.h          |  2 ++
 block/elevator.c     | 20 ++++++++++++--------
 6 files changed, 115 insertions(+), 66 deletions(-)

Thanks
Jianchao