block: move sched_tags allocation/de-allocation outside of locking context

Hi,

There have been a few reports[1] indicating potential lockdep warnings due
to a lock dependency from the percpu allocator to the elevator lock. This
patch series aims to eliminate that dependency.

The series consists of three patches:
The first patch is preparatory patch and just move elevator queue
allocation logic from ->init_sched into blk_mq_init_sched.

The second patch in the series restructures sched_tags allocation and
deallocation during elevator update/switch operations to ensure these
actions are performed entirely outside the ->freeze_lock and ->elevator_
lock. This eliminates the percpu allocator’s lock dependency on the
elevator and freeze lock during scheduler transitions.

The third patch introduces batch allocation and deallocation helpers for
sched_tags. These helpers are used during __blk_mq_update_nr_hw_queues()
to decouple sched_tags memory management from both the elevator and freeze
locks, addressing the lockdep concerns in the nr_hw_queues update path.

[1] https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c71...

Changes since v7:
    - Rebased code on top of block-6.17, commit 5421681bc3ef ("blk-ioc: don't
      hold queue_lock for ioc_lookup_icq()")
Link to v7: https://lore.kernel.org/all/20250701081954.57381-1-nilay@...      

Changes since v6:
    - Add warning when loading elevator tags from an xarray yields nothing
      (Hannes Reinecke)
    - Use elevator tags instead of xarray table as a function argument to
      elv_update_nr_hw_queues (Ming Lei)
Link to v6: https://lore.kernel.org/all/20250630054756.54532-1-nilay@...

Changes since v5:
    - Fixed smatch warning reported by kernel test robot here:
      https://lore.kernel.org/all/202506300509.2S1tygch-lkp@int...
Link to v5: https://lore.kernel.org/all/20250627175544.1063910-1-nila...

Changes since v4:
    - Define a local Xarray variable in __blk_mq_update_nr_hw_queues to store
      sched_tags, instead of storing it in an Xarray defined in 'struct elevator_tags'
      (Ming Lei)
Link to v4: https://lore.kernel.org/all/20250624131716.630465-1-nilay...

Changes since v3:
    - Further split the patchset into three patch series so that we can
      have a separate patch for sched_tags batch allocation/deallocation
      (Ming Lei)
    - Use Xarray to store and load the sched_tags (Ming Lei)
    - Unexport elevator_alloc() as we no longer need to use it outside
      of block layer core (hch)
    - unwind the sched_tags allocation and free tags when we it fails in
      the middle of allocation (hch)
    - Move struct elevator_queue header from commin header to elevator.c
      as there's no user of it outside elevator.c (Ming Lei, hch)
Link to v3: https://lore.kernel.org/all/20250616173233.3803824-1-nila...

Change since v2:
    - Split the patch into a two patch series. The first patch updates
      ->init_sched elevator API change and second patch handles the sched
      tags allocation/de-allocation logic (Ming Lei)
    - Address sched tags allocation/deallocation logic while running in the
      context of nr_hw_queue update so that we can handle all possible
      scenarios in a single patchest (Ming Lei)
Link to v2: https://lore.kernel.org/all/20250528123638.1029700-1-nila...

Changes since v1:
    - As the lifetime of elevator queue and sched tags are same, allocate
      and move sched tags under struct elevator_queue (Ming Lei)
Link to v1: https://lore.kernel.org/all/20250520103425.1259712-1-nila...

Nilay Shroff (3):
  block: move elevator queue allocation logic into blk_mq_init_sched
  block: fix lockdep warning caused by lock dependency in
    elv_iosched_store
  block: fix potential deadlock while running nr_hw_queue update

 block/bfq-iosched.c   |  13 +--
 block/blk-mq-sched.c  | 223 ++++++++++++++++++++++++++++--------------
 block/blk-mq-sched.h  |  12 ++-
 block/blk-mq.c        |  16 ++-
 block/blk.h           |   4 +-
 block/elevator.c      |  38 +++++--
 block/elevator.h      |  16 ++-
 block/kyber-iosched.c |  11 +--
 block/mq-deadline.c   |  14 +--
 9 files changed, 228 insertions(+), 119 deletions(-)

-- 
2.50.1