[PATCHv3 0/4] block: restructure elevator switch path and fix a lockdep splat

[Nilay Shroff <nilay@linux.ibm.com>]

Hi,

This patchset reorganizes the elevator switch path used during both
nr_hw_queues update and elv_iosched_store() operations to address a
recently reported lockdep splat [1].

The warning highlights a locking dependency between ->freeze_lock and
->elevator_lock on pcpu_alloc_mutex, triggered when the Kyber scheduler
dynamically allocates its private scheduling data. The fix is to ensure
that such allocations occur outside the locked sections, thus eliminating
the dependency chain.

While working on this, it also became evident that the nr_hw_queue update
code maintains two disjoint xarrays—one for elevator tags and another
for elevator type—both serving the same purpose. Unifying these into a
single elv_change_ctx structure improves clarity and maintainability.

This series therefore implements four patches:
The first perparatory patch unifies elevator tags and type xarrays. It
combines both xarrays into a single struct elv_change_ctx, simplifying
per-queue elevator state management.

The second patch is aimed to group together all elevator-related 
resources that share the same lifetime and as a first step we move the
elevator tags pointer from struct elv_change_ctx into the newly
inroduced struct elevator_resources. The subsequent patch extends the 
struct elevator_resources to include other elevator-related data.

The third patch introduce ->alloc_sched_data and ->free_sched_data
elevator ops to safely allocate and free scheduler data before acquiring
->freeze_lock and ->elevator_lock, preventing the dependency on pcpu_
alloc_mutex. In this patch we add elevator-data into the struct 
elevator_resources, which is introduced in the previous patch.

The fourth patch converts Kyber scheduler to use the new methods
inroduced in the previous patch. It hooks Kyber’s scheduler data
allocation and teardown logic from ->init_sched and ->exit_sched into
the new methods, ensuring memory operations are performed outside
locked sections.

Together, these changes simplify the elevator switch logic and prevent
the reported lockdep splat.

As always, feedback and suggestions are very welcome!

[1] https://lore.kernel.org/all/CAGVVp+VNW4M-5DZMNoADp6o2VKFhi7KxWpTDkcnVyjO0=-D5+A@mail.gmail.com/

Thanks,
--Nilay

changes fron v2:
  - Introduce helper functions blk_mq_alloc_sched_res_batch() and
    blk_mq_free_sched_res_batch() to encapsulate scheduler resource
    (tags and data) allocation and freeing in batch mode. (Ming Lei)

  - Introduce helper functions blk_mq_alloc_sched_res() and
    blk_mq_free_sched_res() to encapsulate scheduler resource
    allocation and freeing. (Ming Lei)

Link to v2: https://lore.kernel.org/all/20251027173631.1081005-1-nilay@linux.ibm.com/

changes from v1:
  - Keep blk_mq_free_sched_ctx_batch() and blk_mq_alloc_sched_ctx_batch()
    together in the same file (Ming Lei)
  - Since the ctx pointer is stored in xarray after it's dynamically
    allocated, if blk_mq_alloc_sched_ctx_batch() fails to allocate or
    insert ctx pointer in xarray then unwinding the allocation is not
    necessary. Instead looping over the xarray to retrieve the inserted
    ctx pointer and freeing it should be sufficibet. So invoke blk_mq_
    free_sched_ctx_batch() from the blk_mq_alloc_sched_ctx_batch()
    callsite on failure (Ming Lei)
  - As both elevator tags and elevator data shares the same lifetime
    and allocation constraints, abstract both into a new structure
    (Ming Lei)

Link to v1: https://lore.kernel.org/all/20251016053057.3457663-1-nilay@linux.ibm.com/

Nilay Shroff (4):
  block: unify elevator tags and type xarrays into struct elv_change_ctx
  block: move elevator tags into struct elevator_resources
  block: introduce alloc_sched_data and free_sched_data elevator methods
  block: define alloc_sched_data and free_sched_data methods for kyber

 block/blk-mq-sched.c  | 123 +++++++++++++++++++++++++++++++++---------
 block/blk-mq-sched.h  |  34 ++++++++++--
 block/blk-mq.c        |  50 +++++++++--------
 block/blk.h           |   7 ++-
 block/elevator.c      |  85 +++++++++++++++--------------
 block/elevator.h      |  26 ++++++++-
 block/kyber-iosched.c |  30 ++++++++---
 7 files changed, 249 insertions(+), 106 deletions(-)

-- 
2.51.0