[PATCHv3 0/2] fix sbitmap initialization and null_blk shared tagset behavior

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

Hi,

This patchset fixes two subtle issues discovered while unit testing
nr_hw_queue update code using null_blk driver.

The first patch in the series, fixes an issue in the sbitmap initialization
code, where sb->alloc_hint is not explicitly set to NULL when the sbitmap
depth is zero. This can lead to a kernel crash in sbitmap_free(), which
unconditionally calls free_percpu() on sb->alloc_hint — even if it was
never allocated. The crash is caused by dereferencing an invalid pointer
or stale garbage value.

The second patch in the series, prevents runtime updates to submit_queues
or poll_queues when using a shared tagset. Currently, such updates lead 
to the allocation of new hardware queues (hctx) that are never mapped to
any software queues (ctx), rendering them unusable for I/O. This patch
rejects these changes and ensures more consistent behavior. Interestingly,
this unnecessary queue update path helped uncover the issue fixed in first
patch.

As usual, review and feedback are most welcome!

Changes from v2:
    - Updated the second patch to prevent the user from modifying submit
      or poll queues when tagset is shared (Damien Le Moal, Yu Kuai)
Changes from v1:
    - The set->driver_data field should be initialized separately for the
      shared tagset to ensure it is correctly set for both shared and
      non-shared tagset cases. (Damien Le Moal)

Nilay Shroff (2):
  lib/sbitmap: fix kernel crash observed when sbitmap depth is zero
  null_blk: prevent submit and poll queues update for shared tagset

 drivers/block/null_blk/main.c | 32 ++++++++++++++++++++++----------
 lib/sbitmap.c                 |  1 +
 2 files changed, 23 insertions(+), 10 deletions(-)

-- 
2.50.1