[PATCH v2 0/2] blk-iocost: fix hweight cache publication and donation fail-safe

[Kunhai Dai <daikunhai@didiglobal.com>]

This series fixes the blk-iocost hweight cache publication race and adds a
fail-safe for invalid donation weights.

Previous version:
  https://lists.openwall.net/linux-kernel/2026/10/15/1954
Replies:
  https://lists.openwall.net/linux-kernel/2026/10/16/1431
  https://lists.openwall.net/linux-kernel/2026/10/16/2798

Changes since v1:
- split into two patches: the donation fail-safe and the atomic cache;
- add the Fixes tags agreed on the list (e08d02aa5fc9, 7caa47151ab2) and
  Cc stable on the fail-safe;
- drop the donating > 1 guard, which only changed the value printed by the
  warning;
- do not add max(after, 1): the no-race path has not been shown to be
  reachable and the total budget invariant has not been re-validated;
- use GENMASK/FIELD_PREP/FIELD_GET helpers for the packed cache;
- have tracepoints decode the packed cache without refreshing it, and only
  evaluate that read when the tracepoint is enabled;
- explain the activation-time cache update;
- update iocost_monitor.py;
- document the AI-assisted work.

The no-race invalid-weight path is still under investigation: a hand-built
input can trigger the divide at the function level, but runtime
reachability has not been demonstrated.  The first patch is therefore a
fail-safe, not a claim that the path is reachable without a race.

Kunhai Dai (2):
  blk-iocost: abort weight transfer on invalid donation weights
  blk-iocost: publish hweights as one atomic value

 block/blk-iocost.c             | 111 +++++++++++++++++++++++++++------
 include/trace/events/iocost.h  |   7 ++-
 tools/cgroup/iocost_monitor.py |   7 ++-
 3 files changed, 101 insertions(+), 24 deletions(-)

-- 
2.27.0