Users which rely on synchronisation within a BH-disabled section as in
- access to per-CPU data
- per-CPU timers
- synchronisation against another user within a BH-disabled section

rely on the local_lock_t lock in local_bh_disable() on PREEMPT_RT.
Almost all users dropped their dependency. The remaining (identified)
user in networking (pipapo) is in net-next.
What is left is the infrastructure as in tasklet and workqueue (for
bh-worker).
Both are part of this series. The last patch in the series adds an
option to drop the lock.

v1 tasklet https://lore.kernel.org/all/20250812143930.22RBn5BW@linutronix.de
v1 workqueue https://lore.kernel.org/all/20250820103657.vDuDuLx6@linutronix.de
v1 lock-drop https://lore.kernel.org/all/20250613105653.1860729-2-bigeasy@linutronix.de

Sebastian Andrzej Siewior (3):
  workqueue: Provide a handshake for canceling BH workers
  softirq: Provide a handshake for canceling tasklets via polling
  softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT

 kernel/Kconfig.preempt |  13 ++++
 kernel/softirq.c       | 145 ++++++++++++++++++++++++++++++++++-------
 kernel/workqueue.c     |  51 ++++++++++++---
 3 files changed, 175 insertions(+), 34 deletions(-)

-- 
2.51.0