[RFC/WIP PATCH 0/4] hazptr: add shared scan path and lockdep use case

[Kunwu Chan <kunwu.chan@gmail.com>]

Hi all,

This RFC/WIP extends the current v3 hazptr implementation [1] and
adapts the lockdep use case from Boqun Feng's earlier hazptr series [2]
to the current hazptr API.

[1] https://lore.kernel.org/all/20260919000056.3132131-26-paulmck@kernel.org/
[2] https://lore.kernel.org/lkml/20250625031101.12555-1-boqun.feng@gmail.com/

The lockdep conversion replaces the expedited RCU wait in
lockdep_unregister_key() with hazptr_synchronize() for dynamic-key
lookups.

The series also adds a shared-scan kthread for concurrent
hazptr_synchronize() callers, rcuscale support, and an LKMM test
for the acquire-before-scan ordering.

I tested the lockdep path on a 96-CPU ARM64 KVM guest with Boqun's
original mq workload [2]. With a multiqueue virtio-net device
(`-device virtio-net-pci,mq=on,vectors=6`), tc mq add/del x100
completed in 850 ms with hazptr and 860 ms with expedited RCU.
The rmmod workload similarly showed no measurable difference
(1360 ms vs. 1380 ms over 10 runs). There were no crashes or hangs
in these tests.

This is still RFC/WIP. I would appreciate feedback on the shared-scan
design and the lockdep conversion.
 
Kunwu Chan (4):
  hazptr: add shared-scan kthread
  locking/lockdep: use hazptr to wait for dynamic key lookups
  rcuscale: add hazptr scale type
  Documentation/litmus-tests: add hazptr acquire-before-scan test

 .../hazptr/hazptr-acquire-before-scan.litmus  |  49 ++++
 kernel/hazptr.c                               | 274 ++++++++++++++++++
 kernel/locking/lockdep.c                      |  30 +-
 kernel/rcu/rcuscale.c                         |  65 ++++-
 4 files changed, 407 insertions(+), 11 deletions(-)
 create mode 100644 Documentation/litmus-tests/hazptr/hazptr-acquire-before-scan.litmus

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2.43.0