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

[Boqun Feng]

On Tue, Sep 22, 2026 at 03:09:46PM +0800, Kunwu Chan wrote:
> Hi all,
> 

Hello Kunwu,

> 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@xxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/20250625031101.12555-1-boqun.feng@xxxxxxxxx/
> 
> 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.
> 

Thanks a lot for picking up this, much appreciated.

Do you happen to play a bit with rcuscale and see any performance
difference between hazptr_synchronize() vs synchronize_{s}rcu() on
various configs?

> 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.
> 

You could also use /proc/interrupts to measure the IPI counts during the
whole operation, and you should be able to see the difference there.

Regards,
Boqun

> 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
> 
> -- 
> 2.43.0
>