Re: [PATCH hazptr 0/4] Hazard pointer updates

[Boqun Feng <boqun@kernel.org>]

On Sun, Sep 27, 2026 at 11:51:27AM -0400, Mathieu Desnoyers wrote:
> Hi Paul,
> 
> This series applies on top of "hazptr: handle NULL address in
> hazptr_detach" you have in your rcu dev tree.
> 
> This first patch addresses a race identified by Boqun Feng in the
> two-phase wildcard scheme.
> 

Thank you! That looks good from a quick look. I will give a deep look
later on (I was traveling).

For the following versions/patches, could you also Cc Kuwu Chan and
Liang Wang (Cced) ? They are helping on the scan thread and lockdep
integration, so it'll be good to keep them in the loop. Thank you!

> Patches 2-3 are prerequisites for using ptr_eq() in the 4th patch.
> Those were discussed at length in a prior version of hazard pointer
> patches.
> 
> Patch 4 introduces a "try acquire" helper to allow the fast path
> to not rely on wildcards, while keeping the wildcard forward
> progress guarantees in the acquire slow path, used on fast path
> failure.
> 

Nice! I want to point out that later on if the reader can handle the
race with the updater (i.e. the readers don't need the progress
guarantees from hazptr, then we can expose a hazptr_try_acquire() for
exactly that). Although I do have some design trade-off questions for
patch 4.

Regards,
Boqun

> Thanks,
> 
> Mathieu
> 
> Mathieu Desnoyers (4):
>   hazptr: Fix two-phase hazptr_synchronize race with detach
>   compiler.h: Introduce ptr_eq() to preserve address dependency
>   Documentation: RCU: Refer to ptr_eq()
>   hazptr: Introduce "try acquire" fast path, fallback to overflow list
> 
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Boqun Feng <boqun@kernel.org>
> Cc: Bradley Morgan <brads@mainlining.org>
> Cc: Gary Guo <gary@garyguo.net>
> Cc: <rcu@vger.kernel.org>
> Cc: <lkmm@lists.linux.dev>
> 
>  Documentation/RCU/rcu_dereference.rst |  38 +++++++-
>  include/linux/compiler.h              |  63 ++++++++++++
>  include/linux/hazptr.h                |  47 +++++----
>  kernel/hazptr.c                       | 135 +++++++++++++++-----------
>  4 files changed, 203 insertions(+), 80 deletions(-)
> 
> -- 
> 2.43.0
>