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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-09-27 12:15, Boqun Feng wrote:
> 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!

Noted, I've added them to my patch commit messages, so they should
be there for the next round.

> 
>> 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.

Sure, if you have a use-case in mind I'm interested to hear about it!

Thanks,

Mathieu

> 
> 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
>>


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com