From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Jonas Oberhauser <jonas.oberhauser@huawei.com>,
Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
Boqun Feng <boqun.feng@gmail.com>,
"Paul E. McKenney" <paulmck@kernel.org>
Cc: "lkmm@lists.linux.dev" <lkmm@lists.linux.dev>,
Olivier Dion <odion@efficios.com>
Subject: Re: [RFC PATCH 1/1] hpref: Hazard Pointers with Reference Counter
Date: Wed, 7 Jan 2026 09:40:56 -0500 [thread overview]
Message-ID: <9b839ad6-eeb4-4b95-b471-ff1058e1ecd8@efficios.com> (raw)
In-Reply-To: <c7464e0027f9403191d0d3638558bccd@huawei.com>
On 2026-01-07 04:38, Jonas Oberhauser wrote:
>
>> liburcu uatomics and "cmm_*" API initially aimed to replicate the LKMM in userspace (note: this was done circa 2009). However, starting from liburcu 0.15, we extended the uatomics API to support the C11 model
>
> I expect the C11-style-semantics will have a lot of friction with RCU (at the least because of missing dependency ordering).
* RCU publication guarantee:
C11 has the "consume" MO (which we call CMM_CONSUME in liburcu) for the
dependency required by rcu_dereference. AFAIK it is implemented as the
stronger "acquire" MO on most toolchains today though. For those who
really want to keep using a volatile access for rcu_dereference in
liburcu for performance reasons, we introduced the
URCU_DEREFERENCE_USE_VOLATILE define in liburcu. This pairs with a C11
store-release on rcu_assign_pointer.
Note that the CONSUME MO with store-release pairing is required to
let threadsanitizer know about RCU publication guarantee ordering.
* RCU grace period guarantee:
For the read-side critical section (rcu_read_{lock,unlock}) pairing with
synchronize_rcu, we've updated the liburcu read lock/unlock
implementation to use either atomic thread fence as full barrier or
SEQ_CST stores. We've also introduced a urcu/annotate.h header to help
annotating memory access groups. This helps threadsanitizer track the
dependency ordering inherent to RCU grace periods.
> Things like rmb and wmb may also not have a clean formalization in a C11-style semantics.
Right. We've been moving away from cmm_smp_rmb() and cmm_smp_wmb() in
favor of explicit C11 acquire/release dependency chains within the
liburcu implementation as of liburcu 0.15. This facilitates integration
with tools like threadsanitizer. The rmb/wmb APIs are still there for
legacy reasons, but users are encouraged to move to a acquire/release
model when it makes sense.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
next prev parent reply other threads:[~2026-01-07 14:41 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-09-21 16:42 [RFC PATCH 1/1] hpref: Hazard Pointers with Reference Counter Mathieu Desnoyers
2024-09-21 21:07 ` Lai Jiangshan
2024-09-22 7:47 ` Mathieu Desnoyers
2024-09-25 5:57 ` Jonas Oberhauser
2024-09-25 6:35 ` Mathieu Desnoyers
2024-09-25 10:06 ` Jonas Oberhauser
2024-09-25 11:36 ` Mathieu Desnoyers
2024-09-25 12:02 ` Jonas Oberhauser
2024-09-28 11:22 ` Jonas Oberhauser
2024-09-28 11:33 ` Mathieu Desnoyers
2024-09-28 11:50 ` Jonas Oberhauser
2024-09-30 13:31 ` Jonas Oberhauser
2024-10-01 15:24 ` Paul E. McKenney
2026-01-06 14:52 ` Mathieu Desnoyers
2026-01-07 9:38 ` Jonas Oberhauser
2026-01-07 14:40 ` Mathieu Desnoyers [this message]
2026-01-07 17:48 ` Paul E. McKenney
2026-01-07 20:04 ` Mathieu Desnoyers
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