linux-kernel - [RFC PATCH v4 0/4] Hazard Pointers

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Message-Id: <20251218014531.3793471-1-mathieu.desnoyers@efficios.com>
Date: Wed, 17 Dec 2025 20:45:27 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	"Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Nicholas Piggin <npiggin@...il.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>,
	John Stultz <jstultz@...gle.com>,
	Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	maged.michael@...il.com,
	Mateusz Guzik <mjguzik@...il.com>,
	Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
	rcu@...r.kernel.org,
	linux-mm@...ck.org,
	lkmm@...ts.linux.dev
Subject: [RFC PATCH v4 0/4] Hazard Pointers

Hi,

Here is a revisited version of my Hazard Pointers series. Boqun, Joel,
if you guys have time to try it out with your use-cases it would be
great!

This new version does the following:

- It has 8 preallocated hazard pointer slots per CPU (one cache line),
- The hazard pointer user allocates a hazard pointer context variable
  (typically on the stack), which contains the pointer to the slot *and*
  a backup slot,
- When all the per-CPU slots are in use, fallback to the backup slot.
  Chain the backup slot into per-CPU lists, each protected by a raw
  spinlock.
- The hazard pointer synchronize does a piecewise iteration on the
  per-CPU overflow slots lists, releasing the raw spinlock between
  each list item. It uses a 64-bit generation counter to check for
  concurrent list changes, and restart the traversal on generation
  counter mismatch.
- There is a new CONFIG_PREEMPT_HAZPTR config option. When enabled,
  the hazard pointer acquire/release adds and then removes the hazard
  pointer context from a per-task linked list. On context switch, the
  scheduler migrates the per-CPU slots used by the task to the backup
  per-context slots, thus making sure the per-CPU slots are not used
  by preempted and blocked tasks.

It is based on v6.18.1.

Review is very welcome,

Thanks,

Mathieu

Cc: Nicholas Piggin <npiggin@...il.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Will Deacon <will@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: John Stultz <jstultz@...gle.com>
Cc: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Frederic Weisbecker <frederic@...nel.org>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Uladzislau Rezki <urezki@...il.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Zqiang <qiang.zhang1211@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Waiman Long <longman@...hat.com>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: maged.michael@...il.com
Cc: Mateusz Guzik <mjguzik@...il.com>
Cc: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: rcu@...r.kernel.org
Cc: linux-mm@...ck.org
Cc: lkmm@...ts.linux.dev

Mathieu Desnoyers (4):
  compiler.h: Introduce ptr_eq() to preserve address dependency
  Documentation: RCU: Refer to ptr_eq()
  hazptr: Implement Hazard Pointers
  hazptr: Migrate per-CPU slots to backup slot on context switch

 Documentation/RCU/rcu_dereference.rst |  38 +++-
 include/linux/compiler.h              |  63 +++++++
 include/linux/hazptr.h                | 241 ++++++++++++++++++++++++++
 include/linux/sched.h                 |   4 +
 init/init_task.c                      |   3 +
 init/main.c                           |   2 +
 kernel/Kconfig.preempt                |  10 ++
 kernel/Makefile                       |   2 +-
 kernel/fork.c                         |   3 +
 kernel/hazptr.c                       | 150 ++++++++++++++++
 kernel/sched/core.c                   |   2 +
 11 files changed, 512 insertions(+), 6 deletions(-)
 create mode 100644 include/linux/hazptr.h
 create mode 100644 kernel/hazptr.c

-- 
2.39.5