linux-kernel - Re: [PATCH 2/3] futex: Use RCU-based per-CPU reference counting instead of rcuref

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Message-ID: <20250708134708.Rgh8nHcx@linutronix.de>
Date: Tue, 8 Jul 2025 15:47:08 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Waiman Long <llong@...hat.com>
Cc: linux-kernel@...r.kernel.org,
	André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH 2/3] futex: Use RCU-based per-CPU reference counting
 instead of rcuref_t

On 2025-07-08 09:43:44 [-0400], Waiman Long wrote:
> This looks somewhat like what the percpu refcount does (see
> lib/percpu-refcount.c). Could this be used instead of reinventing the wheel
> again?

>From the comment:

  * futex-ref
  *
  * Heavily inspired by percpu-rwsem/percpu-refcount; not reusing any of that
  * code because it just doesn't fit right.
  *
  * Dual counter, per-cpu / atomic approach like percpu-refcount, except it
  * re-initializes the state automatically, such that the fph swizzle is also a
  * transition back to per-cpu.

but I leave it up to Peter if he considers merging that.

> Cheers,
> Longman

Sebastian