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

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Message-ID: <20250708220033.2999-1-hdanton@sina.com>
Date: Wed,  9 Jul 2025 06:00:32 +0800
From: Hillf Danton <hdanton@...a.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH 2/3] futex: Use RCU-based per-CPU reference counting instead of rcuref_t

On Tue, 8 Jul 2025 15:15:58 +0200 Sebastian Andrzej Siewior wrote:
> On 2025-07-08 20:01:56 [+0800], Hillf Danton wrote:
> > On Tue, 8 Jul 2025 11:16:26 +0200 Sebastian Andrzej Siewior wrote:
> > > On 2025-07-08 16:56:39 [+0800], Hillf Danton wrote:
> > > > On Mon,  7 Jul 2025 16:36:22 +0200 Sebastian Andrzej Siewior wrote:
> > > > > +static bool futex_ref_get(struct futex_private_hash *fph)
> > > > > +{
> > > > > +	struct mm_struct *mm = fph->mm;
> > > > > +
> > > > > +	guard(rcu)();
> > > > > +
> > > > Like regular refcount_t, it is buggy to touch fph if futex_atomic drops
> > > > to 0. And more important guard(rcu) does not prevent it from dropping to 0.
> > > 
> > > What is your intention with this? There is an inc-if-not-zero to ensure
> > 
> > I am just simply wondering why get and put do not work without the rcu guard?
> 
> To ensure every get/ put user within this section observed the switch to
> atomics. There is this call-rcu callback which performs the switch. This
> one will be invoked after every user, that was user the per-CPU counter,
> is gone and using the atomic one.
> 
Then percpu refcount sounds like a better option because it is free at least.