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

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Message-ID: <20250708091626.BYR6lG2J@linutronix.de>
Date: Tue, 8 Jul 2025 11:16:26 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Hillf Danton <hdanton@...a.com>
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 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
this does not happen. And it has to drop to zero in order to get
replaced.

> > +	if (smp_load_acquire(&fph->state) == FR_PERCPU) {
> > +		this_cpu_inc(*mm->futex_ref);
> > +		return true;
> > +	}
> > +
> > +	return atomic_long_inc_not_zero(&mm->futex_atomic);
> > +}
> > +
> > +static bool futex_ref_put(struct futex_private_hash *fph)
> > +{
> > +	struct mm_struct *mm = fph->mm;
> > +
> > +	guard(rcu)();
> > +
> > +	if (smp_load_acquire(&fph->state) == FR_PERCPU) {
> > +		this_cpu_dec(*mm->futex_ref);
> > +		return false;
> > +	}
> > +
> > +	return atomic_long_dec_and_test(&mm->futex_atomic);
> > +}

Sebastian