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

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Message-ID: <20250708085640.2918-1-hdanton@sina.com>
Date: Tue,  8 Jul 2025 16:56:39 +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 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.

> +	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);
> +}