linux-kernel - Re: [PATCH v8 03/15] futex: Add basic infrastructure for local task local hash.

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Message-ID: <20250203142923.GJ7145@noisy.programming.kicks-ass.net>
Date: Mon, 3 Feb 2025 15:29:23 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v8 03/15] futex: Add basic infrastructure for local task
 local hash.

On Mon, Feb 03, 2025 at 02:59:23PM +0100, Sebastian Andrzej Siewior wrote:
> +static int futex_hash_allocate(unsigned int hash_slots)
> +{
> +	struct futex_hash_bucket *fhb;
> +	int i;
> +
> +	if (current->mm->futex_hash_bucket)
> +		return -EALREADY;
> +
> +	if (!thread_group_leader(current))
> +		return -EINVAL;
> +
> +	if (hash_slots == 0)
> +		hash_slots = 16;
> +	if (hash_slots < 2)
> +		hash_slots = 2;
> +	if (hash_slots > 131072)
> +		hash_slots = 131072;
> +	if (!is_power_of_2(hash_slots))
> +		hash_slots = rounddown_pow_of_two(hash_slots);

Why not require the argument is 2^n and fail otherwise?