linux-kernel - Re: [PATCH] smp: simplify smp_call_function

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Message-ID: <1a0b35d0-d739-4f1c-9a50-95780eed02e4@efficios.com>
Date: Wed, 8 Oct 2025 13:06:18 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: "Yury Norov (NVIDIA)" <yury.norov@...il.com>,
 Thomas Gleixner <tglx@...utronix.de>, "Paul E. McKenney"
 <paulmck@...nel.org>, Rik van Riel <riel@...riel.com>,
 Ingo Molnar <mingo@...nel.org>, Joel Fernandes <joelagnelf@...dia.com>,
 Roman Kisel <romank@...ux.microsoft.com>,
 "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] smp: simplify smp_call_function_any()

On 2025-10-08 12:57, Yury Norov (NVIDIA) wrote:
> The functions calls get_cpu()/put_cpu() meaningless because the actual
> CPU that would execute the caller's function is not necessarily the
> current one.
> 
> The smp_call_function_single() which is called by
> smp_call_function_any() does the right get/put protection.
> 
> Signed-off-by: Yury Norov (NVIDIA) <yury.norov@...il.com>
> ---
>   kernel/smp.c | 8 ++------
>   1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 02f52291fae4..fa50ed459703 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -754,17 +754,13 @@ EXPORT_SYMBOL_GPL(smp_call_function_single_async);
>   int smp_call_function_any(const struct cpumask *mask,
>   			  smp_call_func_t func, void *info, int wait)
>   {
> -	unsigned int cpu;
> -	int ret;
> +	unsigned int cpu = smp_processor_id();

I wonder whether this passes any moderate testing with kernel debug
options enabled. I would at the very least expect a
raw_smp_processor_id() call here not to trip debug warnings.

AFAIU smp_call_function_any call be called from preemptible context,
right ?

Thanks,

Mathieu

>   
>   	/* Try for same CPU (cheapest) */
> -	cpu = get_cpu();
>   	if (!cpumask_test_cpu(cpu, mask))
>   		cpu = sched_numa_find_nth_cpu(mask, 0, cpu_to_node(cpu));
>   
> -	ret = smp_call_function_single(cpu, func, info, wait);
> -	put_cpu();
> -	return ret;
> +	return smp_call_function_single(cpu, func, info, wait);
>   }
>   EXPORT_SYMBOL_GPL(smp_call_function_any);
>   

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com