linux-kernel - Re: [PATCH 1/7] stop_machine: Introduce stop_machine

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Message-ID: <20260127154948.GLaXjenHeJ7Ay6OFJ5@fat_crate.local>
Date: Tue, 27 Jan 2026 16:49:48 +0100
From: Borislav Petkov <bp@...en8.de>
To: "Chang S. Bae" <chang.seok.bae@...el.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, dave.hansen@...ux.intel.com, peterz@...radead.org,
	david.kaplan@....com
Subject: Re: [PATCH 1/7] stop_machine: Introduce stop_machine_nmi()

On Sun, Jan 25, 2026 at 01:42:16AM +0000, Chang S. Bae wrote:
> +/**
> + * stop_machine_nmi: freeze the machine and run this function in NMI context
> + * @fn: the function to run
> + * @data: the data ptr for the @fn()
> + * @cpus: the cpus to run the @fn() on (NULL = any online cpu)

s/cpu/CPU/g in all text.

> + *
> + * Like stop_machine() but runs the function in NMI context to avoid any risk of
> + * interruption due to NMIs.
> + *
> + * Protects against CPU hotplug.
> + */
> +int stop_machine_nmi(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus);
> +
> +/**
> + * stop_machine_cpuslocked_nmi: freeze and run this function in NMI context
> + * @fn: the function to run
> + * @data: the data ptr for the @fn()
> + * @cpus: the cpus to run the @fn() on (NULL = any online cpu)
> + *
> + * Same as above. Must be called from within a cpus_read_lock() protected
> + * region. Avoids nested calls to cpus_read_lock().
> + */
> +int stop_machine_cpuslocked_nmi(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus);
>  /**
>   * stop_core_cpuslocked: - stop all threads on just one core
>   * @cpu: any cpu in the targeted core
> @@ -160,6 +183,14 @@ int stop_core_cpuslocked(unsigned int cpu, cpu_stop_fn_t fn, void *data);
>  
>  int stop_machine_from_inactive_cpu(cpu_stop_fn_t fn, void *data,
>  				   const struct cpumask *cpus);
> +
> +bool noinstr stop_machine_nmi_handler(void);
> +DECLARE_STATIC_KEY_FALSE(stop_machine_nmi_handler_enable);

Why is the static key in the header if you have an accessor below?

> +static __always_inline bool stop_machine_nmi_handler_enabled(void)
> +{
> +	return static_branch_unlikely(&stop_machine_nmi_handler_enable);
> +}

Just make the accessor the only thing that external code calls.

...

> +DEFINE_STATIC_KEY_FALSE(stop_machine_nmi_handler_enable);
> +static DEFINE_PER_CPU(struct stop_machine_nmi_ctrl, stop_machine_nmi_ctrl);
> +
> +static void enable_nmi_handler(struct multi_stop_data *msdata)
> +{
> +	this_cpu_write(stop_machine_nmi_ctrl.msdata, msdata);
> +	this_cpu_write(stop_machine_nmi_ctrl.nmi_enabled, true);
> +}

Why do we have to enable the NMI handler?

> +void __weak arch_send_self_nmi(void)
> +{
> +	/* Arch code must implement this to support stop_machine_nmi() */
> +}

-- 
Regards/Gruss,
    Boris.

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