linux-kernel - Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

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Date:	Wed, 5 Dec 2012 20:07:03 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
	amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light"
	atomic readers to prevent CPU offline

I'll try to read this series later,

one minor and almost offtopic nit.

On 12/06, Srivatsa S. Bhat wrote:
>
>  static int __ref take_cpu_down(void *_param)
>  {
>  	struct take_cpu_down_param *param = _param;
> +	unsigned long flags;
>  	int err;
>
> +	/*
> +	 *  __cpu_disable() is the step where the CPU is removed from the
> +	 *  cpu_online_mask. Protect it with the light-lock held for write.
> +	 */
> +	write_lock_irqsave(&light_hotplug_rwlock, flags);
> +
>  	/* Ensure this CPU doesn't handle any more interrupts. */
>  	err = __cpu_disable();
> -	if (err < 0)
> +	if (err < 0) {
> +		write_unlock_irqrestore(&light_hotplug_rwlock, flags);
>  		return err;
> +	}
> +
> +	/*
> +	 * We have successfully removed the CPU from the cpu_online_mask.
> +	 * So release the light-lock, so that the light-weight atomic readers
> +	 * (who care only about the cpu_online_mask updates, and not really
> +	 * about the actual cpu-take-down operation) can continue.
> +	 *
> +	 * But don't enable interrupts yet, because we still have work left to
> +	 * do, to actually bring the CPU down.
> +	 */
> +	write_unlock(&light_hotplug_rwlock);
>
>  	cpu_notify(CPU_DYING | param->mod, param->hcpu);
> +
> +	local_irq_restore(flags);
>  	return 0;

This is subjective, but imho _irqsave and the fat comment look confusing.

Currently take_cpu_down() is always called with irqs disabled, so you
do not need to play with interrupts.

10/10 does s/__stop_machine/stop_cpus/ and that patch could simply add
local_irq_disable/enable into take_cpu_down().

But again this is minor and subjective, I won't insist.

Oleg.

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