Subject: Re: [PATCH v3 10/45] smp: Use get/put_online_cpus_atomic() to prevent CPU offline

From: Michael Wang <[email-masked]>
Date: Tue, 02 Jul 2013 13:32:58 +0800

Hi, Srivatsa

On 06/28/2013 03:54 AM, Srivatsa S. Bhat wrote:
[snip]
> @@ -625,8 +632,9 @@ EXPORT_SYMBOL(on_each_cpu_mask);
>   * The function might sleep if the GFP flags indicates a non
>   * atomic allocation is allowed.
>   *
> - * Preemption is disabled to protect against CPUs going offline but not online.
> - * CPUs going online during the call will not be seen or sent an IPI.
> + * We use get/put_online_cpus_atomic() to protect against CPUs going
> + * offline but not online. CPUs going online during the call will
> + * not be seen or sent an IPI.

I was a little confused about this comment, if the offline-cpu still
have chances to become online, then there is chances that we will pick
it from for_each_online_cpu(), isn't it? Did I miss some point?

Regards,
Michael Wang

>   *
>   * You must not call this function with disabled interrupts or
>   * from a hardware interrupt handler or from a bottom half handler.
> @@ -641,26 +649,26 @@ void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info),
>  	might_sleep_if(gfp_flags & __GFP_WAIT);
> 
>  	if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) {
> -		preempt_disable();
> +		get_online_cpus_atomic();
>  		for_each_online_cpu(cpu)
>  			if (cond_func(cpu, info))
>  				cpumask_set_cpu(cpu, cpus);
>  		on_each_cpu_mask(cpus, func, info, wait);
> -		preempt_enable();
> +		put_online_cpus_atomic();
>  		free_cpumask_var(cpus);
>  	} else {
>  		/*
>  		 * No free cpumask, bother. No matter, we'll
>  		 * just have to IPI them one by one.
>  		 */
> -		preempt_disable();
> +		get_online_cpus_atomic();
>  		for_each_online_cpu(cpu)
>  			if (cond_func(cpu, info)) {
>  				ret = smp_call_function_single(cpu, func,
>  								info, wait);
>  				WARN_ON_ONCE(!ret);
>  			}
> -		preempt_enable();
> +		put_online_cpus_atomic();
>  	}
>  }
>  EXPORT_SYMBOL(on_each_cpu_cond);
>