linux-kernel - Re: [PATCH v3 10/45] smp: Use get/put_online_cpus

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Date:	Tue, 02 Jul 2013 13:32:58 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
CC:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	walken@...gle.com, vincent.guittot@...aro.org,
	laijs@...fujitsu.com, David.Laight@...lab.com, rostedt@...dmis.org,
	xiaoguangrong@...ux.vnet.ibm.com, sbw@....edu, fweisbec@...il.com,
	zhong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-arch@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Wang YanQing <udknight@...il.com>,
	Shaohua Li <shli@...ionio.com>,
	Jan Beulich <jbeulich@...e.com>,
	liguang <lig.fnst@...fujitsu.com>
Subject: Re: [PATCH v3 10/45] smp: Use get/put_online_cpus_atomic() to prevent
 CPU offline

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);
> 

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