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 16:47:24 +0800

On 07/02/2013 04:25 PM, Srivatsa S. Bhat wrote:
> Hi Michael,
> 
> On 07/02/2013 11:02 AM, Michael Wang wrote:
>> 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?
>>
> 
> Whether or not the newly onlined CPU is observed in our for_each_online_cpu()
> loop, is dependent on timing. On top of that, there are 2 paths in the code:
> one which uses a temporary cpumask and the other which doesn't. In the former
> case, if a CPU comes online _after_ we populate the temporary cpumask, then
> we won't send an IPI to that cpu, since the temporary cpumask doesn't contain
> that CPU. Whereas, if we observe the newly onlined CPU in the for_each_online_cpu()
> loop itself (either in the former or the latter case), then yes, we will send
> the IPI to that CPU.

So it is not 'during the call' but 'during the call of
on_each_cpu_mask()', correct?

The comment position seems like it declaim that during the call of this
func, online-cpu won't be seem and send IPI...

Regards,
Michael Wang

> 
> Regards,
> Srivatsa S. Bhat
> 
>>
>>>   *
>>>   * 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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