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SubjectRe: [PATCH v3 1/9] smp: Run functions concurrently in smp_call_function_many()
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> On Jul 25, 2019, at 5:36 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Mon, 22 Jul 2019, Nadav Amit wrote:
>>> On Jul 22, 2019, at 11:51 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>>> void on_each_cpu(void (*func) (void *info), void *info, int wait)
>>> {
>>> unsigned long flags;
>>>
>>> preempt_disable();
>>> smp_call_function(func, info, wait);
>>>
>>> smp_call_function() has another preempt_disable as it can be called
>>> separately and it does:
>>>
>>> preempt_disable();
>>> smp_call_function_many(cpu_online_mask, func, info, wait);
>>>
>>> Your new on_each_cpu() implementation does not. So there is a
>>> difference. Whether it matters or not is a different question, but that
>>> needs to be explained and documented.
>>
>> Thanks for explaining - so your concern is for CPUs being offlined.
>>
>> But unless I am missing something: on_each_cpu() calls __on_each_cpu_mask(),
>> which disables preemption and calls __smp_call_function_many().
>>
>> Then __smp_call_function_many() runs:
>>
>> cpumask_and(cfd->cpumask, mask, cpu_online_mask);
>>
>> … before choosing which remote CPUs should run the function. So the only
>> case that I was missing is if the current CPU goes away and the function is
>> called locally.
>>
>> Can it happen? I can add documentation and a debug assertion for this case.
>
> I don't think it can happen:
>
> on_each_cpu()
> on_each_cpu_mask(....)
> preempt_disable()
> __smp_call_function_many()
>
> So if a CPU goes offline between on_each_cpu() and preempt_disable() then
> there is no damage. After the preempt_disable() it can't go away anymore
> and the task executing this cannot be migrated either.
>
> So yes, it's safe, but please add a big fat comment so future readers won't
> be puzzled.

I will do. I will need some more time to respin the next version. I see that
what I build on top of it might require some changes, and I want to minimize
them.

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