Re: [PATCH v3 1/9] smp: Run functions concurrently in smp_call_function_many()

[Thomas Gleixner]

On Mon, 22 Jul 2019, Nadav Amit wrote:
> > On Jul 22, 2019, at 11:51 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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);
> 
> â 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.

Thanks,

	tglx