On Mon, Jul 22, 2019 at 06:41:44PM +0000, Nadav Amit wrote:
> > On Jul 22, 2019, at 11:16 AM, Peter Zijlstra <pet...@infradead.org> wrote:
> >
> > On Fri, Jul 19, 2019 at 11:23:06AM -0700, Dave Hansen wrote:
> >> On 7/18/19 5:58 PM, Nadav Amit wrote:
> >>> @@ -624,16 +622,11 @@ EXPORT_SYMBOL(on_each_cpu);
> >>> void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
> >>> void *info, bool wait)
> >>> {
> >>> - int cpu = get_cpu();
> >>> + preempt_disable();
> >>>
> >>> - smp_call_function_many(mask, func, info, wait);
> >>> - if (cpumask_test_cpu(cpu, mask)) {
> >>> - unsigned long flags;
> >>> - local_irq_save(flags);
> >>> - func(info);
> >>> - local_irq_restore(flags);
> >>> - }
> >>> - put_cpu();
> >>> + __smp_call_function_many(mask, func, func, info, wait);
> >>> +
> >>> + preempt_enable();
> >>> }
> >>
> >> The get_cpu() was missing it too, but it would be nice to add some
> >> comments about why preempt needs to be off. I was also thinking it
> >> might make sense to do:
> >>
> >> cfd = get_cpu_var(cfd_data);
> >> __smp_call_function_many(cfd, ...);
> >> put_cpu_var(cfd_data);
> >>
> >> instead of the explicit preempt_enable/disable(), but I don't feel too
> >> strongly about it.
> >
> > It is also required for cpu hotplug.
>
> But then smpcfd_dead_cpu() will not respect the “cpu” argument. Do you still
> prefer it this way (instead of the current preempt_enable() /
> preempt_disable())?
I just meant that the preempt_disable() (either form) is required for
hotplug (we must not send IPIs to offline CPUs, that gets things upset).
Personally I don't mind the bare preempt_disable() as you have; but I
think Dave's idea of a comment has merrit.
