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

[Peter Zijlstra]

On Mon, Jul 22, 2019 at 06:34:22PM +0000, Nadav Amit wrote:
> > On Jul 22, 2019, at 11:21 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > 
> > On Thu, Jul 18, 2019 at 05:58:29PM -0700, Nadav Amit wrote:
> >> +/*
> >> + * Call a function on all processors.  May be used during early boot while
> >> + * early_boot_irqs_disabled is set.
> >> + */
> >> +static inline void on_each_cpu(smp_call_func_t func, void *info, int wait)
> >> +{
> >> +	on_each_cpu_mask(cpu_online_mask, func, info, wait);
> >> +}
> > 
> > I'm thinking that one if buggy, nothing protects online mask here.
> 
> on_each_cpu_mask() calls __on_each_cpu_mask() which would disable preemption.
> The mask might change, but anyhow __smp_call_function_many() would âandâ it,
> after disabling preemption, with (the potentially updated) cpu_online_mask.

Ah, indeed, as long as we double check the state after disabling
preemption things should be fine.

> What is your concern?

Pavlov reaction to seeing a naked cpu_online_mask :-)