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

[Thomas Gleixner]

On Mon, 22 Jul 2019, Peter Zijlstra 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.

The current implementation has preemption disabled before touching
cpu_online_mask which at least protects against a CPU going away as that
prevents the stomp machine thread from getting on the CPU. But it's not
protected against a CPU coming online concurrently.

Thanks,

	tglx