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

[Nadav Amit]

> On Jul 22, 2019, at 11:37 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> 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.

I still donât understand. If you called cpu_online_mask() and did not
disable preemption before calling it, you are already (today) not protected
against another CPU coming online. Disabling preemption in on_each_cpu()
will not solve it.