linux-kernel - Re: [PATCH v3 1/9] smp: Run functions concurrently in smp_call_function

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Date:   Mon, 22 Jul 2019 19:02:36 +0000
From:   Nadav Amit <namit@...are.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v3 1/9] smp: Run functions concurrently in
 smp_call_function_many()

> On Jul 22, 2019, at 11:51 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> On Mon, 22 Jul 2019, Nadav Amit wrote:
>>> On Jul 22, 2019, at 11:37 AM, Thomas Gleixner <tglx@...utronix.de> 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 don’t 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.
> 
> Disabling preemption _cannot_ protect against a CPU coming online. It only
> can protect against a CPU being offlined.
> 
> The current implementation of on_each_cpu() disables preemption _before_
> touching cpu_online_mask.
> 
> 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);

… 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.