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

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Date:   Mon, 22 Jul 2019 18:40:16 +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 11:37 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.