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

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Message-ID: <C93C241E-E078-4825-8C59-39C84D30F9CB@vmware.com>
Date:   Mon, 22 Jul 2019 18:34:22 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     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>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 11:21 AM, Peter Zijlstra <peterz@...radead.org> 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 “and” it,
after disabling preemption, with (the potentially updated) cpu_online_mask.

What is your concern?