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

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Date:   Mon, 22 Jul 2019 20:51:59 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Nadav Amit <namit@...are.com>
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 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,

	tglx