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Date:   Mon, 22 Jul 2019 20:37:19 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     Nadav Amit <namit@vmware.com>, Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Rik van Riel <riel@surriel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH v3 1/9] smp: Run functions concurrently in
 smp_call_function_many()
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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.

Thanks,

	tglx