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Message-ID: <1326380884.2442.187.camel@twins>
Subject: Re: [PATCH 2/2] mm: page allocator: Do not drain per-cpu lists via
 IPI from page allocator context
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: Mel Gorman <mgorman@suse.de>, Linux-MM <linux-mm@kvack.org>,
        Linux-FSDevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Miklos Szeredi <mszeredi@novell.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>, Greg KH <gregkh@suse.de>,
        Gong Chen <gong.chen@intel.com>
Date: Thu, 12 Jan 2012 16:08:04 +0100
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On Thu, 2012-01-12 at 16:51 +0200, Gilad Ben-Yossef wrote:
> What I can't figure out is why we don't need  get/put_online_cpus()
> pair around each and every call
> to on_each_cpu everywhere? and if we do, perhaps making it a part of
> on_each_cpu is the way to go?
> 
> Something like:
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index f66a1b2..cfa3882 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -691,11 +691,15 @@ void on_each_cpu(void (*func) (void *info), void
> *info, int wait)
>  {
>         unsigned long flags;
> 
> +       BUG_ON(in_atomic());
> +
> +       get_online_cpus();
>         preempt_disable();

Your preempt_disable() here serializes against hotplug..

>         smp_call_function(func, info, wait);
>         local_irq_save(flags);
>         func(info);
>         local_irq_restore(flags);
>         preempt_enable();
> +       put_online_cpus();
>  }
>  EXPORT_SYMBOL(on_each_cpu);