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Date: Fri, 14 Apr 2017 11:56:55 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Sebastian Siewior <bigeasy@linutronix.de>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        "David S. Miller" <davem@davemloft.net>,
        Fenghua Yu <fenghua.yu@intel.com>,
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        Lai Jiangshan <jiangshanlai@gmail.com>, Len Brown <lenb@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>, Tejun Heo <tj@kernel.org>,
        Tony Luck <tony.luck@intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>
Subject: Re: [patch 02/13] workqueue: Provide work_on_cpu_safe()
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On Fri, Apr 14, 2017 at 11:51:07AM +0200, Thomas Gleixner wrote:
> On Fri, 14 Apr 2017, Peter Zijlstra wrote:
> 
> > On Wed, Apr 12, 2017 at 10:07:28PM +0200, Thomas Gleixner wrote:
> > > +long work_on_cpu_safe(int cpu, long (*fn)(void *), void *arg)
> > > +{
> > > +	long ret = -ENODEV;
> > > +
> > > +	get_online_cpus();
> > > +	if (cpu_online(cpu))
> > > +		ret = work_on_cpu(cpu, fn, arg);
> > > +	put_online_cpus();
> > > +	return ret;
> > > +}
> > 
> > But doesn't workqueue have this lovelt 'feature' where it will unbind
> > per-cpu work and run it on random CPUs when hotplug happens?
> > 
> > That is, I think you need a flush_work() before put_online_cpus() if you
> > want to guarantee anything.
> 
> work_on_cpu() is sychnronous, it flushes already.

So much for being awake ...