linux-kernel - Re: [patch 02/13] workqueue: Provide work_on_cpu

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Message-ID: <20170414085459.sp3psluw647ab4lw@hirez.programming.kicks-ass.net>
Date:   Fri, 14 Apr 2017 10:54:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "David S. Miller" <davem@...emloft.net>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Len Brown <lenb@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, Tejun Heo <tj@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [patch 02/13] workqueue: Provide work_on_cpu_safe()

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.