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Subject: Re: [PATCH v5 4/5] cpusets: Update tasks' cpus_allowed mask upon
 updates to root cpuset
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
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Date: Thu, 24 May 2012 11:13:47 +0200
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On Thu, 2012-05-17 at 22:33 +0530, Srivatsa S. Bhat wrote:
> +               /*
> +                * Restore only the top_cpuset because it has to track
> +                * cpu_active_mask always.
> +                * (We don't need to do anything if we come here during resume
> +                * from suspend, since top_cpuset.cpus_allowed will already be
> +                * equal to cpu_active_mask.)
> +                */
> +               if (root == &top_cpuset && !cpumask_equal(root->cpus_allowed,
> +                                                         cpu_active_mask)) {
> +                       mutex_lock(&callback_mutex);
> +                       cpumask_copy(root->cpus_allowed, cpu_active_mask);
> +                       mutex_unlock(&callback_mutex);
> +                       update_tasks_cpumask(root, NULL);
> +               } 

This looks absolutely broken.

Suppose I set an explicit cpu affinity mask on my task, which per not
using cpusets is in the root group.

If I then do a hotplug cycle, I'll find my affinity mask is lost.