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Date: Thu, 16 Jun 2016 15:39:05 -0400
From: Tejun Heo <htejun@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
 Gautham R Shenoy <ego@linux.vnet.ibm.com>,
 Thomas Gleixner <tglx@linutronix.de>,
 Abdul Haleem <abdhalee@linux.vnet.ibm.com>,
 Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>,
 linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] workqueue:Fix affinity of an unbound worker of a
 node with 1 online CPU
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On Thu, Jun 16, 2016 at 02:45:48PM +0200, Peter Zijlstra wrote:
> Subject: workqueue: Fix setting affinity of unbound worker threads
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Thu Jun 16 14:38:42 CEST 2016
> 
> With commit e9d867a67fd03ccc ("sched: Allow per-cpu kernel threads to
> run on online && !active"), __set_cpus_allowed_ptr() expects that only
> strict per-cpu kernel threads can have affinity to an online CPU which
> is not yet active.
> 
> This assumption is currently broken in the CPU_ONLINE notification
> handler for the workqueues where restore_unbound_workers_cpumask()
> calls set_cpus_allowed_ptr() when the first cpu in the unbound
> worker's pool->attr->cpumask comes online. Since
> set_cpus_allowed_ptr() is called with pool->attr->cpumask in which
> only one CPU is online which is not yet active, we get the following
> WARN_ON during an CPU online operation.

Applied to wq/for-4.7-fixes.

Thanks.

-- 
tejun