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