From 77668c8b559e4fe2acf2a0749c7c83cde49a5025 Mon Sep 17 00:00:00 2001
From: Lai Jiangshan <laijs@cn.fujitsu.com>
Date: Fri, 18 Apr 2014 11:04:16 -0400
Subject: [PATCH] workqueue: fix a possible race condition between rescuer and
 pwq-release

There is a race condition between rescuer_thread() and
pwq_unbound_release_workfn().

Even after a pwq is scheduled for rescue, the associated work items
may be consumed by any worker.  If all of them are consumed before the
rescuer gets to them and the pwq's base ref was put due to attribute
change, the pwq may be released while still being linked on
@wq->maydays list making the rescuer dereference already freed pwq
later.

Make send_mayday() pin the target pwq until the rescuer is done with
it.

tj: Updated comment and patch description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # v3.10+
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