From bc0caf099d9df4dd0fad24992b043b40541f4200 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Mon, 1 Apr 2013 11:23:31 -0700
Subject: [PATCH] workqueue: fix race condition in unbound workqueue free path

8864b4e59 ("workqueue: implement get/put_pwq()") implemented pwq
(pool_workqueue) refcnting which frees workqueue when the last pwq
goes away.  It determined whether it was the last pwq by testing
wq->pwqs is empty.  Unfortunately, the test was done outside wq->mutex
and multiple pwq release could race and try to free wq multiple times
leading to oops.

Test wq->pwqs emptiness while holding wq->mutex.

Signed-off-by: Tejun Heo <tj@kernel.org>
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