3.18.47-rt52-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

The RCU sched protection was changed to RCU only and so all IRQ-off and
preempt-off disabled region were changed to the relevant rcu-read-lock
primitives. One was missed and triggered:
|[ BUG: bad unlock balance detected! ]
|4.4.30-rt41 #51 Tainted: G        W
|btattach/345 is trying to release lock (
|Unable to handle kernel paging request at virtual address 6b6b6bbb
|Backtrace:
|[<c016b5a0>] (lock_release) from [<c0804844>] (rt_spin_unlock+0x20/0x30)
|[<c0804824>] (rt_spin_unlock) from [<c0138954>] (put_pwq_unlocked+0xa4/0x118)
|[<c01388b0>] (put_pwq_unlocked) from [<c0138b2c>] (destroy_workqueue+0x164/0x1b0)
|[<c01389c8>] (destroy_workqueue) from [<c078e1ac>] (hci_unregister_dev+0x120/0x21c)
|[<c078e08c>] (hci_unregister_dev) from [<c054f658>] (hci_uart_tty_close+0x90/0xbc)
|[<c054f5c8>] (hci_uart_tty_close) from [<c03a2be8>] (tty_ldisc_close+0x50/0x58)
|[<c03a2b98>] (tty_ldisc_close) from [<c03a2cb4>] (tty_ldisc_kill+0x18/0x78)
|[<c03a2c9c>] (tty_ldisc_kill) from [<c03a3528>] (tty_ldisc_release+0x100/0x134)
|[<c03a3428>] (tty_ldisc_release) from [<c039cd68>] (tty_release+0x3bc/0x460)
|[<c039c9ac>] (tty_release) from [<c020cc08>] (__fput+0xe0/0x1b4)
|[<c020cb28>] (__fput) from [<c020cd3c>] (____fput+0x10/0x14)
|[<c020cd2c>] (____fput) from [<c013e0d4>] (task_work_run+0xa4/0xb8)
|[<c013e030>] (task_work_run) from [<c0121754>] (do_exit+0x40c/0x8b0)
|[<c0121348>] (do_exit) from [<c0122ff8>] (do_group_exit+0x54/0xc4)

Cc: stable-rt@vger.kernel.org
Reported-by: John Keeping <john@metanate.com>
Tested-by: John Keeping <john@metanate.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 kernel/workqueue.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 6122f386dd5d..8af30f579c15 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1109,9 +1109,11 @@ static void put_pwq_unlocked(struct pool_workqueue *pwq)
 		 * As both pwqs and pools are RCU protected, the
 		 * following lock operations are safe.
 		 */
+		rcu_read_lock();
 		local_spin_lock_irq(pendingb_lock, &pwq->pool->lock);
 		put_pwq(pwq);
 		local_spin_unlock_irq(pendingb_lock, &pwq->pool->lock);
+		rcu_read_unlock();
 	}
 }
 
-- 
2.10.2