From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 5 Feb 2016 12:17:14 +0100
Subject: mm: backing-dev: don't disable IRQs in wb_congested_put()
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.6/older/patches-4.6.2-rt5.tar.xz

it triggers:
|BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:930
|in_atomic(): 0, irqs_disabled(): 1, pid: 12, name: rcuc/0
|1 lock held by rcuc/0/12:
| #0:  (rcu_callback){......}, at: [<ffffffff810ce1a6>] rcu_cpu_kthread+0x376/0xb10
|irq event stamp: 23636
|hardirqs last  enabled at (23635): [<ffffffff8173524c>] _raw_spin_unlock_irqrestore+0x6c/0x80
|hardirqs last disabled at (23636): [<ffffffff81173918>] wb_congested_put+0x18/0x90
| [<ffffffff81735434>] rt_spin_lock+0x24/0x60
| [<ffffffff810afed2>] atomic_dec_and_spin_lock+0x52/0x90
| [<ffffffff81173928>] wb_congested_put+0x28/0x90
| [<ffffffff813b833e>] __blkg_release_rcu+0x5e/0x1e0
| [<ffffffff813b8367>] ? __blkg_release_rcu+0x87/0x1e0
| [<ffffffff813b82e0>] ? blkg_conf_finish+0x90/0x90
| [<ffffffff810ce1e7>] rcu_cpu_kthread+0x3b7/0xb10

due to cgwb_lock beeing taken with spin_lock_irqsave() usually.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 mm/backing-dev.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -457,9 +457,9 @@ void wb_congested_put(struct bdi_writeba
 {
 	unsigned long flags;
 
-	local_irq_save(flags);
+	local_irq_save_nort(flags);
 	if (!atomic_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
-		local_irq_restore(flags);
+		local_irq_restore_nort(flags);
 		return;
 	}
 
