From: Tiejun Chen <tiejun.chen@windriver.com>
Date: Wed, 18 Dec 2013 17:51:49 +0800
Subject: rcutree/rcu_bh_qs: Disable irq while calling rcu_preempt_qs()
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.6/older/patches-4.6.2-rt5.tar.xz

Any callers to the function rcu_preempt_qs() must disable irqs in
order to protect the assignment to ->rcu_read_unlock_special. In
RT case, rcu_bh_qs() as the wrapper of rcu_preempt_qs() is called
in some scenarios where irq is enabled, like this path,

do_single_softirq()
    |
    + local_irq_enable();
    + handle_softirq()
    |    |
    |    + rcu_bh_qs()
    |        |
    |        + rcu_preempt_qs()
    |
    + local_irq_disable()

So here we'd better disable irq directly inside of rcu_bh_qs() to
fix this, otherwise the kernel may be freezable sometimes as
observed. And especially this way is also kind and safe for the
potential rcu_bh_qs() usage elsewhere in the future.


Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>
Signed-off-by: Bin Jiang <bin.jiang@windriver.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/rcu/tree.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -259,7 +259,12 @@ static void rcu_preempt_qs(void);
 
 void rcu_bh_qs(void)
 {
+	unsigned long flags;
+
+	/* Callers to this function, rcu_preempt_qs(), must disable irqs. */
+	local_irq_save(flags);
 	rcu_preempt_qs();
+	local_irq_restore(flags);
 }
 #else
 void rcu_bh_qs(void)
