[v2 PATCH] rcutree/rcu_bh_qs: disable irq while calling rcu_preempt_qs()

[Tiejun Chen <tiejun.chen@windriver.com>]

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>
---
 kernel/rcutree.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 7ec834d..6f6d133 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -186,7 +186,12 @@ static void rcu_preempt_qs(int cpu);
 
 void rcu_bh_qs(int cpu)
 {
+	unsigned long flags;
+
+	/* Callers to this function, rcu_preempt_qs(), must disable irqs. */
+	local_irq_save(flags);
 	rcu_preempt_qs(cpu);
+	local_irq_restore(flags);
 }
 #else
 void rcu_bh_qs(int cpu)
-- 
1.7.9.5