rcutree/rcu_bh_qs: disable irq while calling rcu_preempt_qs()

rcutree/rcu_bh_qs: disable irq while calling rcu_preempt_qs()

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.

Cc: stable-rt@vger.kernel.org
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>