Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

From: Sebastian Andrzej Siewior
Date: Fri Jun 28 2019 - 13:45:55 EST

On 2019-06-28 10:30:11 [-0700], Paul E. McKenney wrote:
> > I believe the .blocked field remains set even though we are not any more in a
> > reader section because of deferred processing of the blocked lists that you
> > mentioned yesterday.
> That can indeed happen. However, in current -rcu, that would mean
> that .deferred_qs is also set, which (if in_irq()) would prevent
> the raise_softirq_irqsoff() from being invoked. Which was why I was
> asking the questions about whether in_irq() returns true within threaded
> interrupts yesterday. If it does, I need to find if there is some way
> of determining whether rcu_read_unlock_special() is being called from
> a threaded interrupt in order to suppress the call to raise_softirq()
> in that case.

Please not that:
| void irq_exit(void)
| {

in_irq() returns true
| preempt_count_sub(HARDIRQ_OFFSET);
in_irq() returns false
| if (!in_interrupt() && local_softirq_pending())
| invoke_softirq();

-> invoke_softirq() does
| if (!force_irqthreads) {
| __do_softirq();
| } else {
| wakeup_softirqd();
| }

so for `force_irqthreads' rcu_read_unlock_special() within
wakeup_softirqd() will see false.

> Thanx, Paul