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

Paul E. McKenney
2019-06-28T18:22:27Z

On Fri, Jun 28, 2019 at 07:45:45PM +0200, Sebastian Andrzej Siewior wrote:
> 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.

OK, fair point. How about the following instead, again on -rcu?

Here is the rationale for the new version of the "if" statement:

1. irqs_were_disabled: If interrupts are enabled, we should
instead let the upcoming irq_enable()/local_bh_enable()
do the rescheduling for us.
2. use_softirq: If we aren't using softirq, then
raise_softirq_irqoff() will be unhelpful.
3a. in_interrupt(): If this returns true, the subsequent
call to raise_softirq_irqoff() is guaranteed not to
do a wakeup, so that call will be both very cheap and
quite safe.
3b. Otherwise, if !in_interrupt(), if exp (an expedited RCU grace
period is being blocked), then incurring wakeup overhead
is worthwhile, and if also !.deferred_qs then scheduler locks
cannot be held so the wakeup will be safe.

Does that make more sense?

Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 82c925df1d92..83333cfe8707 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -624,8 +624,9 @@ static void rcu_read_unlock_special(struct task_struct *t)
(rdp->grpmask & rnp->expmask) ||
tick_nohz_full_cpu(rdp->cpu);
// Need to defer quiescent state until everything is enabled.
- if ((exp || in_irq()) && irqs_were_disabled && use_softirq &&
- (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) {
+ if (irqs_were_disabled && use_softirq &&
+ (in_interrupt() ||
+ (exp && !t->rcu_read_unlock_special.b.deferred_qs))) {
// Using softirq, safe to awaken, and we get
// no help from enabling irqs, unlike bh/preempt.
raise_softirq_irqoff(RCU_SOFTIRQ);