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

Joel Fernandes
2019-06-28T19:29:28Z

On Fri, Jun 28, 2019 at 11:22:16AM -0700, Paul E. McKenney wrote:
> 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?

This makes a lot of sense. It would be nice to stick these comments on top of
rcu_read_unlock_special() for future reference.

thanks,

- Joel

>
> 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);
>