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

Paul E. McKenney
2019-06-28T20:07:06Z

On Fri, Jun 28, 2019 at 03:29:23PM -0400, Joel Fernandes wrote:
> 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.

I do have an expanded version in the commit log. I hope to get a more
high-level description in comments.

Thanx, Paul

> 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);
> >
>