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

Joel Fernandes
2019-06-27T15:30:37Z

On Thu, Jun 27, 2019 at 10:34:55AM -0400, Steven Rostedt wrote:
> On Thu, 27 Jun 2019 10:24:36 -0400
> Joel Fernandes <joel@joelfernandes.org> wrote:
>
> > > What am I missing here?
> >
> > This issue I think is
> >
> > (in normal process context)
> > spin_lock_irqsave(rq_lock); // which disables both preemption and interrupt
> > // but this was done in normal process context,
> > // not from IRQ handler
> > rcu_read_lock();
> > <---------- IPI comes in and sets exp_hint
>
> How would an IPI come in here with interrupts disabled?
>
> -- Steve

This is true, could it be rcu_read_unlock_special() got called for some
*other* reason other than the IPI then?

Per Sebastian's stack trace of the recursive lock scenario, it is happening
during cpu_acct_charge() which is called with the rq_lock held.

The only other reasons I know off to call rcu_read_unlock_special() are if
1. the tick indicated that the CPU has to report a QS
2. an IPI in the middle of the reader section for expedited GPs
3. preemption in the middle of a preemptible RCU reader section

1. and 2. are not possible because interrupts are disabled, that's why the
wakeup_softirq even happened.
3. is not possible because we are holding rq_lock in the RCU reader section.

So I am at a bit of a loss how this can happen :-(

Spurious call to rcu_read_unlock_special() may be when it should not have
been called?

thanks,

- Joel