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

Byungchul Park
2019-06-28T10:41:32Z

On Fri, Jun 28, 2019 at 04:31:38PM +0900, Byungchul Park wrote:
> On Thu, Jun 27, 2019 at 01:36:12PM -0700, Paul E. McKenney wrote:
> > On Thu, Jun 27, 2019 at 03:17:27PM -0500, Scott Wood wrote:
> > > On Thu, 2019-06-27 at 11:41 -0700, Paul E. McKenney wrote:
> > > > On Thu, Jun 27, 2019 at 02:16:38PM -0400, Joel Fernandes wrote:
> > > > >
> > > > > I think the fix should be to prevent the wake-up not based on whether we
> > > > > are
> > > > > in hard/soft-interrupt mode but that we are doing the rcu_read_unlock()
> > > > > from
> > > > > a scheduler path (if we can detect that)
> > > >
> > > > Or just don't do the wakeup at all, if it comes to that. I don't know
> > > > of any way to determine whether rcu_read_unlock() is being called from
> > > > the scheduler, but it has been some time since I asked Peter Zijlstra
> > > > about that.
> > > >
> > > > Of course, unconditionally refusing to do the wakeup might not be happy
> > > > thing for NO_HZ_FULL kernels that don't implement IRQ work.
> > >
> > > Couldn't smp_send_reschedule() be used instead?
> >
> > Good point. If current -rcu doesn't fix things for Sebastian's case,
> > that would be well worth looking at. But there must be some reason
> > why Peter Zijlstra didn't suggest it when he instead suggested using
> > the IRQ work approach.
> >
> > Peter, thoughts?
>
> Hello,
>
> Isn't the following scenario possible?
>
> The original code
> -----------------
> rcu_read_lock();
> ...
> /* Experdite */
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> ...
> __rcu_read_unlock();
> if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> rcu_read_unlock_special(t);
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> rcu_preempt_deferred_qs_irqrestore(t, flags);
> barrier(); /* ->rcu_read_unlock_special load before assign */
> t->rcu_read_lock_nesting = 0;
>
> The reordered code by machine
> -----------------------------
> rcu_read_lock();
> ...
> /* Experdite */
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> ...
> __rcu_read_unlock();
> if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> rcu_read_unlock_special(t);
> t->rcu_read_lock_nesting = 0; <--- LOOK AT THIS!!!
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> rcu_preempt_deferred_qs_irqrestore(t, flags);
> barrier(); /* ->rcu_read_unlock_special load before assign */
>
> An interrupt happens
> --------------------
> rcu_read_lock();
> ...
> /* Experdite */
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> ...
> __rcu_read_unlock();
> if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> rcu_read_unlock_special(t);
> t->rcu_read_lock_nesting = 0; <--- LOOK AT THIS!!!
> <--- Handle an (any) irq
> rcu_read_lock();
> /* This call should be skipped */
> rcu_read_unlock_special(t);

Wait.. I got a little bit confused on recordering.

This 'STORE rcu_read_lock_nesting = 0' can happen before
'STORE rcu_read_unlock_special.b.exp_hint = false' regardless of the
order a compiler generated to by the barrier(), because anyway they
are independent so it's within an arch's right.

Then.. is this scenario possible? Or all archs properly deal with
interrupts across this kind of reordering?

Thanks,
Byungchul

> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> rcu_preempt_deferred_qs_irqrestore(t, flags);
> barrier(); /* ->rcu_read_unlock_special load before assign */
>
> We don't have to handle the special thing twice like this which is one
> reason to cause the problem even though another problem is of course to
> call ttwu w/o being aware it's within a context holding pi lock.
>
> Apart from the discussion about how to avoid ttwu in an improper
> condition, I think the following is necessary. I may have something
> missing. It would be appreciated if you let me know in case I'm wrong.
>
> Anyway, logically I think we should prevent reordering between
> t->rcu_read_lock_nesting and t->rcu_read_unlock_special.b.exp_hint not
> only by compiler but also by machine like the below.
>
> Do I miss something?
>
> Thanks,
> Byungchul
>
> ---8<---
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 3c8444e..9b137f1 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -412,7 +412,13 @@ void __rcu_read_unlock(void)
> barrier(); /* assign before ->rcu_read_unlock_special load */
> if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> rcu_read_unlock_special(t);
> - barrier(); /* ->rcu_read_unlock_special load before assign */
> + /*
> + * Prevent reordering between clearing
> + * t->rcu_reak_unlock_special in
> + * rcu_read_unlock_special() and the following
> + * assignment to t->rcu_read_lock_nesting.
> + */
> + smp_wmb();
> t->rcu_read_lock_nesting = 0;
> }
> if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
>
>