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

[Byungchul Park]

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);
			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)) {