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

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Thu, Jun 27, 2019 at 11:11:12AM -0700, Paul E. McKenney wrote:
> On Thu, Jun 27, 2019 at 01:46:27PM -0400, Joel Fernandes wrote:
> > On Thu, Jun 27, 2019 at 1:43 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > >
> > > On Thu, Jun 27, 2019 at 11:40 AM Sebastian Andrzej Siewior
> > > <bigeasy@linutronix.de> wrote:
> > > >
> > > > On 2019-06-27 11:37:10 [-0400], Joel Fernandes wrote:
> > > > > Sebastian it would be nice if possible to trace where the
> > > > > t->rcu_read_unlock_special is set for this scenario of calling
> > > > > rcu_read_unlock_special, to give a clear idea about whether it was
> > > > > really because of an IPI. I guess we could also add additional RCU
> > > > > debug fields to task_struct (just for debugging) to see where there
> > > > > unlock_special is set.
> > > > >
> > > > > Is there a test to reproduce this, or do I just boot an intel x86_64
> > > > > machine with "threadirqs" and run into it?
> > > >
> > > > Do you want to send me a patch or should I send you my kvm image which
> > > > triggers the bug on boot?
> > >
> > > I could reproduce this as well just booting Linus tree with threadirqs
> > > command line and running rcutorture. In 15 seconds or so it locks
> > > up... gdb backtrace shows the recursive lock:
> > 
> > Sorry that got badly wrapped, so I pasted it here:
> > https://hastebin.com/ajivofomik.shell
> 
> Which rcutorture scenario would that be?  TREE03 is thus far refusing
> to fail for me when run this way:
> 
> $ tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 8 --duration 5 --trust-make --configs "TREE03" --bootargs "threadirqs"

Ah, but I was running -rcu.  TREE03 fails at 38 seconds for me on v5.2.

Now to find out which -rcu commit fixed it.  Or at least made it much
less probable, to Sebastian's point.

> If it had failed, I would have tried the patch shown below.  I know that
> Sebastian has some concerns about the bug happening anyway, but we have
> to start somewhere!  ;-)

This patch might thus be completely unnecessary.

							Thanx, Paul

> ------------------------------------------------------------------------
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 82c925df1d92..be7bafc2c0a0 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -624,25 +624,16 @@ 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)) {
> -			// Using softirq, safe to awaken, and we get
> -			// no help from enabling irqs, unlike bh/preempt.
> -			raise_softirq_irqoff(RCU_SOFTIRQ);
> -		} else {
> -			// Enabling BH or preempt does reschedule, so...
> -			// Also if no expediting or NO_HZ_FULL, slow is OK.
> -			set_tsk_need_resched(current);
> -			set_preempt_need_resched();
> -			if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
> -			    !rdp->defer_qs_iw_pending && exp) {
> -				// Get scheduler to re-evaluate and call hooks.
> -				// If !IRQ_WORK, FQS scan will eventually IPI.
> -				init_irq_work(&rdp->defer_qs_iw,
> -					      rcu_preempt_deferred_qs_handler);
> -				rdp->defer_qs_iw_pending = true;
> -				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
> -			}
> +		set_tsk_need_resched(current);
> +		set_preempt_need_resched();
> +		if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
> +		    !rdp->defer_qs_iw_pending && exp) {
> +			// Get scheduler to re-evaluate and call hooks.
> +			// If !IRQ_WORK, FQS scan will eventually IPI.
> +			init_irq_work(&rdp->defer_qs_iw,
> +				      rcu_preempt_deferred_qs_handler);
> +			rdp->defer_qs_iw_pending = true;
> +			irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
>  		}
>  		t->rcu_read_unlock_special.b.deferred_qs = true;
>  		local_irq_restore(flags);
>