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Date:   Thu, 27 Jun 2019 11:11:12 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        rcu <rcu@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs
Reply-To: paulmck@linux.ibm.com
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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"

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!  ;-)

							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);