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From: Byungchul Park <byungchul.park@lge.com>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Scott Wood <swood@redhat.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	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
Date: Fri, 28 Jun 2019 16:31:38 +0900	[thread overview]
Message-ID: <20190628073138.GB13650@X58A-UD3R> (raw)
In-Reply-To: <20190627203612.GD26519@linux.ibm.com>

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)) {




  reply	other threads:[~2019-06-28  7:32 UTC|newest]

Thread overview: 78+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-26 13:54 [RFC] Deadlock via recursive wakeup via RCU with threadirqs Sebastian Andrzej Siewior
2019-06-26 16:25 ` Paul E. McKenney
2019-06-27  7:47   ` Sebastian Andrzej Siewior
2019-06-27 15:52     ` Paul E. McKenney
2019-06-27 14:24   ` Joel Fernandes
2019-06-27 14:34     ` Steven Rostedt
2019-06-27 15:30       ` Joel Fernandes
2019-06-27 15:37         ` Joel Fernandes
2019-06-27 15:40           ` Sebastian Andrzej Siewior
2019-06-27 15:42             ` Joel Fernandes
2019-06-27 17:43             ` Joel Fernandes
2019-06-27 17:46               ` Joel Fernandes
2019-06-27 18:11                 ` Paul E. McKenney
2019-06-27 18:27                   ` Joel Fernandes
2019-06-27 18:51                     ` Joel Fernandes
2019-06-27 19:14                       ` Paul E. McKenney
2019-06-27 19:15                     ` Paul E. McKenney
2019-06-27 18:30                   ` Paul E. McKenney
2019-06-27 20:45                     ` Paul E. McKenney
2019-06-27 15:55         ` Paul E. McKenney
2019-06-27 16:47           ` Joel Fernandes
2019-06-27 17:38             ` Paul E. McKenney
2019-06-27 18:16               ` Joel Fernandes
2019-06-27 18:41                 ` Paul E. McKenney
2019-06-27 20:17                   ` Scott Wood
2019-06-27 20:36                     ` Paul E. McKenney
2019-06-28  7:31                       ` Byungchul Park [this message]
2019-06-28  7:43                         ` Byungchul Park
2019-06-28  8:14                           ` Byungchul Park
2019-06-28  8:24                             ` Byungchul Park
2019-06-28 12:24                               ` Paul E. McKenney
2019-06-28  9:10                           ` Byungchul Park
2019-06-28  9:28                             ` Byungchul Park
2019-06-28 12:21                           ` Paul E. McKenney
2019-06-28 10:40                         ` Byungchul Park
2019-06-28 12:27                           ` Paul E. McKenney
2019-06-28 15:44                           ` Steven Rostedt
2019-06-29 15:12                             ` Andrea Parri
2019-06-29 16:55                               ` Paul E. McKenney
2019-06-29 18:09                                 ` Andrea Parri
2019-06-29 18:21                                   ` Andrea Parri
2019-06-29 19:15                                   ` Paul E. McKenney
2019-06-29 19:35                                     ` Andrea Parri
2019-06-30 23:55                             ` Byungchul Park
2019-06-28 14:15                       ` Peter Zijlstra
2019-06-28 15:54                         ` Paul E. McKenney
2019-06-28 16:04                           ` Peter Zijlstra
2019-06-28 17:20                             ` Paul E. McKenney
2019-07-01  9:42                               ` Peter Zijlstra
2019-07-01 10:24                                 ` Sebastian Andrzej Siewior
2019-07-01 12:23                                   ` Paul E. McKenney
2019-07-01 14:00                                     ` Peter Zijlstra
2019-07-01 16:01                                       ` Paul E. McKenney
2019-06-28 20:01                         ` Scott Wood
2019-07-01  9:45                           ` Peter Zijlstra
2019-06-28 13:54                   ` Peter Zijlstra
2019-06-28 15:30                     ` Paul E. McKenney
2019-06-28 18:40                       ` Sebastian Andrzej Siewior
2019-06-28 18:52                         ` Paul E. McKenney
2019-06-28 19:24                           ` Joel Fernandes
2019-06-28 20:04                             ` Paul E. McKenney
2019-06-28 21:40                               ` Joel Fernandes
2019-06-28 22:25                                 ` Paul E. McKenney
2019-06-28 23:12                                   ` Joel Fernandes
2019-06-29  0:06                                     ` Paul E. McKenney
2019-06-28 16:40                   ` Joel Fernandes
2019-06-28 16:45                     ` Joel Fernandes
2019-06-28 17:30                       ` Paul E. McKenney
2019-06-28 17:41                         ` Paul E. McKenney
2019-06-28 17:45                         ` Sebastian Andrzej Siewior
2019-06-28 18:07                           ` Joel Fernandes
2019-06-28 18:20                             ` Sebastian Andrzej Siewior
2019-07-01  2:08                               ` Joel Fernandes
2019-06-28 18:22                           ` Paul E. McKenney
2019-06-28 19:29                             ` Joel Fernandes
2019-06-28 20:06                               ` Paul E. McKenney
2019-06-28 18:05                         ` Joel Fernandes
2019-06-28 18:23                           ` Paul E. McKenney

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