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)) {
next prev parent 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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