From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: 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 10:41:27 -0700 [thread overview]
Message-ID: <20190628174127.GA32698@linux.ibm.com> (raw)
In-Reply-To: <20190628173011.GX26519@linux.ibm.com>
On Fri, Jun 28, 2019 at 10:30:11AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 28, 2019 at 12:45:59PM -0400, Joel Fernandes wrote:
> > On Fri, Jun 28, 2019 at 12:40:08PM -0400, Joel Fernandes wrote:
> > > On Thu, Jun 27, 2019 at 11:41:07AM -0700, Paul E. McKenney wrote:
> > > [snip]
> > > > > > > And we should document this somewhere for future sanity preservation
> > > > > > > :-D
> > > > > >
> > > > > > Or adjust the code and requirements to make it more sane, if feasible.
> > > > > >
> > > > > > My current (probably wildly unreliable) guess that the conditions in
> > > > > > rcu_read_unlock_special() need adjusting. I was assuming that in_irq()
> > > > > > implies a hardirq context, in other words that in_irq() would return
> > > > > > false from a threaded interrupt handler. If in_irq() instead returns
> > > > > > true from within a threaded interrupt handler, then this code in
> > > > > > rcu_read_unlock_special() needs fixing:
> > > > > >
> > > > > > 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);
> > > > > >
> > > > > > The fix would be replacing the calls to in_irq() with something that
> > > > > > returns true only if called from within a hardirq context.
> > > > > > Thoughts?
> > > > >
> > > > > I am not sure if this will fix all cases though?
> > > > >
> > > > > I think the crux of the problem is doing a recursive wake up. The threaded
> > > > > IRQ probably just happens to be causing it here, it seems to me this problem
> > > > > can also occur on a non-threaded irq system (say current_reader() in your
> > > > > example executed in a scheduler path in process-context and not from an
> > > > > interrupt). Is that not possible?
> > > >
> > > > In the non-threaded case, invoking raise_softirq*() from hardirq context
> > > > just sets a bit in a per-CPU variable. Now, to Sebastian's point, we
> > > > are only sort of in hardirq context in this case due to being called
> > > > from irq_exit(), but the failure we are seeing might well be a ways
> > > > downstream of the actual root-cause bug.
> > >
> > > Hi Paul,
> > > I was talking about calling of rcu_read_unlock_special from a normal process
> > > context from the scheduler.
> > >
> > > In the below traces, it shows that only the PREEMPT_MASK offset is set at the
> > > time of the issue. Both HARD AND SOFT IRQ masks are not enabled, which means
> > > the lock up is from a normal process context.
> > >
> > > I think I finally understood why the issue shows up only with threadirqs in
> > > my setup. If I build x86_64_defconfig, the CONFIG_IRQ_FORCED_THREADING=y
> > > option is set. And booting this with threadirqs, it always tries to
> > > wakeup_ksoftirqd in invoke_softirq.
> > >
> > > I believe what happens is, at an in-opportune time when the .blocked field is
> > > set for the preempted task, an interrupt is received. This timing is quite in
> > > auspicious because t->rcu_read_unlock_special just happens to have its
> > > .blocked field set even though it is not in a reader-section.
>
> Thank you for tracing through this!
>
> > I believe the .blocked field remains set even though we are not any more in a
> > reader section because of deferred processing of the blocked lists that you
> > mentioned yesterday.
>
> That can indeed happen. However, in current -rcu, that would mean
> that .deferred_qs is also set, which (if in_irq()) would prevent
> the raise_softirq_irqsoff() from being invoked. Which was why I was
> asking the questions about whether in_irq() returns true within threaded
> interrupts yesterday. If it does, I need to find if there is some way
> of determining whether rcu_read_unlock_special() is being called from
> a threaded interrupt in order to suppress the call to raise_softirq()
> in that case.
>
> But which version of the kernel are you using here? Current -rcu?
> v5.2-rc1? Something else?
And if this turns out to be current -rcu, and if there is no reasonable
way for rcu_read_unlock_special() to know if it is being invoked from
within a threaded interrupt handler, then the patch below would be one
way out.
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 82c925df1d92..5140e792c1c2 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -625,7 +625,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
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)) {
+ !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);
next prev parent reply other threads:[~2019-06-28 17:41 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
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 [this message]
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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