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Date: Fri, 28 Jun 2019 05:24:07 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Byungchul Park <byungchul.park@....com>
Cc: Scott Wood <swood@...hat.com>,
Joel Fernandes <joel@...lfernandes.org>,
Steven Rostedt <rostedt@...dmis.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
rcu <rcu@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Josh Triplett <josh@...htriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>, kernel-team@....com
Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs
On Fri, Jun 28, 2019 at 05:24:38PM +0900, Byungchul Park wrote:
> On Fri, Jun 28, 2019 at 05:14:32PM +0900, Byungchul Park wrote:
> > On Fri, Jun 28, 2019 at 04:43:50PM +0900, Byungchul Park wrote:
> > > On Fri, Jun 28, 2019 at 04:31:38PM +0900, Byungchul Park wrote:
> > > > 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?
> > > >
> > >
> > > +cc kernel-team@....com
> > > (I'm sorry for more noise on the thread.)
> > >
> > > > 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();
> >
> > Ah. But the problem is this makes rcu_read_unlock() heavier, which is
> > too bad. Need to consider something else. But I'm still curious about
> > if the scenario I told you is correct?
>
> Instead, this patch should be replaced with the following:
>
> ---8<---
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 3c8444e..f103e98 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -624,8 +624,15 @@ static void rcu_read_unlock_special(struct task_struct *t)
>
> local_irq_save(flags);
> irqs_were_disabled = irqs_disabled_flags(flags);
> +
> + WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> + /*
> + * Prevent reordering between rcu_read_unlock_special.b.exp_hint
> + * above and rcu_read_lock_nesting outside of this function.
> + */
> + smp_wmb();
Except that these are manipulated by the current CPU (aside from debug
code), so inter-CPU ordering is not needed. Plus .exp_hint is just a
heuristic.
Or am I missing something subtle here? If so, please provide a
step-by-step explanation of the failure scenario.
Thanx, Paul
> +
> if (preempt_bh_were_disabled || irqs_were_disabled) {
> - WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> /* Need to defer quiescent state until everything is enabled. */
> if (irqs_were_disabled) {
> /* Enabling irqs does not reschedule, so... */
> @@ -638,7 +645,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
> local_irq_restore(flags);
> return;
> }
> - WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> rcu_preempt_deferred_qs_irqrestore(t, flags);
> }
>
>
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