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Date:   Fri, 28 Jun 2019 13:06:06 -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
Message-ID: <20190628200606.GC26519@linux.ibm.com>
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On Fri, Jun 28, 2019 at 03:29:23PM -0400, Joel Fernandes wrote:
> On Fri, Jun 28, 2019 at 11:22:16AM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 28, 2019 at 07:45:45PM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2019-06-28 10:30:11 [-0700], Paul E. McKenney wrote:
> > > > > 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.
> > > 
> > > Please not that:
> > > | void irq_exit(void)
> > > | {
> > > |…
> > > in_irq() returns true
> > > |         preempt_count_sub(HARDIRQ_OFFSET);
> > > in_irq() returns false
> > > |         if (!in_interrupt() && local_softirq_pending())
> > > |                 invoke_softirq();
> > > 
> > > -> invoke_softirq() does
> > > |        if (!force_irqthreads) {
> > > |                 __do_softirq();
> > > |         } else {
> > > |                 wakeup_softirqd();
> > > |         }
> > > 
> > > so for `force_irqthreads' rcu_read_unlock_special() within
> > > wakeup_softirqd() will see false.
> > 
> > OK, fair point.  How about the following instead, again on -rcu?
> > 
> > Here is the rationale for the new version of the "if" statement:
> > 
> > 1.	irqs_were_disabled:  If interrupts are enabled, we should
> > 	instead let the upcoming irq_enable()/local_bh_enable()
> > 	do the rescheduling for us.
> > 2.	use_softirq: If we aren't using softirq, then
> > 	raise_softirq_irqoff() will be unhelpful.
> > 3a.	in_interrupt(): If this returns true, the subsequent
> > 	call to raise_softirq_irqoff() is guaranteed not to
> > 	do a wakeup, so that call will be both very cheap and
> > 	quite safe.
> > 3b.	Otherwise, if !in_interrupt(), if exp (an expedited RCU grace
> > 	period is being blocked), then incurring wakeup overhead
> > 	is worthwhile, and if also !.deferred_qs then scheduler locks
> > 	cannot be held so the wakeup will be safe.
> > 
> > Does that make more sense?
> 
> This makes a lot of sense. It would be nice to stick these comments on top of
> rcu_read_unlock_special() for future reference.

I do have an expanded version in the commit log.  I hope to get a more
high-level description in comments.

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 82c925df1d92..83333cfe8707 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -624,8 +624,9 @@ 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)) {
> > +		if (irqs_were_disabled && use_softirq &&
> > +		    (in_interrupt() ||
> > +		     (exp && !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);
> > 
>