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Date:   Fri, 28 Jun 2019 20:20:45 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@linux.ibm.com>,
        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
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On 2019-06-28 14:07:27 [-0400], Joel Fernandes 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();
> > |         }
> > 
> 
> In my traces which I shared previous email, the wakeup_softirqd() gets
> called.
> 
> I thought force_irqthreads value is decided at boot time, so I got lost a bit
> with your comment.

It does. I just wanted point out that in this case
rcu_unlock() / rcu_read_unlock_special() won't see in_irq() true.

Sebastian