linux-kernel - Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

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Message-ID: <20190628180727.GE240964@google.com>
Date:   Fri, 28 Jun 2019 14:07:27 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        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>
Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

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.