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

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Date:   Thu, 27 Jun 2019 10:34:55 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        rcu@...r.kernel.org, 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 Thu, 27 Jun 2019 10:24:36 -0400
Joel Fernandes <joel@...lfernandes.org> wrote:

> > What am I missing here?  
> 
> This issue I think is
> 
> (in normal process context)
> spin_lock_irqsave(rq_lock); // which disables both preemption and interrupt
> 			   // but this was done in normal process context,
> 			   // not from IRQ handler
> rcu_read_lock();
>           <---------- IPI comes in and sets exp_hint

How would an IPI come in here with interrupts disabled?

-- Steve

> rcu_read_unlock()
>    -> rcu_read_unlock_special
>         -> raise_softirq_irqoff
> 	    -> wakeup_softirq  <--- because in_interrupt returns false.  
> 
> I think the issue is in_interrupt() does not know about threaded interrupts.
> If it did, then the ksoftirqd wake up would not happen.
> 
> Did I get something wrong?