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Date:   Thu, 27 Jun 2019 10:34:55 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        rcu@vger.kernel.org, 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: <20190627103455.01014276@gandalf.local.home>
In-Reply-To: <20190627142436.GD215968@google.com>
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        <20190626162558.GY26519@linux.ibm.com>
        <20190627142436.GD215968@google.com>
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On Thu, 27 Jun 2019 10:24:36 -0400
Joel Fernandes <joel@joelfernandes.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?