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Date:   Thu, 27 Jun 2019 11:30:31 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Steven Rostedt <rostedt@goodmis.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: <20190627153031.GA249127@google.com>
References: <20190626135447.y24mvfuid5fifwjc@linutronix.de>
 <20190626162558.GY26519@linux.ibm.com>
 <20190627142436.GD215968@google.com>
 <20190627103455.01014276@gandalf.local.home>
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On Thu, Jun 27, 2019 at 10:34:55AM -0400, Steven Rostedt wrote:
> 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

This is true, could it be rcu_read_unlock_special() got called for some
*other* reason other than the IPI then?

Per Sebastian's stack trace of the recursive lock scenario, it is happening
during cpu_acct_charge() which is called with the rq_lock held. 

The only other reasons I know off to call rcu_read_unlock_special() are if
1. the tick indicated that the CPU has to report a QS
2. an IPI in the middle of the reader section for expedited GPs
3. preemption in the middle of a preemptible RCU reader section

1. and 2. are not possible because interrupts are disabled, that's why the
wakeup_softirq even happened.
3. is not possible because we are holding rq_lock in the RCU reader section.

So I am at a bit of a loss how this can happen :-(

Spurious call to rcu_read_unlock_special() may be when it should not have
been called?

thanks,

- Joel