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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Thu, 27 Jun 2019 11:37:10 -0400
Message-ID: <CAEXW_YT5LgdP_9SrachU4ZrhV9a7o_DM8eBfgxj=n7yRRyS-TQ@mail.gmail.com>
Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        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>
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On Thu, Jun 27, 2019 at 11:30 AM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> 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 :-(

Sebastian it would be nice if possible to trace where the
t->rcu_read_unlock_special is set for this scenario of calling
rcu_read_unlock_special, to give a clear idea about whether it was
really because of an IPI. I guess we could also add additional RCU
debug fields to task_struct (just for debugging) to see where there
unlock_special is set.

Is there a test to reproduce this, or do I just boot an intel x86_64
machine with "threadirqs" and run into it?