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

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Thu, Jun 27, 2019 at 11:30:31AM -0400, Joel Fernandes 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

4. Some previous reader section was IPIed or preempted, but either
   interrupts, softirqs, or preemption was disabled across the
   rcu_read_unlock() of that previous reader section.

I -think- that this is what Sebastian is seeing.

							Thanx, Paul

> 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