Re: Possible use of RCU while in extended QS: idle vs RCU read-side in interrupt vs rcu_eqs_exit

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 10 Jan 2019 12:11:55 -0500 (EST)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> ----- On Jan 10, 2019, at 8:44 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:
> 
> > ----- On Jan 10, 2019, at 8:08 AM, rostedt rostedt@goodmis.org wrote:
> >   
> >> On Wed, 9 Jan 2019 20:38:51 -0500 (EST)
> >> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> >>   
> >>> Hi Paul,
> >>> 
> >>> I've had a user report that trace_sched_waking() appears to be
> >>> invoked while !rcu_is_watching() in some situation, so I started
> >>> digging into the scheduler idle code.  
> >> 
> >> I'm wondering if this isn't a bug. Do you have the backtrace for where
> >> trace_sched_waking() was called without rcu watching?  
> > 
> > I strongly suspect a bug as well. I'm awaiting a reproducer from the
> > user whom reported this issue so I can add a WARN_ON_ONCE(!rcu_is_watching())
> > in the scheduler code near trace_sched_waking() and gather a backtrace.
> > 
> > It still has to be confirmed, but I suspect this have been triggered
> > within a HyperV guest. It may therefore be related to a virtualized environment.
> > 
> > I'll try to ask more specifically on which environment this was encountered.  
> 
> So it ends up it happens directly on hardware on a Linux laptop. Here is
> the stacktrace:
> 
> vmlinux!try_to_wake_up
> vmlinux!default_wake_function
> vmlinux!pollwake
> vmlinux!__wake_up_common
> vmlinux!__wake_up_common_lock
> vmlinux!__wake_up
> vmlinux!perf_event_wakeup
> vmlinux!perf_pending_event
> vmlinux!irq_work_run_list
> vmlinux!irq_work_run

> vmlinux!smp_irq_work_iterrupt

The first thing this calls is ipi_entering_ack_irq(), which calls
irq_enter(), which calls rcu_irq_enter().

So why isn't rcu watching here?

-- Steve


> vmlinux!irq_work_interrupt
> vmlinux!finish_task_switch
> vmlinux!__schedule
> vmlinux!schedule_idle
> vmlinux!do_idle
> vmlinux!cpu_startup_entry
> vmlinux!start_secondary
> vmlinux!secondary_startup_64
> 
> Does it raise any red flag ?
> 
> Thanks,
> 
> Mathieu
> 
> > 
> > Thanks,
> > 
> > Mathieu
> >   
> >> 
> >> -- Steve
> >>   
> >>> 
> >>> It appears that interrupts are re-enabled before rcu_eqs_exit() is
> >>> invoked when exiting idle code from the scheduler.
> >>> 
> >>> I wonder what happens if an interrupt handler (including scheduler code)
> >>> happens to issue a RCU read-side critical section before rcu_eqs_exit()
> >>> is called ? Is there some code on interrupt entry that ensures rcu eqs
> >>> state is exited in such scenario ?
> >>> 
> >>> Thanks,
> >>> 
> >>> Mathieu  
> > 
> > --
> > Mathieu Desnoyers
> > EfficiOS Inc.
> > http://www.efficios.com  
>