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

[Mathieu Desnoyers]

----- On Jan 10, 2019, at 8:44 AM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx wrote:

> ----- On Jan 10, 2019, at 8:08 AM, rostedt rostedt@xxxxxxxxxxx wrote:
> 
>> On Wed, 9 Jan 2019 20:38:51 -0500 (EST)
>> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> 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
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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com