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

[Steven Rostedt]

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?

-- 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
>