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

[Paul E. McKenney]

On Wed, Jan 09, 2019 at 08:38:51PM -0500, Mathieu Desnoyers 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.
> 
> 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 ?

Interrupt handlers are supposed to invoke irq_enter(), which will in
turn invoke rcu_irq_enter(), which should take care of things.

However, there are cases where a given architecture knows that a given
interrupt handler does not contain RCU readers, and in this case, the
architecture might omit the rcu_irq_enter() or maybe even the whole
irq_enter().  And then it is all fun and games until someone adds an
RCU read-side critical section.  ;-)

							Thanx, Paul