Re: timers: Move clearing of base::timer_running under base::lock

[Paul E. McKenney]

On Tue, Dec 08, 2020 at 09:50:49AM +0100, Sebastian Andrzej Siewior wrote:
> On 2020-12-07 08:06:48 [-0800], Paul E. McKenney wrote:
> > > Yes, but it triggers frequently. Like `rcuc' is somehow is aligned with
> > > the timeout.
> > 
> > Given that a lot of RCU processing is event-driven based on timers,
> > and given that the scheduling-clock interrupts are synchronized for
> > energy-efficiency reasons on many configs, maybe this alignment is
> > expected behavior?
> 
> No, it is the fact that rcu_preempt has a higher priority than
> ksoftirqd. So immediately after the wakeup (of rcu_preempt) there is a
> context switch and expire_timers() has this:
> 
> |   raw_spin_unlock_irq(&base->lock);
> |   call_timer_fn(timer, fn, baseclk);
> |   raw_spin_lock_irq(&base->lock);
> |   base->running_timer = NULL;
> |   timer_sync_wait_running(base);
> 
> So ->running_timer isn't reset and try_to_del_timer_sync() (that
> del_timer_sync() from schedule_timeout()) returns -1 and then the corner
> case is handled where `expiry_lock' is acquired. So everything goes as
> expected.

Makes sense!  Thank you for the explanation!

							Thanx, Paul