On Fri, Mar 19, 2021 at 04:38:48PM -0700, Paul E. McKenney wrote: > > I didn't even think that far. > > My scenario was: > > > > 1. cookie = start_poll_synchronize_rcu() > > > > > > 2. cond_synchronize_rcu() checks the cookie and sees that the > > grace period has not yet expired. So it calls synchronize_rcu() > > which queues a callback. > > > > 3. The grace period for the cookie eventually completes. > > > > 4. The callback queued in 2. gets assigned a new grace period number. > > That new grace period starts. > > > > 5. The new grace period completes and synchronize_rcu() returns. > > > > > > But I think this is due to some deep misunderstanding from my end. > > You mean like this? > > oldstate = start_poll_synchronize_rcu(); > // Why wait? Beat the rush!!! > cond_synchronize_rcu(oldstate); > > This would be a bit silly (why not just call synchronize_rcu()?), > and yes, this would unconditionally get you an extra RCU grace period. > Then again, any call to cond_synchronize_rcu() before the desired grace > period has expired will get you an extra grace period, and maybe more. > > So a given use case either needs to not care about the added latency > or have a high probability of invoking cond_synchronize_rcu() after > the desired grace period has expired. Fair point! Thanks.
