On Fri, Mar 19, 2021 at 11:10:40PM +0100, Frederic Weisbecker wrote: > On Fri, Mar 19, 2021 at 10:51:16AM -0700, Paul E. McKenney wrote: > > On Fri, Mar 19, 2021 at 02:58:54PM +0100, Frederic Weisbecker wrote: > > > It's all a matter of personal taste but if I may suggest some namespace > > > modifications: > > > > > > get_state_synchronize_rcu() -> synchronize_rcu_poll_start_raw() > > > start_poll_synchronize_rcu() -> synchronize_rcu_poll_start() > > > poll_state_synchronize_rcu() -> synchronize_rcu_poll() > > > cond_synchronize_rcu() -> synchronize_rcu_cond() > > > > > > But it's up to you really. > > > > I am concerned about starting anything "synchronize_rcu" if that > > thing doesn't unconditionally wait for a grace period. "What do > > you mean that there was no grace period? Don't you see that call to > > synchronize_rcu_poll_start_raw()???" > > I see, that could indeed be confusing. > > > This objection doesn't apply to cond_synchronize_rcu(), but it is > > already in use, so any name change should be worked with the users. > > All two of them. ;-) > > Probably not worth it. We have cond_resched() as a schedule() counterpart > for a reference after all. Good point! > > > > /** > > > > + * start_poll_state_synchronize_rcu - Snapshot and start RCU grace period > > > > + * > > > > + * Returns a cookie that is used by a later call to cond_synchronize_rcu() > > > > > > It may be worth noting that calling start_poll_synchronize_rcu() and then > > > pass the cookie to cond_synchronize_rcu() soon after may end up waiting for > > > one more grace period. > > > > You mean this sequence of events? > > > > 1. cookie = start_poll_synchronize_rcu() > > > > 2. The grace period corresponding to cookie is almost over... > > > > 3. cond_synchronize_rcu() checks the cookie and sees that the > > grace period has not yet expired. > > > > 4. The grace period corresponding to cookie completes. > > > > 5. Someone else starts a grace period. > > > > 6. cond_synchronize_rcu() invokes synchronize_rcu(), which waits > > for the just-started grace period plus another grace period. > > Thus, there has been no fewer than three full grace periods > > between the call to start_poll_synchronize_rcu() and the > > return from cond_synchronize_rcu(). > > > > Yes, this can happen! And it can be worse, for example, it is quite > > possible that cond_synchronize_rcu() would be preempted for multiple > > grace periods at step 5, in which case it would still wait for almost > > two additional grace periods. > > > > Or are you thinking of something else? > > 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. > > > > + * If a full RCU grace period has elapsed since the earlier call from > > > > + * which oldstate was obtained, return @true, otherwise return @false. > > > > + * Otherwise, invoke synchronize_rcu() to wait for a full grace period. > > > > > > Rephrase suggestion for the last sentence: > > > > > > "In case of failure, it's up to the caller to try polling again later or > > > invoke synchronize_rcu() to wait for a new full grace period to complete." > > > > How about like this? > > > > /** > > * poll_state_synchronize_rcu - Conditionally wait for an RCU grace period > > * > > * @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu() > > * > > * If a full RCU grace period has elapsed since the earlier call from > > * which oldstate was obtained, return @true, otherwise return @false. > > * If @false is returned, it is the caller's responsibilty to invoke this > > * function later on until it does return @true. Alternatively, the caller > > * can explicitly wait for a grace period, for example, by passing @oldstate > > * to cond_synchronize_rcu() or by directly invoking synchronize_rcu(). > > Yes very nice! You got it! Thanx, Paul
