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.
> > > /**
> > > + * 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.
> > > + * 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!
Thanks!
