On Mon, Jan 23, 2023 at 10:22:19AM -0500, Joel Fernandes wrote:
> > What am I missing?
>
> That the acceleration is also done by __note_gp_changes() once the
> grace period ends anyway, so if any acceleration was missed as you
> say, it will be done anyway.
>
> Also it is done by scheduler tick raising softirq:
>
> rcu_pending() does this:
> /* Has RCU gone idle with this CPU needing another grace period? */
> if (!gp_in_progress && rcu_segcblist_is_enabled(&rdp->cblist) &&
> !rcu_rdp_is_offloaded(rdp) &&
> !rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
> return 1;
>
> and rcu_core():
> /* No grace period and unregistered callbacks? */
> if (!rcu_gp_in_progress() &&
> rcu_segcblist_is_enabled(&rdp->cblist) && do_batch) {
> rcu_nocb_lock_irqsave(rdp, flags);
> if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
> rcu_accelerate_cbs_unlocked(rnp, rdp);
> rcu_nocb_unlock_irqrestore(rdp, flags);
> }
>
> So, I am not sure if you need needacc at all. Those CBs that have not
> been assigned grace period numbers will be taken care off :)
But that's only when there is no grace period pending, so it can't happen while
we report a QS.
OTOH without the needacc, those callbacks waiting to be accelerated would be
eventually processed but only on the next tick following the end of a grace
period...if none has started since then. So if someone else starts a new GP
before the current CPU, we must wait another GP, etc...
That's potentially dangerous.
And unfortunately we can't do the acceleration from __note_gp_changes() due
to lock ordering restrictions: nocb_lock -> rnp_lock
>
> Thanks!
>
> -Joel
