Re: [PATCH 03/11] rcu/nocb: Invoke rcu_core() at the start of deoffloading

Boqun Feng
2021-10-14T13:57:35Z

On Thu, Oct 14, 2021 at 12:42:40PM +0100, Valentin Schneider wrote:
> On 14/10/21 00:07, Boqun Feng wrote:
> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >> index e38028d48648..b236271b9022 100644
> >> --- a/kernel/rcu/tree.c
> >> +++ b/kernel/rcu/tree.c
> >> @@ -2717,6 +2717,23 @@ static __latent_entropy void rcu_core(void)
> >> unsigned long flags;
> >> struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> >> struct rcu_node *rnp = rdp->mynode;
> >> + /*
> >> + * On RT rcu_core() can be preempted when IRQs aren't disabled.
> >> + * Therefore this function can race with concurrent NOCB (de-)offloading
> >> + * on this CPU and the below condition must be considered volatile.
> >> + * However if we race with:
> >> + *
> >> + * _ Offloading: In the worst case we accelerate or process callbacks
> >> + * concurrently with NOCB kthreads. We are guaranteed to
> >> + * call rcu_nocb_lock() if that happens.
> >
> > If offloading races with rcu_core(), can the following happen?
> >
> > <offload work>
> > rcu_nocb_rdp_offload():
> > rcu_core():
> > ...
> > rcu_nocb_lock_irqsave(); // no a lock
> > raw_spin_lock_irqsave(->nocb_lock);
> > rdp_offload_toggle():
> > <LOCKING | OFFLOADED set>
> > if (!rcu_segcblist_restempty(...))
> > rcu_accelerate_cbs_unlocked(...);
> > rcu_nocb_unlock_irqrestore();
> > // ^ a real unlock,
> > // and will preempt_enable()
> > // offload continue with ->nocb_lock not held
> >
> > If this can happen, it means an unpaired preempt_enable() and an
> > incorrect unlock. Thoughts? Maybe I'm missing something here?
> >
>
> As Frederic pointed out in an earlier thread [1], this can't happen because
> we still have IRQs disabled, and the offloading process has to be processed
> on the CPU being offloaded. IOW, in the above scenario, rcu_core() can't be
> preempted by the rcu_nocb_rdp_offload() work until it re-enables IRQs at
> rcu_nocb_unlock_irqrestore().
>
> (hopefully Paul or Frederic will correct me if I've just spewed garbage)
>
> [1]: https://lore.kernel.org/lkml/20210930105340.GA232241@lothringen/
>

Thanks! I think Frederic and you are right: this cannot happen. Thank
you both for looking into this ;-)

Regards,
Boqun

> > Regards,
> > Boqun
> >