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

Valentin Schneider
2021-10-14T11:42:50Z

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/

> Regards,
> Boqun
>