Re: [PATCH 2/2] rcu/nocb: Fix missed RCU barrier on deoffloading

[Neeraj Upadhyay]

> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 16865475120b..2605dd234a13 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -891,7 +891,18 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>  	swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
>  					    nocb_cb_wait_cond(rdp));
>  	if (kthread_should_park()) {
> -		kthread_parkme();
> +		/*
> +		 * kthread_park() must be preceded by an rcu_barrier().
> +		 * But yet another rcu_barrier() might have sneaked in between
> +		 * the barrier callback execution and the callbacks counter
> +		 * decrement.
> +		 */
> +		if (rdp->nocb_cb_sleep) {

Is READ_ONCE() not required here?


- Neeraj

> +			rcu_nocb_lock_irqsave(rdp, flags);
> +			WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
> +			rcu_nocb_unlock_irqrestore(rdp, flags);
> +			kthread_parkme();
> +		}
>  	} else if (READ_ONCE(rdp->nocb_cb_sleep)) {
>  		WARN_ON(signal_pending(current));
>  		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));