Re: [PATCH 1/3] rcu/nocb: Use switch/case on NOCB timer state machine

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Oct 02, 2024 at 04:57:36PM +0200, Frederic Weisbecker wrote:
> It's more convenient to benefit from the fallthrough feature of
> switch / case to handle the timer state machine. Also a new state is
> about to be added that will take advantage of it.
> 
> No intended functional change.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>  kernel/rcu/tree_nocb.h | 33 +++++++++++++++++++++++----------
>  1 file changed, 23 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 97b99cd06923..2fb803f863da 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -271,22 +271,35 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
>  
>  	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
>  
> -	/*
> -	 * Bypass wakeup overrides previous deferments. In case of
> -	 * callback storms, no need to wake up too early.
> -	 */
> -	if (waketype == RCU_NOCB_WAKE_LAZY &&
> -	    rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) {

In the old code, if this "if" branch is not taken,

> -		mod_timer(&rdp_gp->nocb_timer, jiffies + rcu_get_jiffies_lazy_flush());
> -		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> -	} else if (waketype == RCU_NOCB_WAKE_BYPASS) {
> +	switch (waketype) {
> +	case RCU_NOCB_WAKE_BYPASS:
> +		/*
> +		 * Bypass wakeup overrides previous deferments. In case of
> +		 * callback storms, no need to wake up too early.
> +		 */
>  		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
>  		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> -	} else {

... it will end up in this else branch, however,

> +		break;
> +	case RCU_NOCB_WAKE_LAZY:
> +		if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) {
> +			mod_timer(&rdp_gp->nocb_timer, jiffies + rcu_get_jiffies_lazy_flush());
> +			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> +		}
> +		/*
> +		 * If the timer is already armed, a non-lazy enqueue may have happened
> +		 * in-between. Don't delay it and fall-through.
> +		 */
> +		break;

... here we break instead of fallthrough when waketype ==
RCU_NOCB_WAKE_LAZY and rdp->nocb_defer_wakeup != RCU_NOCB_WAKE_NOT, this
seems to me a functional change, is this intented?

Regards,
Boqun

> +	case RCU_NOCB_WAKE:
> +		fallthrough;
> +	case RCU_NOCB_WAKE_FORCE:
>  		if (rdp_gp->nocb_defer_wakeup < RCU_NOCB_WAKE)
>  			mod_timer(&rdp_gp->nocb_timer, jiffies + 1);
>  		if (rdp_gp->nocb_defer_wakeup < waketype)
>  			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> +		break;
> +	default:
> +		WARN_ON_ONCE(1);
>  	}
>  
>  	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
> -- 
> 2.46.0
>