Re: [PATCH 11/13] rcu/nocb: Only cancel nocb timer if not polling

[Paul E. McKenney]

On Tue, Feb 23, 2021 at 01:10:09AM +0100, Frederic Weisbecker wrote:
> No need to disarm the nocb_timer if rcu_nocb is polling because it
> shouldn't be armed either.
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>

OK, so it does make sense to move that del_timer() under the following
"if" statement, then.  ;-)

> ---
>  kernel/rcu/tree_plugin.h | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 9da67b0d3997..d8b50ff40e4b 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2186,18 +2186,18 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
>  	my_rdp->nocb_gp_gp = needwait_gp;
>  	my_rdp->nocb_gp_seq = needwait_gp ? wait_gp_seq : 0;
>  	if (bypass) {
> -		raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
> -		// Avoid race with first bypass CB.
> -		if (my_rdp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
> -			WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
> -			del_timer(&my_rdp->nocb_timer);
> -		}
>  		if (!rcu_nocb_poll) {
> +			raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
> +			// Avoid race with first bypass CB.
> +			if (my_rdp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
> +				WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
> +				del_timer(&my_rdp->nocb_timer);
> +			}
>  			// At least one child with non-empty ->nocb_bypass, so set
>  			// timer in order to avoid stranding its callbacks.
>  			mod_timer(&my_rdp->nocb_bypass_timer, j + 2);
> +			raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
>  		}
> -		raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
>  	}
>  	if (rcu_nocb_poll) {
>  		/* Polling, so trace if first poll in the series. */
> -- 
> 2.25.1
>