Re: [PATCH v4 5/8] rcu: clear defer_qs_pending at expedited IPI entry

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Jun 25, 2026 at 08:42:58PM -0400, Joel Fernandes wrote:
> The per-CPU clear in __note_gp_changes() (from an earlier commit)
> runs only when the local CPU notices a normal-GP advance.  When an
> expedited GP arrives at a CPU whose defer_qs_pending is already
> PENDING, rcu_read_unlock_special() may skip irq_work queuing due to the
> pending gate.
> 
> Clear defer_qs_pending on the IPI target right in rcu_exp_handler().
> This makes it possible for any arming attempt that follows
> the IPI within the current GP to be able to queue irq_work again,
> allowing completion expedited GPs quickly than waiting for one scheduler
> tick.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
>  kernel/rcu/tree_exp.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index a43469da3926..5be80d255d6a 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -763,6 +763,12 @@ static void rcu_exp_handler(void *unused)
>  			 READ_ONCE(rdp->cpu_no_qs.b.exp)))
>  		return;
>  
> +	/*
> +	 * Clear defer_qs_pending so arming attempts following this IPI
> +	 * within the current GP can queue irq_work again.
> +	 */
> +	rcu_defer_qs_clear(rdp);

Suppose that there is also a normal grace period pending.  Will clearing
this cause extra unnecessary activity with respect to that normal
grace period?  If so, why is that OK?  If not, what prevents it?

							Thanx, Paul

> +
>  	/*
>  	 * Second, the common case of not being in an RCU read-side
>  	 * critical section.  If also enabled or idle, immediately
> -- 
> 2.34.1
>