Re: [RFC PATCH 08/10] rcu: Detect expedited grace period completion in rcu_pending()

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Apr 17, 2026 at 04:11:56PM -0700, Puranjay Mohan wrote:
> rcu_pending() is the gatekeeper that decides whether rcu_core() should
> run on the current CPU's timer tick. Currently it checks if the CPU has
> callbacks ready to invoke or a grace period has completed or started.
> 
> It does not check that an expedited GP has completed. After an expedited
> GP, callbacks remain in RCU_WAIT_TAIL (not yet advanced to
> RCU_DONE_TAIL) and So rcu_core() never runs to advance them.
> 
> Add a check using rcu_segcblist_nextgp() combined with
> poll_state_synchronize_rcu_full() to detect when any pending callbacks'
> grace period has completed.
> 
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> ---
>  kernel/rcu/tree.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 0e43866dc4cd..309273a37b0a 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3671,6 +3671,7 @@ EXPORT_SYMBOL_GPL(cond_synchronize_rcu_full);
>  static int rcu_pending(int user)
>  {
>  	bool gp_in_progress;
> +	struct rcu_gp_oldstate gp_state;
>  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  	struct rcu_node *rnp = rdp->mynode;
>  
> @@ -3701,6 +3702,12 @@ static int rcu_pending(int user)
>  	    rcu_segcblist_ready_cbs(&rdp->cblist))
>  		return 1;
>  
> +	/* Has a GP (normal or expedited) completed for pending callbacks? */
> +	if (!rcu_rdp_is_offloaded(rdp) &&
> +	    rcu_segcblist_nextgp(&rdp->cblist, &gp_state) &&
> +	    poll_state_synchronize_rcu_full(&gp_state))
> +		return 1;

Do we need the overhead of at least one and at worst two full memory
barriers on every ticks that have pending callbacks? I think that there can be
a racy check here, some unordered version of poll_state_synchronize_rcu_full()
perhaps, and leave the ordering duty to rcu_core().

Thanks.

> +
>  	/* Has RCU gone idle with this CPU needing another grace period? */
>  	if (!gp_in_progress && rcu_segcblist_is_enabled(&rdp->cblist) &&
>  	    !rcu_rdp_is_offloaded(rdp) &&
> -- 
> 2.52.0
> 

-- 
Frederic Weisbecker
SUSE Labs