Re: [PATCH 2/3] rcu/exp: Remove confusing needless full barrier on task unblock

Joel Fernandes
2025-02-25T21:59:12Z

On Fri, Feb 14, 2025 at 12:25:58AM +0100, Frederic Weisbecker wrote:
> A full memory barrier in the RCU-PREEMPT task unblock path advertizes
> to order the context switch (or rather the accesses prior to
> rcu_read_unlock()) with the expedited grace period fastpath.
>
> However the grace period can not complete without the rnp calling into
> rcu_report_exp_rnp() with the node locked. This reports the quiescent
> state in a fully ordered fashion against updater's accesses thanks to:
>
> 1) The READ-SIDE smp_mb__after_unlock_lock() barrier accross nodes
> locking while propagating QS up to the root.
>
> 2) The UPDATE-SIDE smp_mb__after_unlock_lock() barrier while holding the
> the root rnp to wait/check for the GP completion.
>
> 3) The (perhaps redundant given step 1) and 2)) smp_mb() in rcu_seq_end()
> before the grace period completes.
>
> This makes the explicit barrier in this place superflous. Therefore
> remove it as it is confusing.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
> kernel/rcu/tree_plugin.h | 1 -
> 1 file changed, 1 deletion(-)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 3c0bbbbb686f..d51cc7a5dfc7 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -534,7 +534,6 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
> WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
> (!empty_norm || rnp->qsmask));
> empty_exp = sync_rcu_exp_done(rnp);
> - smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */

I was wondering though, this is a slow path and the smp_mb() has been there
since 2009 or so. Not sure if it is super valuable to remove it at this
point. But we/I should definitely understand it.

I was wondering if you could also point to the fastpath that this is racing
with, it is not immediately clear (to me) what this smp_mb() is pairing with :(

thanks,

- Joel

> np = rcu_next_node_entry(t, rnp);
> list_del_init(&t->rcu_node_entry);
> t->rcu_blocked_node = NULL;
> --
> 2.46.0
>