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@xxxxxxxxxx> > --- > 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 >
