On Tue, Feb 25, 2025 at 04:59:08PM -0500, Joel Fernandes wrote: > 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 :( My guess was one of the lock acquisitions or dyntick checks in __sync_rcu_exp_select_node_cpus(), but I am not seeing anything there. In this context, "fastpath" would be one of the early exits, for example, the "continue" statements in the second for_each_leaf_node_cpu_mask() loop. But again, I am not seeing anything that appears to need that smp_mb(). As you say, that smp_mb() is not on a fastpath, so we need to check carefully before removing it. Thanx, Paul > thanks, > > - Joel > > > > > > > np = rcu_next_node_entry(t, rnp); > > list_del_init(&t->rcu_node_entry); > > t->rcu_blocked_node = NULL; > > -- > > 2.46.0 > >
