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

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Message-ID: <f75ce493-f23e-4f55-b32f-77a7868f4e2d@paulmck-laptop>
Date: Tue, 25 Feb 2025 16:08:03 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 2/3] rcu/exp: Remove confusing needless full barrier on
 task unblock

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@...nel.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 :(

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
> >