linux-kernel - Re: [PATCH 2/5] rcu/exp: Remove confusing needless full barrier on task unblock

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Message-ID: <5bd01e08-85cd-4538-a75d-c230bed7b0dd@paulmck-laptop>
Date: Wed, 19 Mar 2025 07:03:46 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 2/5] rcu/exp: Remove confusing needless full barrier on
 task unblock

On Wed, Mar 19, 2025 at 10:01:36AM +0100, Frederic Weisbecker wrote:
> Le Tue, Mar 18, 2025 at 10:18:12AM -0700, Paul E. McKenney a écrit :
> > On Fri, Mar 14, 2025 at 03:36:39PM +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>
> > 
> > Still cannot see a problem with this, but still a bit nervous.
> 
> Where is the challenge in life if we manage to fall alseep within a minute
> at bedtime?

;-) ;-) ;-)

Suppose that there was an issue with this that we are somehow not spotting.
How would you go about debugging it?

							Thanx, Paul

> > Acked-by: Paul E. McKenney <paulmck@...nel.org>
> 
> Thanks!