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

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Message-ID: <Z9qH8MHMtL-l_X-G@p200300d06f3e98759ed3c196478e337b.dip0.t-ipconnect.de>
Date: Wed, 19 Mar 2025 10:01:36 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...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

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?

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

Thanks!