linux-kernel - Re: [PATCH 2/4] rcu: Remove useless WRITE_ONCE() on rcu_data.exp_deferred

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Message-ID: <20210916210514.GA40064@lothringen>
Date:   Thu, 16 Sep 2021 23:05:14 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, rcu@...r.kernel.org,
        urezki@...il.com, boqun.feng@...il.com,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        joel@...lfernandes.org
Subject: Re: [PATCH 2/4] rcu: Remove useless WRITE_ONCE() on
 rcu_data.exp_deferred_qs

On Thu, Sep 16, 2021 at 09:43:40AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 16, 2021 at 02:10:46PM +0200, Frederic Weisbecker wrote:
> > This variable is never written nor read remotely. Remove this confusion.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > ---
> >  kernel/rcu/tree_exp.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index f3947c49eee7..4266610b4587 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -255,7 +255,7 @@ static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
> >   */
> >  static void rcu_report_exp_rdp(struct rcu_data *rdp)
> >  {
> > -	WRITE_ONCE(rdp->exp_deferred_qs, false);
> > +	rdp->exp_deferred_qs = false;
> 
> Are you sure that this can never be invoked from an interrupt handler?
> And that rdp->exp_deferred_qs is never read from an interrupt handler?
> If either can happen, then the WRITE_ONCE() does play a role, right?

Well, the only effect I can imagine is that it can partly prevent from an
interrupt to report concurrently the quiescent state during the few
instructions before we mask interrupts and lock the node.

That's a micro performance benefit that avoid a second call to
rcu_report_exp_cpu_mult() with the extra locking and early exit.

But then that racy interrupt can still happen before we clear exp_deferred_qs.
In this case __this_cpu_cmpxchg() would have been more efficient.

Thanks.

> 							Thanx, Paul
> 
> >  	rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
> >  }
> >  
> > -- 
> > 2.25.1
> >