From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
rcu@vger.kernel.org, urezki@gmail.com, boqun.feng@gmail.com,
Neeraj Upadhyay <neeraju@codeaurora.org>,
joel@joelfernandes.org
Subject: Re: [PATCH 2/4] rcu: Remove useless WRITE_ONCE() on rcu_data.exp_deferred_qs
Date: Thu, 16 Sep 2021 23:05:14 +0200 [thread overview]
Message-ID: <20210916210514.GA40064@lothringen> (raw)
In-Reply-To: <20210916164340.GF4156@paulmck-ThinkPad-P17-Gen-1>
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@kernel.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
> >
next prev parent reply other threads:[~2021-09-16 21:05 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-16 12:10 [PATCH 0/4] rcu: Unify a bit [non-]PREEMPT expedited quiescent state report Frederic Weisbecker
2021-09-16 12:10 ` [PATCH 1/4] rcu: Ignore rdp.cpu_no_qs.b.exp on premptible RCU's rcu_qs() Frederic Weisbecker
2021-09-16 12:10 ` [PATCH 2/4] rcu: Remove useless WRITE_ONCE() on rcu_data.exp_deferred_qs Frederic Weisbecker
2021-09-16 16:43 ` Paul E. McKenney
2021-09-16 21:05 ` Frederic Weisbecker [this message]
2021-09-17 18:10 ` Paul E. McKenney
2021-09-17 22:00 ` Frederic Weisbecker
2021-09-16 12:10 ` [PATCH 3/4] rcu: Move rcu_data.cpu_no_qs.b.exp reset to rcu_export_exp_rdp() Frederic Weisbecker
2021-09-16 12:10 ` [PATCH 4/4] rcu: Remove rcu_data.exp_deferred_qs and convert to rcu_data.cpu no_qs.b.exp Frederic Weisbecker
2021-09-16 16:40 ` [PATCH 0/4] rcu: Unify a bit [non-]PREEMPT expedited quiescent state report Paul E. McKenney
2021-09-28 18:13 ` Paul E. McKenney
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