linux-kernel - Re: [PATCH v2 3/5] tree/nocb: Adjust RCU_NOCB_WAKE

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Message-ID: <CAEXW_YSfJHt+hUYzPWs4BAvi0JL2UzHzLxKwb3=aRt9V7aQ4QQ@mail.gmail.com>
Date:   Wed, 30 Aug 2023 17:13:13 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        paulmck@...nel.org, Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [PATCH v2 3/5] tree/nocb: Adjust RCU_NOCB_WAKE_* macros from
 weaker to stronger

On Tue, Aug 29, 2023 at 6:53 AM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> On Sat, Jul 29, 2023 at 02:27:33PM +0000, Joel Fernandes (Google) wrote:
> > This is needed to make the next patch work correctly as we rely on the
> > strength of the wakeup when comparing deferred-wakeup types across
> > different CPUs.
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> >  kernel/rcu/tree.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index 192536916f9a..0f40a9c2b78d 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -288,8 +288,8 @@ struct rcu_data {
> >
> >  /* Values for nocb_defer_wakeup field in struct rcu_data. */
> >  #define RCU_NOCB_WAKE_NOT    0
> > -#define RCU_NOCB_WAKE_BYPASS 1
> > -#define RCU_NOCB_WAKE_LAZY   2
> > +#define RCU_NOCB_WAKE_LAZY   1
> > +#define RCU_NOCB_WAKE_BYPASS 2
> >  #define RCU_NOCB_WAKE                3
> >  #define RCU_NOCB_WAKE_FORCE  4
>
> Good change but make sure to audit all the occurences of
> RCU_NOCB_WAKE_LAZY and RCU_NOCB_WAKE_BYPASS. For example this breaks
> do_nocb_deferred_wakeup_timer() that will now ignore RCU_NOCB_WAKE_LAZY
> timers.
>

Ah I did do an audit, but missed this one. So then I think
do_nocb_deferred_wakeup_timer() should pass the weakest one
(RCU_NOCB_WAKE_LAZY) to do_nocb_deferred_wakeup_common().

Thanks a lot Frederic! I will take a deeper dive into this and author
users of RCU_NOCB_WAKE* and repost soon.

 - Joel