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

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Date:   Tue, 29 Aug 2023 12:53:20 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.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 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.

Thanks.