linux-kernel - Re: [PATCH bpf-next 1/4] rcu: factor out migrate_enable_rcu and migrate_disable

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Message-ID: <CAADnVQJygR6Pb1SQq=tJUpHVx7wwnSX1A78mXGha+bQArowtHQ@mail.gmail.com>
Date: Tue, 14 Oct 2025 07:59:59 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Jakub Sitnicki <jakub@...udflare.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	jiang.biao@...ux.dev, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/4] rcu: factor out migrate_enable_rcu and migrate_disable_rcu

On Tue, Oct 14, 2025 at 4:27 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> Factor out migrate_enable_rcu/migrate_disable_rcu from
> rcu_read_lock_dont_migrate/rcu_read_unlock_migrate.
>
> These functions will be used in the following patches.
>
> It's a little weird to define them in rcupdate.h. Maybe we should move
> them to sched.h?
>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> ---
>  include/linux/rcupdate.h | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index c5b30054cd01..43626ccc07e2 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -988,18 +988,32 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
>         preempt_enable_notrace();
>  }
>
> -static __always_inline void rcu_read_lock_dont_migrate(void)
> +/* This can only be used with rcu_read_lock held */
> +static inline void migrate_enable_rcu(void)
> +{
> +       WARN_ON_ONCE(!rcu_read_lock_held());
> +       if (IS_ENABLED(CONFIG_PREEMPT_RCU))
> +               migrate_enable();
> +}
> +
> +/* This can only be used with rcu_read_lock held */
> +static inline void migrate_disable_rcu(void)
>  {
> +       WARN_ON_ONCE(!rcu_read_lock_held());
>         if (IS_ENABLED(CONFIG_PREEMPT_RCU))
>                 migrate_disable();
> +}
> +
> +static __always_inline void rcu_read_lock_dont_migrate(void)
> +{
>         rcu_read_lock();
> +       migrate_disable_rcu();
>  }
>
>  static inline void rcu_read_unlock_migrate(void)
>  {
> +       migrate_enable_rcu();
>         rcu_read_unlock();
> -       if (IS_ENABLED(CONFIG_PREEMPT_RCU))
> -               migrate_enable();
>  }

Sorry. I don't like any of it. It obfuscates the code
without adding any benefits.

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