linux-kernel - Re: [PATCH bpf-next 1/7] rcu: add rcu_migrate_enable and rcu_migrate

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Message-ID: <eb93f12d-2232-4b7e-a7c6-71082a69f1f6@paulmck-laptop>
Date: Fri, 15 Aug 2025 08:31:17 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Menglong Dong <menglong8.dong@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	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>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/7] rcu: add rcu_migrate_enable and
 rcu_migrate_disable

On Fri, Aug 15, 2025 at 04:02:14PM +0300, Alexei Starovoitov wrote:
> On Fri, Aug 15, 2025 at 9:18 AM Menglong Dong <menglong8.dong@...il.com> wrote:
> >
> > migrate_disable() is called to disable migration in the kernel, and it is
> > used togather with rcu_read_lock() oftenly.
> >
> > However, with PREEMPT_RCU disabled, it's unnecessary, as rcu_read_lock()
> > will disable preemption, which will also disable migration.
> >
> > Introduce rcu_migrate_enable() and rcu_migrate_disable(), which will do
> > the migration enable and disable only when the rcu_read_lock() can't do
> > it.
> >
> > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > ---
> >  include/linux/rcupdate.h | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 120536f4c6eb..0d9dbd90d025 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -72,6 +72,16 @@ static inline bool same_state_synchronize_rcu(unsigned long oldstate1, unsigned
> >  void __rcu_read_lock(void);
> >  void __rcu_read_unlock(void);
> >
> > +static inline void rcu_migrate_enable(void)
> > +{
> > +       migrate_enable();
> > +}
> 
> Interesting idea.
> I think it has to be combined with rcu_read_lock(), since this api
> makes sense only when used together.
> 
> rcu_read_lock_dont_migrate() ?
> 
> It will do rcu_read_lock() + migrate_disalbe() in PREEMPT_RCU
> and rcu_read_lock() + preempt_disable() otherwise?

That could easily be provided.  Or just make one, and if it starts
having enough use cases, it could be pulled into RCU proper.

> Also I'm not sure we can rely on rcu_read_lock()
> disabling preemption in all !PREEMPT_RCU cases.
> iirc it's more nuanced than that.

For once, something about RCU is non-nuanced.  But don't worry, it won't
happen again.  ;-)

In all !PREEMPT_RCU, preemption must be disabled across all RCU read-side
critical sections in order for RCU to work correctly.

							Thanx, Paul