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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CADxym3Y_1ushP8cYj8r7y0xin3zX3kObsM8_dCMUVtu54Tz9yw@mail.gmail.com>
Date: Sun, 17 Aug 2025 09:55:27 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, 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 9:02 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> 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() ?

Yeah, it looks much better. So we can introduce:

  rcu_read_lock_dont_migrate()
  rcu_read_unlock_do_ migrate()

>
> It will do rcu_read_lock() + migrate_disalbe() in PREEMPT_RCU
> and rcu_read_lock() + preempt_disable() otherwise?
>
> 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.