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] [day] [month] [year] [list]

Message-ID: <CADxym3bkqdXScTBvQMOb-JTDZTmAqdm_m_we4Rds6W=rgByauQ@mail.gmail.com>
Date: Sun, 17 Aug 2025 10:01:23 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: paulmck@...nel.org
Cc: Alexei Starovoitov <alexei.starovoitov@...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 11:31 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> 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.

Hi, do you mean that we should start with a single
use case? In this series, I started it with the BPF
subsystem. Most of the situations are similar, which will
call rcu_read_lock+migrate_disable and run bpf prog.

>
> > 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.

Great! I worried about this part too.

Thanks!
Menglong Dong

>
>                                                         Thanx, Paul