linux-kernel - Re: [PATCH 3/8] refscale: Add local_bh

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: <20251112091452.AxsV206Y@linutronix.de>
Date: Wed, 12 Nov 2025 10:14:52 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org
Subject: Re: [PATCH 3/8] refscale: Add local_bh_disable() readers

On 2025-11-11 11:21:04 [-0800], Paul E. McKenney wrote:
> On Tue, Nov 11, 2025 at 04:38:03PM +0100, Sebastian Andrzej Siewior wrote:
> > On 2025-11-02 14:49:43 [-0800], Paul E. McKenney wrote:
> > > --- a/kernel/rcu/refscale.c
> > > +++ b/kernel/rcu/refscale.c
> > > @@ -636,6 +636,37 @@ static const struct ref_scale_ops jiffies_ops = {
> > >  	.name		= "jiffies"
> > >  };
> > >  
> > > +static void ref_bh_section(const int nloops)
> > > +{
> > > +	int i;
> > > +
> > > +	preempt_disable();
> > > +	for (i = nloops; i >= 0; i--) {
> > > +		local_bh_disable();
> > 
> > This (preempt off followed by bh off) may cause warnings. That would be
> > bh is disabled on the CPU, it gets preempted by _this_ which disables
> > first preemption and then bh. 
> > I hid the code under CONFIG_PREEMPT_RT_NEEDS_BH_LOCK so it shouldn't be
> > a problem in the long term I think. So just if you see a warning here
> > under RT you know why :)
> 
> Huh.  Would migrate_disable() be appropriate?  Or I suppose I could just
> let it migrate on RT.  So how about the fix shown below?

Depends on what you want to achieve. Even with that bh-disable you can
be preempted but you can't migrate to another CPU.
That preempt-disable() will keep the RCU read section open during
bh-disable/ enable but migrate_disable() won't. But this not something I
need to explain to you ;) 
If that (to be within a RCU read section) is you intention you could
explicitly add a rcu_read_lock() there.
The change you suggested won't have the problem I mentioned.

> 							Thanx, Paul

Sebastian