netdev - Re: [PATCH v3 net 11/16] ipv6: input: convert to dev_net

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Message-ID: <39a1fde2-63f7-4092-870f-ae20156fbb9e@paulmck-laptop>
Date: Tue, 4 Feb 2025 13:17:08 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>,
	"David S . Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
	Kuniyuki Iwashima <kuniyu@...zon.com>,
	Simon Horman <horms@...nel.org>, eric.dumazet@...il.com,
	rcu@...r.kernel.org
Subject: Re: [PATCH v3 net 11/16] ipv6: input: convert to dev_net_rcu()

On Tue, Feb 04, 2025 at 10:06:15PM +0100, Eric Dumazet wrote:
> On Tue, Feb 4, 2025 at 10:00 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Tue, 4 Feb 2025 21:10:59 +0100 Eric Dumazet wrote:
> > > > Test output:
> > > > https://netdev-3.bots.linux.dev/vmksft-net-dbg/results/978202/61-l2tp-sh/
> > > > Decoded:
> > > > https://netdev-3.bots.linux.dev/vmksft-net-dbg/results/978202/vm-crash-thr2-0
> > >
> > > Oh well. So many bugs.
> >
> > TBH I'm slightly confused by this, and the previous warnings.
> >
> > The previous one was from a timer callback.
> >
> > This one is with BH disabled.
> >
> > I thought BH implies RCU protection. We certainly depend on that
> > in NAPI for XDP. And threaded NAPI does the exact same thing as
> > xfrm_trans_reinject(), a bare local_bh_disable().
> >
> > RCU folks, did something change or is just holes in my brain again?
> 
> Nope, BH does not imply rcu_read_lock()

You are both right?  ;-)

The synchronize_rcu() function will wait for all types of RCU readers,
including BH-disabled regions of code.  However, lockdep can distinguish
between the various sorts of readers.  So for example

	lockdep_assert_in_rcu_read_lock_bh();

will complain unless you did rcu_read_lock_bh(), even if you did something
like disable_bh().  If you don't want to distinguish and are happy with
any type of RCU reader, you can use

	lockdep_assert_in_rcu_reader();

I have been expecting that CONFIG_PREEMPT_RT=y kernels will break this
any day now, but so far so good.  ;-)

							Thanx, Paul