netdev - Re: [PATCH] net: ipmr: fix suspicious RCU warning

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Date:   Tue, 19 Nov 2019 17:12:17 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     David Miller <davem@...emloft.net>
Cc:     anders.roxell@...aro.org, kuznet@....inr.ac.ru,
        yoshfuji@...ux-ipv6.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: ipmr: fix suspicious RCU warning

On Tue, Nov 19, 2019 at 02:50:48PM -0800, David Miller wrote:
> From: Anders Roxell <anders.roxell@...aro.org>
> Date: Mon, 18 Nov 2019 10:09:25 +0100
> 
> > @@ -108,9 +108,18 @@ static void igmpmsg_netlink_event(struct mr_table *mrt, struct sk_buff *pkt);
> >  static void mroute_clean_tables(struct mr_table *mrt, int flags);
> >  static void ipmr_expire_process(struct timer_list *t);
> >  
> > +#ifdef CONFIG_PROVE_LOCKING
> > +int ip_mr_initialized;
> > +void ip_mr_now_initialized(void) { ip_mr_initialized = 1; }
> > +#else
> > +const int ip_mr_initialized = 1;
> > +void ip_mr_now_initialized(void) { }
> > +#endif
> 
> This seems excessive and a bit not so pretty.
> 
> > +
> >  #ifdef CONFIG_IP_MROUTE_MULTIPLE_TABLES
> >  #define ipmr_for_each_table(mrt, net) \
> > -	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list)
> > +	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
> > +			(lockdep_rtnl_is_held() || !ip_mr_initialized))
> >  
> >  static struct mr_table *ipmr_mr_table_iter(struct net *net,
> >  					   struct mr_table *mrt)
> 
> The problematic code path is ipmr_rules_init() done during ipmr_net_init().
> 
> You can just wrap this call around RCU locking or take the RTNL mutex.

Agreed, that would work quite well.

							Thanx, Paul

> That way you don't need to rediculous ip_mr_initialized knob which frankly
> doesn't even seem accurate to me.  It's a centralized global variable
> which is holding state about multiple network namespace objects which makes
> absolutely no sense at all, it's wrong.