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

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Date:   Tue, 19 Nov 2019 14:50:48 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     anders.roxell@...aro.org
Cc:     kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org, paulmck@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: ipmr: fix suspicious RCU warning

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.

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.