netdev - Re: [PATCH net 2/2 RESEND] ipmr: Add lockdep expression to ipmr_for_each

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Message-ID: <20200512093231.7ce29f30@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 12 May 2020 09:32:31 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
Cc:     sfr@...b.auug.org.au, Amol Grover <frextrite@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Joel Fernandes <joel@...lfernandes.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH net 2/2 RESEND] ipmr: Add lockdep expression to
 ipmr_for_each_table macro

On Tue, 12 May 2020 10:47:05 +0530 Madhuparna Bhowmik wrote:
> > >  #ifdef CONFIG_IP_MROUTE_MULTIPLE_TABLES
> > > -#define ipmr_for_each_table(mrt, net) \
> > > -	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
> > > -				lockdep_rtnl_is_held())
> > > +#define ipmr_for_each_table(mrt, net)					\
> > > +	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list,	\
> > > +				lockdep_rtnl_is_held() ||		\
> > > +				lockdep_is_held(&pernet_ops_rwsem))  
> > 
> > This is a strange condition, IMHO. How can we be fine with either
> > lock.. This is supposed to be the writer side lock, one can't have 
> > two writer side locks..
> > 
> > I think what is happening is this:
> > 
> > ipmr_net_init() -> ipmr_rules_init() -> ipmr_new_table()
> > 
> > ipmr_new_table() returns an existing table if there is one, but
> > obviously none can exist at init.  So a better fix would be:
> > 
> > #define ipmr_for_each_table(mrt, net)					\
> > 	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list,	\
> > 				lockdep_rtnl_is_held() ||		\
> > 				list_empty(&net->ipv4.mr_tables))
> >  
> (adding Stephen)
> 
> Hi Jakub,
> 
> Thank you for your suggestion about this patch.
> Here is a stack trace for ipmr.c:
> 
> [...]

Thanks!

> > Thoughts?  
> 
> Do you think a similar fix (the one you suggested) is also applicable
> in the ip6mr case.

Yes, looking at the code it seems ip6mr has the exact same flow for
netns init.