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From: Jakub Kicinski <kuba@kernel.org>
To: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Cc: sfr@canb.auug.org.au, Amol Grover <frextrite@gmail.com>,
	"David S . Miller" <davem@davemloft.net>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kernel-mentees@lists.linuxfoundation.org,
	Joel Fernandes <joel@joelfernandes.org>,
	"Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH net 2/2 RESEND] ipmr: Add lockdep expression to ipmr_for_each_table macro
Date: Tue, 12 May 2020 09:32:31 -0700	[thread overview]
Message-ID: <20200512093231.7ce29f30@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> (raw)
In-Reply-To: <20200512051705.GB9585@madhuparna-HP-Notebook>

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.

WARNING: multiple messages have this Message-ID (diff)
From: Jakub Kicinski <kuba@kernel.org>
To: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Cc: sfr@canb.auug.org.au, "Paul E . McKenney" <paulmck@kernel.org>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	Joel Fernandes <joel@joelfernandes.org>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	linux-kernel-mentees@lists.linuxfoundation.org,
	"David S . Miller" <davem@davemloft.net>
Subject: Re: [Linux-kernel-mentees] [PATCH net 2/2 RESEND] ipmr: Add lockdep expression to ipmr_for_each_table macro
Date: Tue, 12 May 2020 09:32:31 -0700	[thread overview]
Message-ID: <20200512093231.7ce29f30@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> (raw)
In-Reply-To: <20200512051705.GB9585@madhuparna-HP-Notebook>

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.
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  reply	other threads:[~2020-05-12 16:32 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-09  7:22 [PATCH net 1/2 RESEND] ipmr: Fix RCU list debugging warning Amol Grover
2020-05-09  7:22 ` [Linux-kernel-mentees] " Amol Grover
2020-05-09  7:22 ` [PATCH net 2/2 RESEND] ipmr: Add lockdep expression to ipmr_for_each_table macro Amol Grover
2020-05-09  7:22   ` [Linux-kernel-mentees] " Amol Grover
2020-05-09 21:19   ` Jakub Kicinski
2020-05-09 21:19     ` [Linux-kernel-mentees] " Jakub Kicinski
2020-05-12  5:17     ` Madhuparna Bhowmik
2020-05-12  5:17       ` [Linux-kernel-mentees] " Madhuparna Bhowmik
2020-05-12 16:32       ` Jakub Kicinski [this message]
2020-05-12 16:32         ` Jakub Kicinski
2020-05-13  5:34         ` Madhuparna Bhowmik
2020-05-13  5:34           ` [Linux-kernel-mentees] " Madhuparna Bhowmik
2020-05-12 17:17       ` Amol Grover
2020-05-12 17:17         ` [Linux-kernel-mentees] " Amol Grover
2020-05-12 21:59         ` Jakub Kicinski
2020-05-12 21:59           ` [Linux-kernel-mentees] " Jakub Kicinski
2020-05-13 11:54       ` Stephen Rothwell
2020-05-13 11:54         ` [Linux-kernel-mentees] " Stephen Rothwell

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