linux-kernel - Re: [PATCH] rtnetlink: Synchronze net in rtnl

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Message-ID: <d20c023b-33d5-4c48-a404-e2ba05f98a99@arista.com>
Date:   Mon, 25 Feb 2019 23:21:58 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, linux-kernel@...r.kernel.org
Cc:     0x7f454c46@...il.com, "David S. Miller" <davem@...emloft.net>,
        Florian Westphal <fw@...len.de>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        netdev@...r.kernel.org
Subject: Re: [PATCH] rtnetlink: Synchronze net in rtnl_unregister()

Hi Eric,

On 2/25/19 11:09 PM, Eric Dumazet wrote:
> On 02/25/2019 01:27 PM, Dmitry Safonov wrote:
>> While it's possible to document that rtnl_unregister() requires
>> synchronize_net() afterwards - unlike rtnl_unregister_all(), I believe
>> the module exit is very much slow-path.
> 
> rtnl_unregister_all() needs the sychronize_rcu() at this moment
> because of the kfree(tab), not because of the kfree_rcu(link, rcu);

I may be wrong here, but shouldn't we wait for grace period to elapse by
the reason that rtnl_msg_handlers are protected by RCU, not only by rtnl?
Like, without synchronize_net() in rtnl_unregister() - what prevents
module exit race to say, rtnetlink_rcv_msg()=>rtnl_get_link()?


>> --- a/net/core/rtnetlink.c
>> +++ b/net/core/rtnetlink.c
>> @@ -308,7 +308,9 @@ int rtnl_unregister(int protocol, int msgtype)
>>  	rcu_assign_pointer(tab[msgindex], NULL);
>>  	rtnl_unlock();
>>  
>> -	kfree_rcu(link, rcu);
>> +	synchronize_net();
>> +
>> +	kfree(link);
> 
> 
> I really do not see a difference here (other than this being much slower of course)
> 
> If the caller needs rcu_barrier(), then add it in the caller ?

Well, sure - but it seems confusing that rtnl_unregister() will require
synchronize_rcu(), while rtnl_unregister_all() will not.
And I thought no one would care about another synchronize_rcu() in exit
path.

Thanks,
          Dmitry