netdev - Re: [PATCH net-next 0/7] rtnetlink: allow to run selected handlers without rtnl

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Message-ID: <20170809081928.GA26899@breakpoint.cc>
Date:   Wed, 9 Aug 2017 10:19:28 +0200
From:   Florian Westphal <fw@...len.de>
To:     David Miller <davem@...emloft.net>
Cc:     fw@...len.de, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 0/7] rtnetlink: allow to run selected handlers
 without rtnl

David Miller <davem@...emloft.net> wrote:
> From: Florian Westphal <fw@...len.de>
> Date: Tue,  8 Aug 2017 18:02:29 +0200
> 
> > Unfortunately RTNL mutex is a performance issue, e.g. a cpu adding
> > an ip address prevents other cpus from seemingly unrelated tasks
> > such as dumping tc classifiers.
> 
> It is related if somehow the TC entries refer to IP addresses.
>
> Someone could create something like that.

Actually I am not following.  Why would read-only accesses need rtnl
locking wrt. any other operation (provided of course rtnl lock doesn't
protect the data structure)?

> > Initial no-rtnl spots are ip6 fib add/del and netns new/getid.
> 
> I could see the netns stuff being ok, but IPv6 route add/del I'm
> not so sure of.

[..]

> There really is a hierachy of these dependencies.  Device state, up
> to neighbour table state, up to protocol address state, up to routes,
>    up to FIB tables, etc. etc. etc.
> 
>    I'd really like to make this operate more freely, but this is an
>    extremely delicate area which has been bottled up like this for
>    two decades so good luck :-)

Would you accept a v2 if i don't touch ipv6 routes for the time being?

I would then audit those again.  At the very least inet6_rtm_getroute should
be able to work without rtnl lock (i.e., use a different lock if
needed to protect vs. concurrent modifications).