netdev - Re: [Patch net-next] fib: move fib_rules_cleanup

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Message-ID: <20150327120135.GC12265@casper.infradead.org>
Date:	Fri, 27 Mar 2015 12:01:35 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	Cong Wang <cwang@...pensource.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [Patch net-next] fib: move fib_rules_cleanup_ops() under rtnl
 lock

On 03/26/15 at 04:47pm, Alexander Duyck wrote:
> On 03/26/2015 04:05 PM, Cong Wang wrote:
> >On Thu, Mar 26, 2015 at 3:32 PM, Cong Wang <cwang@...pensource.com> wrote:
> >>On the other hand, the name rules_mod_lock already tells it is
> >>just a protection for ops (module) register.
> >I even doubt we really need rules_mod_lock, it is per netns,
> >which is newly allocated when registering pernet and upper
> >layer should guarantee no concurrent unregistering, so we
> >probably only need to take rtnl lock.
> 
> I'm adding Thomas as he was the original author for the code and might have
> a better idea of what needs to be rtnl locked and what doesn't.  You should
> probably CC him as well on the v2 patch.
> 
> As far as why I am so focused on moving fib4_rules_exit it is because we
> don't want to call that delete function until after the table has been
> cleared.  Otherwise you end up triggering the external_flush and unmerge
> code on a full table instead of an empty one.  The result is you end up
> allocating a bunch of memory before you then turn around and free it.  So
> even if you retain the rtnl_lock changes it would still be best to move
> fib4_rules_exit call to the region after you have freed the FIB tables, but
> before you free fib_table_hash.

I agree with Alex. Reordering fib4_rules_exit() makes sense. Not only
to fix this issue but also for the purpose of correct ordering of
allocation and releasing.

It is definitely also safe to move fib_rules_cleanup_ops() out of
rules_mod_lock. It is the responsibility of whoever registers the
rules that no rule is in use he calls fib_rules_unregister().

I don't see why fib_rules should hold rtnl_lock upon delete for the
caller. If the caller requires this protection it's up to him to
provide it.
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