Subject: Re: [Patch net-next] fib: move fib_rules_cleanup_ops() under rtnl lock

From: Alexander Duyck <[email-masked]>
Date: Fri, 27 Mar 2015 15:12:48 -0700

On 03/27/2015 02:17 PM, Cong Wang wrote:
> On Fri, Mar 27, 2015 at 2:08 PM, Alexander Duyck
> <[email-masked]> wrote:
>> This locking issue, if present, is separate from the original issue you
>> reported.  I'm going to submit a patch to fix your original issue and you
>> can chase this locking issue down separately if that is what you want to do.
> Make sure you really read my changelog, in case you don't:
>
> "
> ops->rules_list is protected by rtnl_lock + RCU,
> there is no reason to take net->rules_mod_lock here.
> Also, ops->delete() needs to be called with rtnl_lock
> too. The problem exists before, just it is exposed
> recently due to the fib local/main table change.
> "
>
> Sometimes people more easily miss the most obvious thing,
> which is the first sentences of my changelog.

I got that, but you are arguing in circles.  In the case of fib4 we 
already held the rtnl lock when all of this was called.  The delete bit 
only really applies to fib4 since that is the only rules setup that 
seems to implement that function.  As I said your "fix" was obscuring 
the original issue.  The original issue was that we were allocating in a 
cleanup path.  That is the first thing that needs to be fixed.

The rtnl_lock or not is a secondary issue.  It may be a fix but it 
doesn't really address the original problem which was allocating in a 
cleanup path.

>
>> This way if someone ever decides to backport it they can actually fix the
>> original issue without pulling in speculative fixes for the rtnl locking
>> problem since we were already holding the lock for fib4.
>>
> Backporting is my guess of Thomas's point, you go too far beyond it.

Backporting wasn't his issue.  From what I can tell he was okay with 
pulling the fib_rules_cleanup_ops outside of the rules_mode_lock, I am 
as well since I believe that is only there because that used to be in a 
loop that would walk through a list looking for ops in order to delete 
it.  Since the list walk is gone you could just hold the lock for the 
list_del_rcu and you are good.

The point he was trying to get at is that you should not make the 
rtnl_lock a part of fib_rules_unregster.  If someone is calling it in 
clean-up and requires it they should be taking the rtnl_lock like we did 
in fib4.  The issue is fib_rules_unregister is also called in the 
exception path for init and the rtnl_lock isn't necessary in that path.

> Also, you have a different definition of original issue.

Yes.  You reported a sleeping function called from invalid context, and 
you were fixing it by splitting up the rtnl_lock/unlock section in fib4 
unnecessarily which opens us up to other possible races, and left the 
function expensive and bloated as it was performing allocations in a 
clean-up path.

I've submitted patches for the issue I cared about so once those patches 
are applied feel free to try and address the rtnl_lock issue separately, 
however I would prefer it if you didn't split up the locking between the 
table freeing and the unregister as it should really all be done as one 
transaction without having to release and reacquire the RTNL lock in the 
middle of it.

- Alex