Re: [patch net-next v2 09/11] ipv4: fib: Add an API to request a FIB dump

On 23.11.2016 20:53, Ido Schimmel wrote:

On Wed, Nov 23, 2016 at 06:47:03PM +0100, Hannes Frederic Sowa wrote:

quoted

Hmm, I think you need to read the sequence counter under rtnl_lock to
have an ordering with the rest of the updates to the RCU trie. Otherwise
you don't know if the fib trie has the correct view regarding to the
incoming notifications as a whole. This is also necessary during restarts.

I spent quite a lot of time thinking about this specific issue, but I
couldn't convince myself that the read should be done under RTNL and I'm
not sure I understand your reasoning. Can you please elaborate?

If, before each notification sent, we call atomic_inc() and then call
atomic_read() at the end, then how can we be tricked?

The race I am suspecting to happen is:

<CPU0> fib_register()

<CPU1> delete route by notifier
<CPU1> enqueue delete cmd into ordered queue

<CPU0> starts dump
<CPU0> sees deleted route by CPU1 because route not yet removed from RCU
<CPU0> enqueues route for addition

sometimes later in the ordered queue:

delete route -> route not in hw, nop
add route from dump -> route added to hardware

The result should actually have been that route isn't in hw.

Bye,
Hannes