Subject: Re: [PATCH net-next v5 2/6] bonding: remove the no effect lock for bond_3ad_lacpdu_recv()

From: Ding Tianhong <[email-masked]>
Date: Thu, 26 Sep 2013 14:19:26 +0800

On 2013/9/26 12:23, Jay Vosburgh wrote:
> Ding Tianhong <[email-masked]> wrote:
> 
>> On 2013/9/25 18:33, Veaceslav Falico wrote:
>>> On Wed, Sep 25, 2013 at 05:52:19PM +0800, Ding Tianhong wrote:
>>>> There is no pointer needed read lock protection, remove the unnecessary lock
>>>> and improve performance for the 3ad recv path.
> 
> 	How much does removing the lock around the LACPDU receive
> processing improve performance?  This is not high rate traffic; the
> "fast" rate is one LACPDU per second (per port); the default rate is one
> every 30 seconds.
> 

agree.

>>> I don't really understand it. Here's the code path:
>>>
>>> rx_handler (holding rcu_read_lock()) -> bond_handle_frame() ->
>>> bond->recv_probe -> bond_3ad_lacpdu_recv(). So we're holding only the
>>> rcu_read_lock() there. What stops us from racing with
>>> bond_3ad_unbind_slave(), for example?
>>>
>>> As in:
>>>
>>> CPU0                CPU1
>>> --------            -----------
>>> ...                bond_3ad_unbind_slave()
>>> bond_3ad_rx_indication()    ...
>>> if (!port->slave) {        ...            //slave is ok
>>>                 port->slave = NULL;
>>> ad_marker_info_received()    ...
>>> ad_marker_send()        ...
>>> slave = port->slave;        ...
>>> skb->dev = slave->dev;        ...
>>>        ^^^ NULL pointer dereference.
>>>
>>>
>>> I'm not saying that this approach is wrong, maybe I'm missing something,
>>> but when removing locks it's usually a good thing to do - to comment it in
>>> depth in the commit message why it's not already needed.
>>>
>>
>> no, it will not happend, pls review the cold:
>> 	netdev_rx_handler_unregister(slave_dev);
>> 	write_lock_bh(&bond->lock);
>>
>> 	/* Inform AD package of unbinding of slave. */
>> 	if (bond->params.mode == BOND_MODE_8023AD) {
>> 		/* must be called before the slave is
>> 		 * detached from the list
>> 		 */
>> 		bond_3ad_unbind_slave(slave);
>> 	}
>> netdev_rx_handler_unregiste() will remvoe the rx_handle before the bond_3ad_unbind_slave(),
>> it was safe to run bond_3ad_rx_indication(). 
> 
> 	I'm not sure this is safe if bond_3ad_rx_indication is started
> prior to the unbind, e.g.,
> 
> CPU 0				CPU 1
> ----				-----
> bond_3ad_rx_indication
> [ pass port->slave test ]
> [ ... ]				rx_handler_unregister
> 
> [ state machine lock could be
>   contended, forcing us to wait ]
> __get_state_machine_lock
> 
> 				write_lock(&bond->lock)
> 				bond_3ad_unbind_slave()
> 				[ ... ]
> 				port->slave = NULL;
> 
> [ got the lock ]
> ad_rx_machine(lacpdu, port)
> [ detect loopback ]
> pr_err(... port->slave->bond->dev->name)
> 
> 	or that ad_marker case that Veaceslav describes.
> 
> 	-J
> 

yes, I miss one thing here, there  is no rcu_read_lock() here, so when enter
bond_3ad_unbind_slave(), bond_3ad_rx_indication() was running.
we should miss the patch, thanks.

>> Best regards
>> Ding Tianhong

>>
>>>>
>>>> Signed-off-by: Ding Tianhong <[email-masked]>
>>>> Cc: Nikolay Aleksandrov <[email-masked]>
>>>> ---
>>>> drivers/net/bonding/bond_3ad.c | 2 --
>>>> 1 file changed, 2 deletions(-)
>>>>
>>>> diff --git a/drivers/net/bonding/bond_3ad.c b/drivers/net/bonding/bond_3ad.c
>>>> index 7a3860f..c134f43 100644
>>>> --- a/drivers/net/bonding/bond_3ad.c
>>>> +++ b/drivers/net/bonding/bond_3ad.c
>>>> @@ -2494,9 +2494,7 @@ int bond_3ad_lacpdu_recv(const struct sk_buff *skb, struct bonding *bond,
>>>>     if (!lacpdu)
>>>>         return ret;
>>>>
>>>> -    read_lock(&bond->lock);
>>>>     ret = bond_3ad_rx_indication(lacpdu, slave, skb->len);
>>>> -    read_unlock(&bond->lock);
>>>>     return ret;
>>>> }
>>>>
>>>> -- 
>>>> 1.8.2.1
> 
> ---
> 	-Jay Vosburgh, IBM Linux Technology Center, [email-masked]
> 
> 
> .
>