netdev - Re: [PATCH net-next v5 2/6] bonding: remove the no effect lock for bond_3ad_lacpdu

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Date:	Thu, 26 Sep 2013 10:51:19 +0800
From:	Ding Tianhong <dingtianhong@...wei.com>
To:	Veaceslav Falico <vfalico@...hat.com>
CC:	Jay Vosburgh <fubar@...ibm.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	"David S. Miller" <davem@...emloft.net>,
	Nikolay Aleksandrov <nikolay@...hat.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v5 2/6] bonding: remove the no effect lock for
 bond_3ad_lacpdu_recv()

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.
> 
> 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(). 

Best regards
Ding Tianhong


>>
>> Signed-off-by: Ding Tianhong <dingtianhong@...wei.com>
>> Cc: Nikolay Aleksandrov <nikolay@...hat.com>
>> ---
>> 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
>>
>>
>>
>> -- 
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> 
> .
> 


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