[v3,net-next,2/3] bonding: fix __get_first_agg RCU usage

Currently, the RCU read lock usage is just wrong - it gets the slave struct
under RCU and continues to use it when RCU lock is released.

However, it's still safe to do this cause we didn't need the
rcu_read_lock() initially - all of the __get_first_agg() callers are either
holding RCU read lock or the RTNL lock, so that we can't sync while in it.

So, remove the useless rcu locking and add a comment.

Fixes: be79bd048 ("bonding: add RCU for bond_3ad_state_machine_handler()")
CC: dingtianhong@huawei.com
CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
---

Notes:
    v2 -> v3:
    Use the rcu primitives.
    
    v1 -> v2:
    Don't use RCU primitives as we can hold RTNL.

 drivers/net/bonding/bond_3ad.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

On 2014/1/10 17:18, Veaceslav Falico wrote:
> Currently, the RCU read lock usage is just wrong - it gets the slave struct
> under RCU and continues to use it when RCU lock is released.
> 
> However, it's still safe to do this cause we didn't need the
> rcu_read_lock() initially - all of the __get_first_agg() callers are either
> holding RCU read lock or the RTNL lock, so that we can't sync while in it.
> 
> So, remove the useless rcu locking and add a comment.
> 
> Fixes: be79bd048 ("bonding: add RCU for bond_3ad_state_machine_handler()")
> CC: dingtianhong@huawei.com
> CC: Jay Vosburgh <fubar@us.ibm.com>
> CC: Andy Gospodarek <andy@greyhouse.net>
> Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
> ---
> 
> Notes:
>     v2 -> v3:
>     Use the rcu primitives.
>     
>     v1 -> v2:
>     Don't use RCU primitives as we can hold RTNL.
> 
>  drivers/net/bonding/bond_3ad.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/net/bonding/bond_3ad.c b/drivers/net/bonding/bond_3ad.c
> index 9ff55eb..27dac0e 100644
> --- a/drivers/net/bonding/bond_3ad.c
> +++ b/drivers/net/bonding/bond_3ad.c
> @@ -143,6 +143,7 @@ static inline struct bonding *__get_bond_by_port(struct port *port)
>   *
>   * Return the aggregator of the first slave in @bond, or %NULL if it can't be
>   * found.
> + * The caller must hold RCU lock.
>   */
>  static inline struct aggregator *__get_first_agg(struct port *port)
>  {
> @@ -153,9 +154,7 @@ static inline struct aggregator *__get_first_agg(struct port *port)
>  	if (bond == NULL)
>  		return NULL;
>  
> -	rcu_read_lock();
>  	first_slave = bond_first_slave_rcu(bond);
> -	rcu_read_unlock();
>  
I am afraid the lockdep check will calling some warming:
bond_3ad_unbind_slave -> __get_first_agg -> bond_first_slave_rcu -> netdev_lower_get_first_private_rcu -> list_first_or_null_rcu

but the bond_3ad_unbind_slave is not in RCU.

Regards
Ding
>  	return first_slave ? &(SLAVE_AD_INFO(first_slave).aggregator) : NULL;
>  }
> 


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On Fri, Jan 10, 2014 at 06:43:41PM +0800, Ding Tianhong wrote:
>On 2014/1/10 17:18, Veaceslav Falico wrote:
>> -	rcu_read_lock();
>>  	first_slave = bond_first_slave_rcu(bond);
>> -	rcu_read_unlock();
>>
>I am afraid the lockdep check will calling some warming:
>bond_3ad_unbind_slave -> __get_first_agg -> bond_first_slave_rcu -> netdev_lower_get_first_private_rcu -> list_first_or_null_rcu
>
>but the bond_3ad_unbind_slave is not in RCU.

Yep, right, I'm always colliding with my next patchset which removes it
completely, so it doesn't whine.

Will resend.

>
>Regards
>Ding
>>  	return first_slave ? &(SLAVE_AD_INFO(first_slave).aggregator) : NULL;
>>  }
>>
>
>
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