Subject: Re: [PATCH v3 net-next 2/3] bonding: fix __get_first_agg RCU usage

From: Ding Tianhong <[email-masked]>
Date: Fri, 10 Jan 2014 18:43:41 +0800

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: [email-masked]
> CC: Jay Vosburgh <[email-masked]>
> CC: Andy Gospodarek <[email-masked]>
> Signed-off-by: Veaceslav Falico <[email-masked]>
> ---
> 
> 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;
>  }
>