Subject: Re: [PATCH v2 net-next 2/3] bonding: fix __get_first_agg RCU usage

From: Ding Tianhong <[email-masked]>
Date: Thu, 9 Jan 2014 19:58:17 +0800

On 2014/1/9 19:20, 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:
>     v1 -> v2:
>     Don't use RCU primitives as we can hold RTNL.
> 
>  drivers/net/bonding/bond_3ad.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/bonding/bond_3ad.c b/drivers/net/bonding/bond_3ad.c
> index cf5fab8..d2782c8 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 either hold RCU or RTNL 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();
> +	first_slave = bond_first_slave(bond);
>  
>  	return first_slave ? &(SLAVE_AD_INFO(first_slave).aggregator) : NULL;
>  }
> 

Hi, Veaceslav:

Do you mean the bond_first_slave is safe in rcu_read_xxlock()?

#define bond_first_slave(bond) \
	(bond_has_slaves(bond) ? \
		netdev_adjacent_get_private(bond_slave_list(bond)->next) : \
		NULL)

Regards
Ding