Subject: Re: [PATCH net-next 5/6] bonding: restructure and simplify bond_for_each_slave_next()

From: Nikolay Aleksandrov <[email-masked]>
Date: Tue, 03 Sep 2013 12:22:10 +0200

On 08/30/2013 12:05 PM, Ding Tianhong wrote:
> remove the wordy int and add bond_for_each_slave_next_rcu() for future use.
> 
> Signed-off-by: Ding Tianhong <[email-masked]>
> Cc: Nikolay Aleksandrov <[email-masked]>
> ---
>  drivers/net/bonding/bond_alb.c  |  3 +--
>  drivers/net/bonding/bond_main.c |  6 ++----
>  drivers/net/bonding/bonding.h   | 19 +++++++++++++++----
>  3 files changed, 18 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/bonding/bond_alb.c b/drivers/net/bonding/bond_alb.c
> index 3a5db7b..d266c56 100644
> --- a/drivers/net/bonding/bond_alb.c
> +++ b/drivers/net/bonding/bond_alb.c
> @@ -383,7 +383,6 @@ static struct slave *rlb_next_rx_slave(struct bonding *bond)
>  {
>  	struct alb_bond_info *bond_info = &(BOND_ALB_INFO(bond));
>  	struct slave *rx_slave, *slave, *start_at;
> -	int i = 0;
>  
>  	if (bond_info->next_rx_slave)
>  		start_at = bond_info->next_rx_slave;
> @@ -392,7 +391,7 @@ static struct slave *rlb_next_rx_slave(struct bonding *bond)
>  
>  	rx_slave = NULL;
>  
> -	bond_for_each_slave_from(bond, slave, i, start_at) {
> +	bond_for_each_slave_from(bond, slave, start_at) {
>  		if (SLAVE_IS_OK(slave)) {
>  			if (!rx_slave) {
>  				rx_slave = slave;
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index 4264a76..8c9902a 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -904,7 +904,6 @@ static struct slave *bond_find_best_slave(struct bonding *bond)
>  	struct slave *new_active, *old_active;
>  	struct slave *bestslave = NULL;
>  	int mintime = bond->params.updelay;
> -	int i;
>  
>  	new_active = bond->curr_active_slave;
>  
> @@ -923,7 +922,7 @@ static struct slave *bond_find_best_slave(struct bonding *bond)
>  	/* remember where to stop iterating over the slaves */
>  	old_active = new_active;
>  
> -	bond_for_each_slave_from(bond, new_active, i, old_active) {
> +	bond_for_each_slave_from(bond, new_active, old_active) {
>  		if (new_active->link == BOND_LINK_UP) {
>  			return new_active;
>  		} else if (new_active->link == BOND_LINK_BACK &&
> @@ -2891,7 +2890,6 @@ do_failover:
>  static void bond_ab_arp_probe(struct bonding *bond)
>  {
>  	struct slave *slave, *next_slave;
> -	int i;
>  
>  	read_lock(&bond->curr_slave_lock);
>  
> @@ -2923,7 +2921,7 @@ static void bond_ab_arp_probe(struct bonding *bond)
>  
>  	/* search for next candidate */
>  	next_slave = bond_next_slave(bond, bond->current_arp_slave);
> -	bond_for_each_slave_from(bond, slave, i, next_slave) {
> +	bond_for_each_slave_from(bond, slave, next_slave) {
>  		if (IS_UP(slave->dev)) {
>  			slave->link = BOND_LINK_BACK;
>  			bond_set_slave_active_flags(slave);
> diff --git a/drivers/net/bonding/bonding.h b/drivers/net/bonding/bonding.h
> index 9898493..a3ab47f 100644
> --- a/drivers/net/bonding/bonding.h
> +++ b/drivers/net/bonding/bonding.h
> @@ -119,14 +119,25 @@
>   * bond_for_each_slave_from - iterate the slaves list from a starting point
>   * @bond:	the bond holding this list.
>   * @pos:	current slave.
> - * @cnt:	counter for max number of moves
>   * @start:	starting point.
>   *
>   * Caller must hold bond->lock
>   */
> -#define bond_for_each_slave_from(bond, pos, cnt, start) \
> -	for (cnt = 0, pos = start; pos && cnt < (bond)->slave_cnt; \
> -	     cnt++, pos = bond_next_slave(bond, pos))
> +#define bond_for_each_slave_from(bond, pos, start) \
> +	for (int cnt = 0, pos = start; pos && cnt < (bond)->slave_cnt; \
> +	    cnt++, pos = bond_next_slave(bond, pos))
> +
Please read below the argument against using the nested cnt definition.

> +/**
> + * bond_for_each_slave_from_rcu - iterate the slaves list from a starting point
> + * @bond:	the bond holding this list.
> + * @pos:	current slave.
> + * @start:	starting point.
> + *
> + * Caller must hold rcu_read_lock
> + */
> +#define bond_for_each_slave_from_rcu(bond, pos, start) \
> +	for (int cnt = 0, pos = start; pos && cnt < (bond)->slave_cnt; \
> +	    cnt++, pos = bond_next_slave_rcu(bond, pos))
>  
I don't think you can rely on slave_cnt in RCU, you may go overboard and pass
twice over the same slave if a slave gets removed, or the opposite. Also this
definition of cnt is troublesome because the name is quite common, I don't know
if it's accepted, but it if it is at least change the name to something like
__cnt or anything that is less likely to be defined.
The cnt argument goes for bond_for_each_slave_from as well.

>  /**
>   * bond_for_each_slave - iterate over all slaves
>