netdev - Re: [PATCH net-next] bonding: use rtnl_deref in bond_change_rx

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Date:	Wed, 16 Jul 2014 11:05:48 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Veaceslav Falico <vfalico@...il.com>
Cc:	netdev@...r.kernel.org, Jay Vosburgh <j.vosburgh@...il.com>,
	Andy Gospodarek <andy@...yhouse.net>
Subject: Re: [PATCH net-next] bonding: use rtnl_deref in
 bond_change_rx_flags()

On Wed, 2014-07-16 at 09:24 +0200, Veaceslav Falico wrote:
> As it's always called with RTNL held, via dev_set_allmulti/promiscuity.
> 
> CC: Jay Vosburgh <j.vosburgh@...il.com>
> CC: Andy Gospodarek <andy@...yhouse.net>
> Signed-off-by: Veaceslav Falico <vfalico@...il.com>
> ---
>  drivers/net/bonding/bond_main.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index d643807..2998710 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -498,7 +498,7 @@ static int bond_set_promiscuity(struct bonding *bond, int inc)
>  	int err = 0;
>  
>  	if (bond_uses_primary(bond)) {
> -		struct slave *curr_active = bond_deref_active_protected(bond);
> +		struct slave *curr_active = rtnl_dereference(bond->curr_active_slave);
>  
>  		/* write lock already acquired */

This seems dubious to me. What really protects curr_active_slave being
modified ?

Considering the presence of the previous comment, I assumed the sync
point was the write lock. Not rtnl.

If write lock is never held without rtnl, then maybe the write lock is
useless, I don't know.

But after your patch its not really consistent and we increase
confusion.

>  		if (curr_active)
> @@ -524,7 +524,7 @@ static int bond_set_allmulti(struct bonding *bond, int inc)
>  	int err = 0;
>  
>  	if (bond_uses_primary(bond)) {
> -		struct slave *curr_active = bond_deref_active_protected(bond);
> +		struct slave *curr_active = rtnl_dereference(bond->curr_active_slave);
>  
>  		/* write lock already acquired */
>  		if (curr_active)


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