Re: [net-next-2.6 PATCH] net: zero kobject in rx_queue_release

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 15 novembre 2010 à 22:59 -0800, John Fastabend a écrit :
> netif_set_real_num_rx_queues() can decrement and increment
> the number of rx queues. For example ixgbe does this as
> features and offloads are toggled. Presumably this could
> also happen across down/up on most devices if the available
> resources changed (cpu offlined).
> 
> The kobject needs to be zero'd in this case so that the
> state is not preserved across kobject_put()/kobject_init_and_add().
> 
> This resolves the following error report.
> 
> ixgbe 0000:03:00.0: eth2: NIC Link is Up 10 Gbps, Flow Control: RX/TX
> kobject (ffff880324b83210): tried to init an initialized object, something is seriously wrong.
> Pid: 1972, comm: lldpad Not tainted 2.6.37-rc18021qaz+ #169
> Call Trace:
>  [<ffffffff8121c940>] kobject_init+0x3a/0x83
>  [<ffffffff8121cf77>] kobject_init_and_add+0x23/0x57
>  [<ffffffff8107b800>] ? mark_lock+0x21/0x267
>  [<ffffffff813c6d11>] net_rx_queue_update_kobjects+0x63/0xc6
>  [<ffffffff813b5e0e>] netif_set_real_num_rx_queues+0x5f/0x78
>  [<ffffffffa0261d49>] ixgbe_set_num_queues+0x1c6/0x1ca [ixgbe]
>  [<ffffffffa0262509>] ixgbe_init_interrupt_scheme+0x1e/0x79c [ixgbe]
>  [<ffffffffa0274596>] ixgbe_dcbnl_set_state+0x167/0x189 [ixgbe]
> 
> Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
> ---
> 
>  net/core/net-sysfs.c |    9 +++++++--
>  1 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index 3ba526b..960c075 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c
> @@ -711,13 +711,18 @@ static void rx_queue_release(struct kobject *kobj)
>  
> 
>  	map = rcu_dereference_raw(queue->rps_map);
> -	if (map)
> +	if (map) {
> +		rcu_assign_pointer(queue->rps_map, NULL);

Hmm, yes this works, but I am not sure queue->rps_map can be read by
other cpus at this point.

rcu_assign_pointer() is a documented interface with implied semantic : I
put a NULL pointer on a RCU protected variable, and avoid a memory
barrier because NULL is special.

If this patch is for current kernel, I advise using RCU_INIT_POINTER()
instead to make clear we only want to set the pointer to NULL, and avoid
sparse warnings :)




>  		call_rcu(&map->rcu, rps_map_release);
> +	}
>  
>  	flow_table = rcu_dereference_raw(queue->rps_flow_table);
> -	if (flow_table)
> +	if (flow_table) {
> +		rcu_assign_pointer(queue->rps_flow_table, NULL);


same here ?

>  		call_rcu(&flow_table->rcu, rps_dev_flow_table_release);
> +	}
>  
> +	memset(kobj, 0, sizeof(*kobj));

Is it the regular way to perform this, no kobject_{clear|del|deinit}() ?

>  	dev_put(queue->dev);
>  }
>  
> 

Thanks