Re: [PATCH net-next] rocker: fix neigh tbl index increment race

[Simon Horman <simon.horman@netronome.com>]

On Fri, Jun 12, 2015 at 09:24:40PM -0700, sfeldma@gmail.com wrote:
> From: Scott Feldman <sfeldma@gmail.com>
> 
> rocker->neigh_tbl_next_index is used to generate unique indices for neigh
> entries programmed into the device.  The way new indices were generated was
> racy with the new prepare-commit transaction model.  A simple fix here
> removes the race.  The race was with two processes getting the same index,
> one process using prepare-commit, the other not:
> 
> Proc A					Proc B
> 
> PREPARE phase
> get neigh_tbl_next_index
> 
> 					NONE phase
> 					get neigh_tbl_next_index
> 					neigh_tbl_next_index++
> 
> COMMIT phase
> neigh_tbl_next_index++
> 
> Both A and B got the same index.  The fix is to store and increment
> neigh_tbl_next_index in the PREPARE (or NONE) phase and use value in COMMIT
> phase:
> 
> Proc A					Proc B
> 
> PREPARE phase
> get neigh_tbl_next_index
> neigh_tbl_next_index++
> 
> 					NONE phase
> 					get neigh_tbl_next_index
> 					neigh_tbl_next_index++
> 
> COMMIT phase
> // use value stashed in PREPARE phase
> 
> Reported-by: Simon Horman <simon.horman@netronome.com>
> Signed-off-by: Scott Feldman <sfeldma@gmail.com>

Reviewed-by: Simon Horman <simon.horman@netronome.com>

> ---
>  drivers/net/ethernet/rocker/rocker.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/rocker/rocker.c b/drivers/net/ethernet/rocker/rocker.c
> index c6a6e3c..a9d1559 100644
> --- a/drivers/net/ethernet/rocker/rocker.c
> +++ b/drivers/net/ethernet/rocker/rocker.c
> @@ -2901,10 +2901,10 @@ static void _rocker_neigh_add(struct rocker *rocker,
>  			      enum switchdev_trans trans,
>  			      struct rocker_neigh_tbl_entry *entry)
>  {
> -	entry->index = rocker->neigh_tbl_next_index;
> +	if (trans != SWITCHDEV_TRANS_COMMIT)
> +		entry->index = rocker->neigh_tbl_next_index++;
>  	if (trans == SWITCHDEV_TRANS_PREPARE)
>  		return;
> -	rocker->neigh_tbl_next_index++;
>  	entry->ref_count++;
>  	hash_add(rocker->neigh_tbl, &entry->entry,
>  		 be32_to_cpu(entry->ip_addr));
> -- 
> 1.7.10.4
>