Re: [PATCH] drivers: net: xgene: fix: Out of order descriptor bytes read

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

On Thu, 2015-01-22 at 12:03 -0800, Iyappan Subramanian wrote:
> This patch fixes the following kernel crash,
> 
> 	WARNING: CPU: 2 PID: 0 at net/ipv4/tcp_input.c:3079 tcp_clean_rtx_queue+0x658/0x80c()
> 	Call trace:

> 
> Software writes poison data into the descriptor bytes[15:8] and upon
> receiving the interrupt, if those bytes are overwritten by the hardware with
> the valid data, software also reads bytes[7:0] and executes receive/tx
> completion logic.
> 
> If the CPU executes the above two reads in out of order fashion, then the
> bytes[7:0] will have older data and causing the kernel panic.  We have to
> force the order of the reads and thus this patch introduces read memory
> barrier between these reads.
> 
> Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
> Signed-off-by: Keyur Chudgar <kchudgar@apm.com>
> Tested-by: Mark Langsdorf <mlangsdo@redhat.com>
> ---
>  drivers/net/ethernet/apm/xgene/xgene_enet_main.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/net/ethernet/apm/xgene/xgene_enet_main.c b/drivers/net/ethernet/apm/xgene/xgene_enet_main.c
> index 83a5028..3622cdb 100644
> --- a/drivers/net/ethernet/apm/xgene/xgene_enet_main.c
> +++ b/drivers/net/ethernet/apm/xgene/xgene_enet_main.c
> @@ -369,6 +369,8 @@ static int xgene_enet_process_ring(struct xgene_enet_desc_ring *ring,
>  		if (unlikely(xgene_enet_is_desc_slot_empty(raw_desc)))
>  			break;
>  
> +		/* read fpqnum field after dataaddr field */
> +		smp_rmb();
>  		if (is_rx_desc(raw_desc))
>  			ret = xgene_enet_rx_frame(ring, raw_desc);
>  		else

Reading your changelog, it looks like you need a plain rmb() here.