[dpdk-dev] [PATCH] examples/l3fwd: fix segfault with gcc 5.x

[Ananyev, Konstantin]

Hi Tomasz,

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Tomasz Kulasek
> Sent: Monday, April 04, 2016 3:45 PM
> To: dev at dpdk.org
> Subject: [dpdk-dev] [PATCH] examples/l3fwd: fix segfault with gcc 5.x
> 
> It seems that with gcc >5.x and -O2/-O3 optimization breaks packet grouping
> algorithm.
> 
> When last packet pointer "lp" and "pnum->u64" buffer points the same
> memory buffer, high optimization can cause unpredictable results. It seems
> that assignment of precalculated group sizes may interfere with
> initialization of new group size when lp points value inside current group
> and didn't should be changed.
> 
> With gcc >5.x and optimization we cannot be sure which assignment will be
> done first, so the group size can be counted incorrectly.
> 
> This patch eliminates intersection of assignment of initial group size
> (lp[0] = 1) and precalculated group sizes when gptbl[v].idx < 4.
> 
> Fixes: 94c54b4158d5 ("examples/l3fwd: rework exact-match")
> 
> Signed-off-by: Tomasz Kulasek <tomaszx.kulasek at intel.com>
> ---
>  examples/l3fwd/l3fwd_sse.h |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/examples/l3fwd/l3fwd_sse.h b/examples/l3fwd/l3fwd_sse.h
> index f9cf50a..1afa1f0 100644
> --- a/examples/l3fwd/l3fwd_sse.h
> +++ b/examples/l3fwd/l3fwd_sse.h
> @@ -283,9 +283,9 @@ port_groupx4(uint16_t pn[FWDSTEP + 1], uint16_t *lp, __m128i dp1, __m128i dp2)
> 
>  	/* if dest port value has changed. */
>  	if (v != GRPMSK) {
> -		lp = pnum->u16 + gptbl[v].idx;
> -		lp[0] = 1;
>  		pnum->u64 = gptbl[v].pnum;
> +		pnum->u16[FWDSTEP] = 1;

Hmm, but  FWDSTEP and gptbl[v].idx are not always equal.
Actually could you explain a bit more - what exactly is reordered by gcc 5.x,
and how to reproduce it?
i.e what sequence of input packets will trigger an error? 
Konstantin

> +		lp = pnum->u16 + gptbl[v].idx;
>  	}
> 
>  	return lp;
> --
> 1.7.9.5