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

[De Lara Guarch, Pablo]

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ananyev,
> Konstantin
> Sent: Monday, April 04, 2016 4:35 PM
> To: Kulasek, TomaszX
> Cc: dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] examples/l3fwd: fix segfault with gcc 5.x
> 
> 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?

Hi Konstantin,

I could see the issue when having two flows in one port, one going to port 0 and the other to port 1 (using Exact Match).
There is no issue when there is just one flow per port, using an older gcc version (< 5.0) or using O0/O1 (and of course, using LPM).

Pablo


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