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

[Kulasek, TomaszX]

Hi Konstantin,

> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Monday, April 4, 2016 17:35
> To: Kulasek, TomaszX <tomaszx.kulasek at intel.com>
> 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?
> Konstantin
> 
> > +		lp = pnum->u16 + gptbl[v].idx;
> >  	}
> >
> >  	return lp;
> > --
> > 1.7.9.5


Eg. For this case, when group is changed:

	{
		/* 0xb: a == b, b == c, c != d, d == e */
		.pnum = UINT64_C(0x0002000100020003),
		.idx = 3,
		.lpv = 2,
	},

We expect:

	pnum->u16 = { 3, 2, 1, 2, x }
	lp = pnum->u16 + 3;
	// should be lp[0] == 2

but for gcc 5.2

	lp = pnum->u16 + gptbl[v].idx;
	lp[0] = 1;
	pnum->u64 = gptbl[v].pnum;

gives, for some reason lp[0] == 1, even if pnum->u16[3] == 2.

It causes, that group is shorter and fails trying to send next group with messy length.

We should set lp[0] = 1 only when needed (gptbl[v].idx == 4), so this is why I set pnum->u16[4] = 1. I set it up always to prevent condition. For idx < 4 we don't need to set lp[0].

The problem is that both pointers operates on the same memory buffer and, it seems like gcc optimization will produce (it is wrong):

	lp = pnum->u16 + gptbl[v].idx;
	pnum->u64 = gptbl[v].pnum;
	lp[0] = 1;

except:

	lp = pnum->u16 + gptbl[v].idx;
	lp[0] = 1;
	pnum->u64 = gptbl[v].pnum;

This issue is with gcc 5.x and application seems to fail for the patterns where gptbl[v].idx < 4.

Tomasz