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

Kulasek, TomaszX tomaszx.kulasek at intel.com
Tue Apr 5 14:02:09 CEST 2016



> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Monday, April 4, 2016 21:05
> 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
> 
> 
> 
> > -----Original Message-----
> > From: Kulasek, TomaszX
> > Sent: Monday, April 04, 2016 5:20 PM
> > To: Ananyev, Konstantin
> > Cc: dev at dpdk.org
> > Subject: RE: [dpdk-dev] [PATCH] examples/l3fwd: fix segfault with gcc
> > 5.x
> >
> > 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.
> 
> 
> Thanks for explanation Tomasz.
> So it reordered:
> lp[0] = 1;
> pnum->u64 = gptbl[v].pnum;
> correct?
> My first thought was to insert a rte_complier_barrier() between these two
> lines, but actually your approach looks cleaner.
> Konstantin

Yes.


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