From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: "Kulasek, TomaszX" <tomaszx.kulasek@intel.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [dpdk-dev] [PATCH] examples/l3fwd: fix segfault with gcc 5.x
Date: Mon, 4 Apr 2016 19:05:18 +0000 [thread overview]
Message-ID: <2601191342CEEE43887BDE71AB97725836B2E22E@irsmsx105.ger.corp.intel.com> (raw)
In-Reply-To: <3042915272161B4EB253DA4D77EB373A14E7D5BA@IRSMSX102.ger.corp.intel.com>
> -----Original Message-----
> From: Kulasek, TomaszX
> Sent: Monday, April 04, 2016 5:20 PM
> To: Ananyev, Konstantin
> Cc: dev@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@intel.com>
> > Cc: dev@dpdk.org
> > Subject: RE: [dpdk-dev] [PATCH] examples/l3fwd: fix segfault with gcc 5.x
> >
> > Hi Tomasz,
> >
> > > -----Original Message-----
> > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Tomasz Kulasek
> > > Sent: Monday, April 04, 2016 3:45 PM
> > > To: dev@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@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
next prev parent reply other threads:[~2016-04-04 19:05 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-04 14:45 Tomasz Kulasek
2016-04-04 15:34 ` Ananyev, Konstantin
2016-04-04 15:51 ` De Lara Guarch, Pablo
2016-04-04 16:20 ` Kulasek, TomaszX
2016-04-04 19:05 ` Ananyev, Konstantin [this message]
2016-04-05 12:02 ` Kulasek, TomaszX
2016-04-04 19:06 ` Ananyev, Konstantin
2016-04-06 9:27 ` Thomas Monjalon
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