[dpdk-dev] [PATCH v2] ring: enforce reading the tails before ring operations

[Gavin Hu (Arm Technology China)]

> -----Original Message-----
> From: Ilya Maximets <i.maximets at samsung.com>
> Sent: Thursday, March 7, 2019 4:52 PM
> To: Gavin Hu (Arm Technology China) <Gavin.Hu at arm.com>;
> dev at dpdk.org
> Cc: nd <nd at arm.com>; thomas at monjalon.net; jerinj at marvell.com;
> hemant.agrawal at nxp.com; Nipun.gupta at nxp.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli at arm.com>; olivier.matz at 6wind.com
> Subject: Re: [PATCH v2] ring: enforce reading the tails before ring
> operations
> 
> On 07.03.2019 9:45, gavin hu wrote:
> > In weak memory models, like arm64, reading the {prod,cons}.tail may get
> > reordered after reading or writing the ring slots, which corrupts the ring
> > and stale data is observed.
> >
> > This issue was reported by NXP on 8-A72 DPAA2 board. The problem is
> most
> > likely caused by missing the acquire semantics when reading cons.tail (in
> > SP enqueue) or prod.tail (in SC dequeue) which makes it possible to read
> a
> > stale value from the ring slots.
> >
> > For MP (and MC) case, rte_atomic32_cmpset() already provides the
> required
> > ordering. This patch is to prevent reading and writing the ring slots get
> > reordered before reading {prod,cons}.tail for SP (and SC) case.
> 
> Read barrier rte_smp_rmb() is OK to prevent reading the ring get
> reordered
> before reading the tail. However, to prevent *writing* the ring get
> reordered
> *before reading* the tail you need a full memory barrier, i.e.
> rte_smp_mb().

ISHLD(rte_smp_rmb is DMB(ishld) orders LD/LD and LD/ST, while WMB(ST Option) orders ST/ST.
For more details, please refer to: Table B2-1 Encoding of the DMB and DSB <option> parameter  in
https://developer.arm.com/docs/ddi0487/latest/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile 

> 
> >
> > Signed-off-by: gavin hu <gavin.hu at arm.com>
> > Reviewed-by: Ola Liljedahl <Ola.Liljedahl at arm.com>
> > Tested-by: Nipun Gupta <nipun.gupta at nxp.com>
> > ---
> >  lib/librte_ring/rte_ring_generic.h | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> >
> > diff --git a/lib/librte_ring/rte_ring_generic.h
> b/lib/librte_ring/rte_ring_generic.h
> > index ea7dbe5..1bd3dfd 100644
> > --- a/lib/librte_ring/rte_ring_generic.h
> > +++ b/lib/librte_ring/rte_ring_generic.h
> > @@ -90,9 +90,11 @@ __rte_ring_move_prod_head(struct rte_ring *r,
> unsigned int is_sp,
> >  			return 0;
> >
> >  		*new_head = *old_head + n;
> > -		if (is_sp)
> > -			r->prod.head = *new_head, success = 1;
> > -		else
> > +		if (is_sp) {
> > +			r->prod.head = *new_head;
> > +			rte_smp_rmb();
> > +			success = 1;
> > +		} else
> >  			success = rte_atomic32_cmpset(&r->prod.head,
> >  					*old_head, *new_head);
> >  	} while (unlikely(success == 0));
> > @@ -158,9 +160,11 @@ __rte_ring_move_cons_head(struct rte_ring *r,
> unsigned int is_sc,
> >  			return 0;
> >
> >  		*new_head = *old_head + n;
> > -		if (is_sc)
> > -			r->cons.head = *new_head, success = 1;
> > -		else
> > +		if (is_sc) {
> > +			r->cons.head = *new_head;
> > +			rte_smp_rmb();
> > +			success = 1;
> > +		} else
> >  			success = rte_atomic32_cmpset(&r->cons.head,
> *old_head,
> >  					*new_head);
> >  	} while (unlikely(success == 0));
> >